The study of physiology prepares you for a wide range of careers. You can apply your knowledge of physiology directly, choosing a career in the biomedical sphere. Physiology graduates can also use the general skills and knowledge they have acquired to pursue a career in a variety of workplaces. Just some of the careers in which Physiology graduates are currently employed are shown below with relevant links where available.

Please note: An additional qualification may be required for some of the careers shown below.

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Careers in Physiology (Physiology Majors) - Monash University

The liver is a vital organ of vertebrates and some other animals.[2] In the human, it is located in the upper right quadrant of the abdomen, below the diaphragm. The liver has a wide range of functions, including detoxification of various metabolites, protein synthesis, and the production of biochemicals necessary for digestion.[3]

The liver is a gland and plays a major role in metabolism with numerous functions in the human body, including regulation of glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification.[3] It is an accessory digestive gland and produces bile, an alkaline compound which aids in digestion via the emulsification of lipids. The gallbladder, a small pouch that sits just under the liver, stores bile produced by the liver.[4] The liver's highly specialized tissue consisting of mostly hepatocytes regulates a wide variety of high-volume biochemical reactions, including the synthesis and breakdown of small and complex molecules, many of which are necessary for normal vital functions.[5] Estimates regarding the organ's total number of functions vary, but textbooks generally cite it being around 500.[6]

Terminology related to the liver often starts in hepat- from the Greek word for liver.[7]

There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. Artificial livers are yet to be developed to promote long term replacement in the absence of the liver. As of now,[8]liver transplantation is the only option for complete liver failure.

The liver is a reddish brown wedge-shaped organ with four lobes of unequal size and shape. A human liver normally weighs 1.441.66kg (3.23.7lb).[9] It is both the heaviest internal organ and the largest gland in the human body. Located in the right upper quadrant of the abdominal cavity, it rests just below the diaphragm, to the right of the stomach and overlies the gallbladder.[4]

The liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood rich in digested nutrients from the entire gastrointestinal tract and also from the spleen and pancreas.[8] These blood vessels subdivide into small capillaries known as liver sinusoids, which then lead to a lobule.

Lobules are the functional units of the liver. Each lobule is made up of millions of hepatic cells (hepatocytes) which are the basic metabolic cells. The lobules are held together by fine areolar tissue which extends into the structure of the liver, by accompanying the vessels (veins and arteries) ducts and nerves through the hepatic portal, as a fibrous capsule called Glisson's capsule.[10] The whole surface of the liver is covered in a serous coat derived from peritoneum and this has an inner fibrous coat (Glisson's capsule) to which it is firmly adhered. The fibrous coat is of areolar tissue and follows the vessels and ducts to support them.

Gross anatomy traditionally divided the liver into two portions a right and a left lobe, as viewed from the front (diaphragmatic) surface; but the underside (the visceral surface) shows it to be divided into four lobes and includes the caudate and quadrate lobes.[11]

The falciform ligament, visible on the front of the liver, divides the liver into a left and a much larger right lobe. From the visceral surface, the two additional lobes are located between the right and left lobes, one in front of the other. A line can be imagined running from the left of the vena cava and all the way forward to divide the liver and gallbladder into two halves.[12] This line is called Cantlie's line.[13]

Other anatomical landmarks exist, such as the ligamentum venosum and the round ligament of the liver (ligamentum teres), which further divide the left side of the liver in two sections. An important anatomical landmark, the porta hepatis, also known as the transverse fissure of the liver, divides this left portion into four segments, which can be numbered starting at the caudate lobe as I in an anticlockwise manner. From this visceral view, seven segments can be seen, because the eighth segment is only visible in the parietal view.[14]

On the diaphragmatic surface, apart from a large triangular bare area where it connects to the diaphragm, the liver is covered by a thin double-layered membrane, the peritoneum, that help reduces friction against other organs.[15] This surface covers the convex shape of the two lobes where it accommodates the shape of the diaphragm. The peritoneum folds back on itself to form the falciform ligament and the right and left triangular ligaments.[16]

These peritoneal ligaments are not related to the anatomic ligaments in joints, and the right and left triangular ligaments have no known functional importance, though they serve as surface landmarks.[16] The falciform ligament functions to attach the liver to the posterior portion of the anterior body wall.

The visceral surface or inferior surface, is uneven and concave. It is covered in peritoneum apart from where it attaches the gallbladder and the porta hepatis.[15]

There are several impressions on the surface of the liver which accommodate the various adjacent structures and organs. Underneath the right lobe and to the right of the gallbladder fossa, are two impressions, one behind the other and separated by a ridge. The one in front is a shallow colic impression, formed by the hepatic flexure and the one behind is a deeper renal impression accommodating part of the right kidney and part of the suprarenal gland.[17]

The suprarenal impression is a small triangular depressed area on the liver. It is located close to the right of the fossa between the bare area and the caudate lobe and immediately above the renal impression. The greater part of the suprarenal impression is devoid of peritoneum and it lodges the right suprarenal gland.[18]

Medial to the renal impression is a third and slightly marked impression, lying between it and the neck of the gall-bladder. This is caused by the descending portion of the duodenum, and is known as the duodenal impression.[18]

The inferior surface of the left lobe of the liver presents behind and to the left the gastric impression.[18] This is moulded over the upper front surface of the stomach, and to the right of this is a rounded eminence, the tuber omentale, which fits into the concavity of the lesser curvature of the stomach and lies in front of the anterior layer of the lesser omentum.

Microscopically, each liver lobe is seen to be made up of hepatic lobules. The lobules are roughly hexagonal, and consist of plates of hepatocytes radiating from a central vein.[19][pageneeded]The central vein joins to the hepatic vein to carry blood out from the liver. A distinctive component of a lobule is the portal triad, which can be found running along each of the lobule's corners. The portal triad, misleadingly named, consists of five structures: a branch of the hepatic artery, a branch of the hepatic portal vein, and a bile duct, as well as lymphatic vessels and a branch of the vagus nerve.[20] Between the hepatocyte plates are liver sinusoids, which are enlarged capillaries through which blood from the hepatic portal vein and hepatic artery enters via the portal triads, then drains to the central vein.[19][pageneeded]

Histology, the study of microscopic anatomy, shows two major types of liver cell: parenchymal cells and non-parenchymal cells. 7085% of the liver volume is occupied by parenchymal hepatocytes. Non-parenchymal cells constitute 40% of the total number of liver cells but only 6.5% of its volume.[21] The liver sinusoids are lined with two types of cell, sinusoidal endothelial cells, and phagocytic Kupffer cells.[22]Hepatic stellate cells are non-parenchymal cells found in the perisinusoidal space, between a sinusoid and a hepatocyte.[21] Additionally, intrahepatic lymphocytes are often present in the sinusoidal lumen.[21]

The central area or hilum, known as the porta hepatis is where the common bile duct, hepatic portal vein, and the hepatic artery proper enter the liver. The duct, vein, and artery divide into left and right branches, and the areas of the liver supplied by these branches constitute the functional left and right lobes.The functional lobes are separated by the imaginary plane, Cantlie's line, joining the gallbladder fossa to the inferior vena cava. The plane separates the liver into the true right and left lobes. The middle hepatic vein also demarcates the true right and left lobes. The right lobe is further divided into an anterior and posterior segment by the right hepatic vein. The left lobe is divided into the medial and lateral segments by the left hepatic vein.

The fissure for the round ligament of the liver (ligamentum teres) also separates the medial and lateral segments. The medial segment is also called the quadrate lobe. In the widely used Couinaud (or "French") system, the functional lobes are further divided into a total of eight subsegments based on a transverse plane through the bifurcation of the main portal vein.[23] The caudate lobe is a separate structure which receives blood flow from both the right- and left-sided vascular branches.[24][25] The Couinaud classification of liver anatomy divides the liver into eight functionally independent segments. Each segment has its own vascular inflow, outflow and biliary drainage. In the centre of each segment there is a branch of the portal vein, hepatic artery and bile duct. In the periphery of each segment there is vascular outflow through the hepatic veins.[26] The division of the liver into independent units means that segments can be resected without damaging the remaining segments.[27] To preserve the viability of the liver following surgery, resections follow the vessels defining the peripheries of each segment. This means that resection lines parallel the hepatic veins, leaving the portal veins, bile ducts, and hepatic arteries intact.[23]

The classification system uses the vascular supply in the liver to separate the functional units (numbered I to VIII):

The remainder of the units (II to VIII) are numbered in a clockwise fashion:[26]

Units V to VIII make up the right part of the liver:[26]

Organogenesis, the development of the organs takes place from the third to the eighth week in human embryogenesis. The origins of the liver lie in both the ventral portion of the foregut endoderm (endoderm being one of the 3 embryonic germ layers) and the constituents of the adjacent septum transversum mesenchyme. In the human embryo, the hepatic diverticulum is the tube of endoderm that extends out from the foregut into the surrounding mesenchyme. The mesenchyme of septum transversum induces this endoderm to proliferate, to branch, and to form the glandular epithelium of the liver. A portion of the hepatic diverticulum (that region closest to the digestive tube) continues to function as the drainage duct of the liver, and a branch from this duct produces the gallbladder.[28] Besides signals from the septum transversum mesenchyme, fibroblast growth factor from the developing heart also contributes to hepatic competence, along with retinoic acid emanating from the lateral plate mesoderm. The hepatic endodermal cells undergo a morphological transition from columnar to pseudostratified resulting in thickening into the early liver bud. Their expansion forms a population of the bipotential hepatoblasts.[29]Hepatic stellate cells are derived from mesenchyme.[30]

After migration of hepatoblasts into the septum transversum mesenchyme, the hepatic architecture begins to be established, with liver sinusoids and bile canaliculi appearing. The liver bud separates into the lobes. The left umbilical vein becomes the ductus venosus and the right vitelline vein becomes the portal vein. The expanding liver bud is colonized by hematopoietic cells. The bipotential hepatoblasts begin differentiating into biliary epithelial cells and hepatocytes. The biliary epithelial cells differentiate from hepatoblasts around portal veins, first producing a monolayer, and then a bilayer of cuboidal cells. In ductal plate, focal dilations emerge at points in the bilayer, become surrounded by portal mesenchyme, and undergo tubulogenesis into intrahepatic bile ducts. Hepatoblasts not adjacent to portal veins instead differentiate into hepatocytes and arrange into cords lined by sinudoidal epithelial cells and bile canaliculi. Once hepatoblasts are specified into hepatocytes and undergo further expansion, they begin acquiring the functions of a mature hepatocyte, and eventually mature hepatocytes appear as highly polarized epithelial cells with abundant glycogen accumulation. In the adult liver, hepatocytes are not equivalent, with position along the portocentrovenular axis within a liver lobule dictating expression of metabolic genes involved in drug metabolism, carbohydrate metabolism, ammonia detoxification, and bile production and secretion. WNT/-catenin has now been identified to be playing a key role in this phenomenon.[29]

In the growing fetus, a major source of blood to the liver is the umbilical vein which supplies nutrients to the growing fetus. The umbilical vein enters the abdomen at the umbilicus, and passes upward along the free margin of the falciform ligament of the liver to the inferior surface of the liver. There it joins with the left branch of the portal vein. The ductus venosus carries blood from the left portal vein to the left hepatic vein and then to the inferior vena cava, allowing placental blood to bypass the liver.

In the fetus, the liver does not perform the normal digestive processes and filtration of the infant liver because nutrients are received directly from the mother via the placenta. The fetal liver releases some blood stem cells that migrate to the fetal thymus, creating the T-cells or T-lymphocytes. After birth, the formation of blood stem cells shifts to the red bone marrow.

After two to five days, the umbilical vein and ductus venosus are completely obliterated; the former becomes the round ligament of liver and the latter becomes the ligamentum venosum. In the disorders of cirrhosis and portal hypertension, the umbilical vein can open up again.

At birth the liver comprises roughly 4% of body weight and is at average 120 g. Over the course of development, it will increase to 1.41.6kg but will only take up 2.53.5% of body weight.[31]

The various functions of the liver are carried out by the liver cells or hepatocytes. The liver is thought to be responsible for up to 500 separate functions, usually in combination with other systems and organs. Currently, there is no artificial organ or device capable of reproducing all the functions of the liver. Some functions can be carried out by liver dialysis, an experimental treatment for liver failure.

The liver receives a dual blood supply from the hepatic portal vein and hepatic arteries. The hepatic portal vein delivers approximately 75% of the liver's blood supply, and carries venous blood drained from the spleen, gastrointestinal tract, and its associated organs. The hepatic arteries supply arterial blood to the liver, accounting for the remaining quarter of its blood flow. Oxygen is provided from both sources; approximately half of the liver's oxygen demand is met by the hepatic portal vein, and half is met by the hepatic arteries.[32]

Blood flows through the liver sinusoids and empties into the central vein of each lobule. The central veins coalesce into hepatic veins, which leave the liver and drain into the inferior vena cava.[20]

The biliary tract is derived from the branches of the bile ducts. The biliary tract, also known as the biliary tree, is the path by which bile is secreted by the liver then transported to the first part of the small intestine, the duodenum. The bile produced in the liver is collected in bile canaliculi, small grooves between the faces of adjacent hepatocytes. The canaliculi radiate to the edge of the liver lobule, where they merge to form bile ducts. Within the liver, these ducts are termed intrahepatic bile ducts, and once they exit the liver they are considered extrahepatic. The intrahepatic ducts eventually drain into the right and left hepatic ducts, which exit the liver at the transverse fissure, and merge to form the common hepatic duct. The cystic duct from the gallbladder joins with the common hepatic duct to form the common bile duct.[20]

Bile either drains directly into the duodenum via the common bile duct, or is temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the second part of the duodenum together at the hepatopancreatic ampulla, also known as the ampulla of Vater.

The liver plays a major role in carbohydrate, protein, amino acid, and lipid metabolism.

The liver performs several roles in carbohydrate metabolism: The liver synthesizes and stores approximately 100g of glycogen via glycogenesis, the formation of glycogen from glucose. When needed, the liver releases glucose into the blood by performing glycogenolysis, the breakdown of glycogen into glucose.[33] The liver is also responsible for gluconeogenesis, which is the synthesis of glucose from certain amino acids, lactate or glycerol. Adipose and liver cells produce glycerol by breakdown of fat, which the liver uses for gluconeogenesis.[33]

The liver is responsible for the mainstay of protein metabolism, synthesis as well as degradation. It is also responsible for a large part of amino acid synthesis. The liver plays a role in the production of clotting factors as well as red blood cell production. Some of the proteins synthesized by the liver include coagulation factors I (fibrinogen), II (prothrombin), V, VII, VIII, IX, X, XI, XIII, as well as protein C, protein S and antithrombin. In the first trimester fetus, the liver is the main site of red blood cell production. By the 32nd week of gestation, the bone marrow has almost completely taken over that task. The liver is a major site of production for thrombopoietin, a glycoprotein hormone that regulates the production of platelets by the bone marrow.[34]

The liver plays several roles in lipid metabolism: it performs cholesterol synthesis, lipogenesis, the production of triglycerides, and a bulk of the body's lipoproteins are synthesized in the liver.

The liver plays a key role in digestion, as it produces and excretes bile (a yellowish liquid) required for emulsifying fats and help the absorption of vitamin K from the diet. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder.

The liver also produces insulin-like growth factor 1 (IGF-1), a polypeptide protein hormone that plays an important role in childhood growth and continues to have anabolic effects in adults.

The liver is responsible for the breakdown of insulin and other hormones. The liver breaks down bilirubin via glucuronidation, facilitating its excretion into bile. The liver is responsible for the breakdown and excretion of many waste products. It plays a key role in breaking down or modifying toxic substances (e.g., methylation) and most medicinal products in a process called drug metabolism. This sometimes results in toxication, when the metabolite is more toxic than its precursor. Preferably, the toxins are conjugated to avail excretion in bile or urine. The liver breaks down ammonia into urea as part of the urea cycle, and the urea is excreted in the urine.[19]

The oxidative capacity of the liver decreases with aging and therefore any medications that require oxidation (for instance, benzodiazepines) are more likely to accumulate to toxic levels. However, medications with shorter half-lives, such as lorazepam and oxazepam, are preferred in most cases when benzodiazepines are required in regard to geriatric medicine.

The liver is a vital organ and supports almost every other organ in the body. Because of its strategic location and multidimensional functions, the liver is also prone to many diseases.[36] The bare area of the liver is a site that is vulnerable to the passing of infection from the abdominal cavity to the thoracic cavity.

Hepatitis is a common condition of inflammation of the liver. The most usual cause of this is viral, and the most common of these infections are hepatitis A, B, C, D, and E. Some of these infections are sexually transmitted. Inflammation can also be caused by other viruses in the Herpesviridae family such as the herpes simplex virus. Infection with hepatitis B virus or hepatitis C virus is the main cause of liver cancer.[37]

Hepatic encephalopathy is caused by an accumulation of toxins in the bloodstream that are normally removed by the liver. This condition can result in coma and can prove fatal.

Other disorders caused by excessive alcohol consumption are grouped under alcoholic liver diseases and these include alcoholic hepatitis, fatty liver, and cirrhosis. Liver damage can also be caused by drugs, particularly paracetamol and drugs used to treat cancer.

BuddChiari syndrome is a condition caused by blockage of the hepatic veins (including thrombosis) that drain the liver. It presents with the classical triad of abdominal pain, ascites and liver enlargement.[38]

Primary biliary cirrhosis is an autoimmune disease of the liver.[39][40] It is marked by slow progressive destruction of the small bile ducts of the liver, with the intralobular ducts (Canals of Hering) affected early in the disease.[41] When these ducts are damaged, bile and other toxins build up in the liver (cholestasis) and over time damages the liver tissue in combination with ongoing immune related damage. This can lead to scarring (fibrosis) and cirrhosis.

Many diseases of the liver are accompanied by jaundice caused by increased levels of bilirubin in the system. The bilirubin results from the breakup of the hemoglobin of dead red blood cells; normally, the liver removes bilirubin from the blood and excretes it through bile.

There are also many pediatric liver diseases, including biliary atresia, alpha-1 antitrypsin deficiency, alagille syndrome, progressive familial intrahepatic cholestasis, Langerhans cell histiocytosis and hepatic hemangioma a benign tumour the most common type of liver tumour, thought to be congenital. Diseases that interfere with liver function will lead to derangement of these processes. However, the liver has a great capacity to regenerate and has a large reserve capacity. In most cases, the liver only produces symptoms after extensive damage.

Hepatomegaly refers to an enlarged liver and can be due to many causes. It can be palpated in a liver span measurement.

Liver diseases may be diagnosed by liver function testsblood tests that can identify various markers. For example, acute-phase reactants are produced by the liver in response to injury or inflammation.

The classic symptoms of liver damage include the following:

The diagnosis of liver disease is made by liver function tests, groups of blood tests, that can readily show the extent of liver damage. If infection is suspected, then other serological tests will be carried out. Sometimes, an ultrasound or a CT scan is needed to produce an image of the liver.

Physical examination of the liver can only reveal its size and any tenderness, and some form of imaging will also be needed.[43]

Axial CT image showing anomalous hepatic veins coursing on the subcapsular anterior surface of the liver.[44]

Maximum intensity projection (MIP) CT image as viewed anteriorly showing the anomalous hepatic veins coursing on the anterior surface of the liver

Lateral MIP view in the same patient

A CT scan in which the liver and portal vein are shown.

Damage to the liver is sometimes determined with a biopsy, particularly when the cause of liver damage is unknown. In the 21st century they have been largely replaced by high-resolution radiographic scans. The latter do not require ultrasound guidance, lab involvement, microscopic analysis, organ damage, pain, or patient sedation; and the results are available immediately on a computer screen.[citation needed]

In a biopsy, a needle is inserted into the skin just below the rib cage and a tissue sample obtained. The tissue is sent to the laboratory, where it is analyzed under a microscope. Sometimes, a radiologist may assist the physician performing a liver biopsy by providing ultrasound guidance.[45]

The liver is the only human internal organ capable of natural regeneration of lost tissue; as little as 25% of a liver can regenerate into a whole liver.[46] This is, however, not true regeneration but rather compensatory growth in mammals.[47] The lobes that are removed do not regrow and the growth of the liver is a restoration of function, not original form. This contrasts with true regeneration where both original function and form are restored. In some other species, such as fish, the liver undergoes true regeneration by restoring both shape and size of the organ.[48] In the liver, large areas of the tissues are formed but for the formation of new cells there must be sufficient amount of material so the circulation of the blood becomes more active.[49]

This is predominantly due to the hepatocytes re-entering the cell cycle. That is, the hepatocytes go from the quiescent G0 phase to the G1 phase and undergo mitosis. This process is activated by the p75 receptors.[50] There is also some evidence of bipotential stem cells, called hepatic oval cells or ovalocytes (not to be confused with oval red blood cells of ovalocytosis), which are thought to reside in the canals of Hering. These cells can differentiate into either hepatocytes or cholangiocytes. Cholangiocytes are the epithelial lining cells of the bile ducts.[51] They are cuboidal epithelium in the small interlobular bile ducts, but become columnar and mucus secreting in larger bile ducts approaching the porta hepatis and the extrahepatic ducts.

Scientific and medical works about liver regeneration often refer to the Greek Titan Prometheus who was chained to a rock in the Caucasus where, each day, his liver was devoured by an eagle, only to grow back each night. The myth suggests the ancient Greeks may have known about the livers remarkable capacity for self-repair.[52]

Human liver transplants were first performed by Thomas Starzl in the United States and Roy Calne in Cambridge, England in 1963 and 1967, respectively.

Liver transplantation is the only option for those with irreversible liver failure. Most transplants are done for chronic liver diseases leading to cirrhosis, such as chronic hepatitis C, alcoholism, autoimmune hepatitis, and many others. Less commonly, liver transplantation is done for fulminant hepatic failure, in which liver failure occurs over days to weeks.

Liver allografts for transplant usually come from donors who have died from fatal brain injury. Living donor liver transplantation is a technique in which a portion of a living person's liver is removed and used to replace the entire liver of the recipient. This was first performed in 1989 for pediatric liver transplantation. Only 20 percent of an adult's liver (Couinaud segments 2 and 3) is needed to serve as a liver allograft for an infant or small child.

More recently, adult-to-adult liver transplantation has been done using the donor's right hepatic lobe, which amounts to 60 percent of the liver. Due to the ability of the liver to regenerate, both the donor and recipient end up with normal liver function if all goes well. This procedure is more controversial, as it entails performing a much larger operation on the donor, and indeed there have been at least two donor deaths out of the first several hundred cases. A recent publication has addressed the problem of donor mortality, and at least 14 cases have been found.[53] The risk of postoperative complications (and death) is far greater in right-sided operations than that in left-sided operations.

With the recent advances of noninvasive imaging, living liver donors usually have to undergo imaging examinations for liver anatomy to decide if the anatomy is feasible for donation. The evaluation is usually performed by multidetector row computed tomography (MDCT) and magnetic resonance imaging (MRI). MDCT is good in vascular anatomy and volumetry. MRI is used for biliary tree anatomy. Donors with very unusual vascular anatomy, which makes them unsuitable for donation, could be screened out to avoid unnecessary operations.

MDCT image. Arterial anatomy contraindicated for liver donation

MDCT image. Portal venous anatomy contraindicated for liver donation

MDCT image. 3D image created by MDCT can clearly visualize the liver, measure the liver volume, and plan the dissection plane to facilitate the liver transplantation procedure.

Phase contrast CT image. Contrast is perfusing the right liver but not the left due to a left portal vein thrombus.

In Greek mythology, Prometheus was punished by the gods for revealing fire to humans, by being chained to a rock where a vulture (or an eagle) would peck out his liver, which would regenerate overnight. (The liver is the only human internal organ that actually can regenerate itself to a significant extent.) Many ancient peoples of the Near East and Mediterranean areas practiced a type of divination called haruspicy, where they tried to obtain information by examining the livers of sheep and other animals.

In Plato, and in later physiology, the liver was thought to be the seat of the darkest emotions (specifically wrath, jealousy and greed) which drive men to action.[54] The Talmud (tractate Berakhot 61b) refers to the liver as the seat of anger, with the gallbladder counteracting this.

The Persian, Urdu, and Hindi languages ( or or jigar) refer to the liver in figurative speech to indicate courage and strong feelings, or "their best"; e.g., "This Mecca has thrown to you the pieces of its liver!".[55] The term jan e jigar, literally "the strength (power) of my liver", is a term of endearment in Urdu. In Persian slang, jigar is used as an adjective for any object which is desirable, especially women. In the Zulu language, the word for liver (isibindi) is the same as the word for courage.

The legend of Liver-Eating Johnson says that he would cut out and eat the liver of each man killed after dinner.

In the motion picture The Message, Hind bint Utbah is implied or portrayed eating the liver of Hamza ibn Abd al-Muttalib during the Battle of Uhud. Although there are narrations that suggest that Hind did "taste", rather than eat, the liver of Hamza, the authenticity of these narrations has to be questioned.

On November 26, 1987, the city of Ferrol, Spain, inaugurated what is believed to be the only monument to the liver in the world. The then - major, Jaime Quintanilla, also happened to be a doctor, and thought appropriate to promote the monument. At an approximate cost of $3.200, the monument stands in the village of Baln. A plaque reads (In Galician language, free translation): "The Liver [is the] basis of Life", and below "Through History, Mankind tried to cure all illness. By helping it on this duty, you are doing a great job. We are grateful for it".[56]

The liver of mammals, fowl, and fish are commonly eaten as food by humans. Domestic pig, ox, lamb, calf, chicken, and goose livers are widely available from butchers and supermarkets.

Liver can be baked, boiled, broiled, fried, stir-fried, or eaten raw (asbeh nayeh or sawda naye in Lebanese cuisine, liver sashimi). In many preparations, pieces of liver are combined with pieces of meat or kidneys, like in the various forms of Middle Eastern mixed grill (e.g. meurav Yerushalmi). Liver is often made into spreads. Well-known examples include liver pt, foie gras, chopped liver, and leverpastej. Liver sausages such as Braunschweiger and liverwurst are also a valued meal. Liver sausages may also be used as spreads. A traditional South African delicacy, namely skilpadjies, is made of minced lamb's liver wrapped in netvet (caul fat), and grilled over an open fire.

Animal livers are rich in iron and vitamin A, and cod liver oil is commonly used as a dietary supplement. Traditionally, some fish livers were valued as food, especially the stingray liver. It was used to prepare delicacies, such as poached skate liver on toast in England, as well as the beignets de foie de raie and foie de raie en croute in French cuisine.[57]

The liver is found in all vertebrates, and is typically the largest visceral (internal) organ. Its form varies considerably in different species, and is largely determined by the shape and arrangement of the surrounding organs. Nonetheless, in most species it is divided into right and left lobes; exceptions to this general rule include snakes, where the shape of the body necessitates a simple cigar-like form. The internal structure of the liver is broadly similar in all vertebrates.[58]

An organ sometimes referred to as a liver is found associated with the digestive tract of the primitive chordate Amphioxus. Although it performs many functions of a liver, it is not considered a true liver but a homolog of the vertebrate liver.[59][60][61] The amphioxus hepatic caecum produces the liver-specific proteins vitellogenin, antithrombin, plasminogen, alanine aminotransferase, and insulin/Insulin-like growth factor (IGF)[62]

View of the various organs and blood-vessels in proximity with liver.

Liver lifted to show gall bladder and stomach in situ.

Cross section showing the liver as the large brown mass in the left of the images, right of the individual.

Cross section of an inferior portion of the liver, showing gallbladder and various structures.

Human liver. Visceral surface of liver.

Human liver. Horizontal section to newborn

Showing ligaments and bare area

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Liver - Wikipedia, the free encyclopedia

106

Nobel Prizes in Physiology or Medicine have been awarded between 1901 and 2015.

Medicine Prizes have been given to one Laureate only.

women have been awarded the Medicine Prize so far.

years was the age of the youngest Medicine Laureate ever, Frederick G. Banting, who was awarded the 1923 Medicine Prize for the discovery of insulin.

years was the age of the oldest Medicine Laureate ever, Peyton Rous, when he was awarded the Medicine Prize in 1966 for his discovery of tumour-inducing viruses.

All facts and figures about the Nobel Prize in Physiology or Medicine

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Nobelprize.org

[Photographs: Robyn Lee and J. Kenji Lopez-Alt]

I haven't always been comfortable with foie gras, though I've spent a good chunk of my life working with it. At first, the discomfort was with the taste. I tried it first as a teenager in the form of a cold terrine that tasted mostly of cat food to me. Then again, I also hated mayonnaise, brussels sprouts, and fish at the time, so my young opinion could hardly be trusted. Later on, as my culinary career expanded, I learned to love it.

I learned to appreciate how it spreads like the world's most decadent and flavorful butter when served cold as a torchon. I learned to appreciate how when it's served hot, it's crisp, sweet, and savory, and melts in your mouth like no other food in the world. And then I learned how it's produced. How in order to get the liver to expand to a good 600% of its natural size, the ducks must be force-fed in a practice known as gavage, wherein a long metal tube (like the one on the right) is forcibly inserted into the duck's mouth up to three times a day and a large amount of food is crammed into its gullet until the liver becomes so large that it takes up the vast majority of the bird's body cavity.

My immediate reaction was a slight gag, followed by revulsion, as I imagined the discomfort of having a tube shoved down my own throat. It's a fair and common reaction, though as I later learned, not the correct one. But we'll get to that.

Even if you haven't eaten foie, pretty much everyone is familiar with the abhorrent images of mistreated ducks peddled by PETA and sites like nofoiegras.org, and indeed they are truly disturbing. Ducks crammed into wire cages just big enough to stand in with their filth-encrusted heads sticking out a hole in the front. Their feathers are scraggly and wiry (if present at all), there's often blood coming out of their nostrils, and their faces and feathers are caked with vomit and corn meal. A duck drinks scummy water out of a communal trough running in front of it while just upstream one of its less fortuitous bunkmates sits dead with its head lolling sideways, half submerged in the cloudy green water.

I've no doubt that farms like this exist in the world, and it is a terrible, atrocious tragedy. If this is how all foieor even all meatis produced, I'd become a vegetarian today. But video or photographic footage of one badly managed farm or even a thousand badly managed farms does not prove that the production of foie gras, as a practice, is necessarily harmful to the health or mental well-being of a duck. Foie gras production should be judged not by the worst farms, but by the best, because those are the ones that I'm going to choose to buy my foie from if at all.

So the real question is: is the production of foie gras torturous under even the best of conditions?

Those on one side would answer yes. How could force feeding an animal ever be considered anything but torture? On the other hand are those who claim that American foie farms are positively idyllic with ducks waddling around spacious pens, even queuing up for their gavage, that for a duck, none of the things we consider uncomfortable stress them out in the least. But who's right?

To answer this question, me and a few fellow Serious Eaters (yes, including Dumpling) set out on a brisk fall morning for La Belle Farms in the idyllic Hudson Valley in what was promised to us as a 100% full-access, bottom to top tour of the operation. We'd be free to see anything we liked, no doors would be locked, and we'd be taking cameras and notebooks with us.

Owner Herman Lee, who immigrated from Hong Kong in 1973 to attend the Fashion Institute of Technology before going on to start Bella Poultry, one of the most well-respected chicken operations in the Northeast, began raising Moulard ducks for foie gras in 2000, after spending several years studying the industry both at home and abroad.

In his office, he seems comfortable, almost eager to get started, to show us what his farm does. We waste no time in getting down to the fabrication room, where teams of workers are just beginning to pull ducks out of a walk-in cooler. Freshly killed and plucked the day before, they're now ready to be eviscerated and broken down into their various parts.

With a red-coated USDA inspector watching their every move (the USDA inspector is there every day), the crew gets to work.

"We'll process about 500 ducks today," says Bob Ambrose, Herman's business partner and head of Bella Bella Gourmet, La Belle's value-added line of prepared foods. "The ducks are all stunned in electrified water before we slaughter them, so they're completely unconscious, then we air-chill them and allow them to dry overnight," he explains. The stunning makes for a quick, painless death, while air-chilling and drying prevents them from taking on any extra water weight, diluting the flavor of their relatively lean meat.

The animals are unloaded one at a time onto a conveyer belt where the skilled workers go at them, each one making a few vital cuts, assembly line-style. As the first liver is removed, Robyn, our intrepid photographer, gives an audible gasp. "Whoah, that's big!"

Indeed, if you've never seen a whole lobe of foie gras before, the size of it can be a bit shocking. Weighing in at around a pound, each liver is roughly the size of a small football. That's close to 10% of the duck's total body weight, and it takes up the vast majority of the lower half of its body. The livers are passed to a woman who sorts them into two different grades, depending on the amount of bruises and blemishes they have. Large, clean livers get the "A" designation, while the rest are sorted into "B" and "Petite" trays.

Bob is quick to point out that "any mishandling of the ducksrough treatment, that kind of thingwill cause bruising, reducing its price," he explains. "So we've got a strong incentive to be gentle with the birds." Duck handlers, who are mostly female (apparently ducks take better to women) work on an bonus-based program where their pay is bumped for every "A" grade lobe one of their charge produces. It's the first time I've heard of a farm that offers workers a monetary incentive to be gentler with the animals. Bob insists that it works, and that the most experienced feeders can increase the number of A lobes from the normal 55% up to over 70%.

At a wholesale price of around $30 a pound for A's, the liver is the most prized part of the duck, but it's hardly enough to sustain the business.

"We use and sell every part of the duck except the heads and feet," explains Bob. The breasts, known as magret are removed and individually packaged to be sold fresh to chefs and gourmet butchers. Some of them are cured and dried into duck prosciutto, or smoked to a sweet, ham-like flavor. The excess fat (of which there is plenty) gets rendered down and sold to restaurants. The legs are cooked in the traditional french confit style, while the wings are smoked and slow-cooked.

The entire processing room gets sprayed down and disinfected every day. Next door is the killing room, where the ducks are zipped assembly-line style from the stunning station to the killing/bleeding station to the machines that pluck their feathers, which resemble industrial-sized washing machines lined with rubber fingers. The room is absolutely spotless, the countertops and conveyor belts a gleaming stainless steel.

So far, so good. It's about as clean and organized an operation as I've ever seen in a farm. We put on full-length disposable jumpsuits to protect our street clothes along with face masks, hair nets, and rubber boots to protect the ducks from outside germs, and head towards the sheds where the ducks spend the bulk of their 3 1/2 month lives.*

*That's significantly longer than the 4 1/2 weeks a normal chicken spends on this earth before slaughter.

The real questions would be answered within the walls of these long, corrugated aluminum boxes. I'd consulted with a veterinarian and done some reading on the subject of illnesses in waterfowl, so even before we entered the shed, I had a good idea of what to look for to recognize sick or distressed birds. I wanted to be sure that I could judge for myself how well-off these ducks were.

Labored breathing, discharge from the nostrils, infected or cloudy eyes are all signs of sickness or stress. Bleeding beaks or feet and missing feathers would indicate rough treatment or fighting amongst themselves. I walked into the shed prepared for the worst, and instead was quite stunned.

Far from the cramped, cruel conditions shown in the videos and photographs I'd seen, here was an enormous shed, full of birds free to roam as they pleased. They congregated in groups, quietly quacking at each other, roamed freely over the sawdust-strewn floor, even stretched their wings for a flap now and then. Granted, it did smella distinct barnyard aroma with a hint of ammonia (the chicken shed we visited afterwards had a much stronger ammonia smell to it), but as anyone who's worked on an animal farm will tell you, all farms smell, just as before the introduction of modern plumbing, all cities smelled as well.

Incidentally, all the birds here are male. The female Moulards don't grow livers as well as males, and are therefore not as profitable. Like the other foie farms in this country, La Belle sends their female ducklings to Trinidad within weeks of hatching where they are raised for meat.

It's true, there could have been more natural sunlight (a few large screened windows with fans in them were spaced along each side of the structure), and the air could have been fresher, but all in all, asides from the truly free-range chickens I've seen in backyards and a few small farms in New England and New York, and some of the boutique chef-run "education center" style farms, these were probably the most well-accommodated farm animals I'd ever seen. When asked about the light and air situation, Herman explained that "the animals are kept off of antibiotics, so we have to keep them minimally exposed to the outdoors." They'd let them out if they could, but wild bird populations can easily introduce deadly bacteria to domestic flocks, he says.

The birds seemed to show a mild aversion to us, flocking together and giving us a wide berth as we walked through the shed. Chichi quickly spotted a single dead bird, which we inquired about. La Belle shows a mortality rate of around 1% in their ducks, which may seem large, but it's less than 1/5th the mortality of regular chicken or duck farms, and about 10 times lower than the mortality and injury rate of the backyard chickens I'm acquainted with.

Eventually the ducks became a little less edgy, and I was able to move in for a closer look. All signs pointed to completely healthy animals. Their beaks were clean, their eyes were bright, they had no trouble vocalizing, and their feathers were for the most part completely intact. They seemed to waddle around with a positive swagger, congregating at the water dripper and feeding stations.

The facts so far: for at least the first 12 weeks of their lives, these ducks are sitting pretty in a stress-free, spacious environment. The next shed is where the ducks spend their last 25 dayswhere the gavage takes place.

Before we went inside, we were told that this was the only part of the tour where we would not be allowed to take photographs or video. Ah, I thoughta sure sign that what we are about to see is going to put us off our lunches (or tasting menus, as the case may be). But Bob explains: It's not that they have anything to hide with the procedure itself, it's that they've recently began employing a new custom-designed piece of technology that they don't want the two competing farms to get their hands on. We'd see it in action in a moment.

We entered another long shed, this one filled from end to end with 5-foot by 7-foot pens, each one holding about 10 ducks. Again, the ducks tended to congregate together, leaving more than half of the space in their pen empty. Occasionally, one would waddle out of the group for a stretch. Just as in the other sheds, these ducks seemed healthy, albeit much larger (these guys were on their third week of gavage, just a few days away from slaughter).

We walked down row after row of pens until we got to one where a worker was just about to start feeding. At La Belle, the ducks are fed three times a day for a total of up to 240 grams of their custom-designed feed. As we watched, the workera petit womanclimbed into the pen and sat on an overturned box. One at a time, she pulled a duck towards her and held it between her legs with its neck arched upwards. She gently squeezed the base of the duck's neck ("checking to make sure that he's finished all his food from the last feeding," says Bob), then eases a flexible plastic tube down the ducks throat. A machine whirls, a small bulge forms where the food is deposited, and the duck walks off, giving its head one shake, but otherwise seemingly unaffected.

While most other farms in the world still use metal tubes to feed their duck, La Belle has recently switched to a custom-made flexible plastic version. This is the piece of technology that they didn't want us filming.

However, a quick search on YouTube turned up this video, which is not dissimilar from our own experience (this video shows geese in France):

According to Bob, when the feeder feels the duck's esophagus, if there's any food remaining, she'll skip that feeding. So while the ducks are technically force-fed, there is a level of built-in anatomical control so that the ducks can't take in any more food than they can physically handle. That's more respect than most fast food chains show for their human customers.

La Belle has also started a program to reduce their workers' load. Many farms require that the same feeder work with the same ducks for the entire gavage process to reduce stress on the animal. For a worker, this means three long feeding shifts per day, every day, for 25 days.

A few years ago, they discovered that it's not the actual worker that the ducks grow accustomed to, it's just their sight and smell. They found that by having two different workers wear the same set of clothes, the ducks would respond to the second as if they were still the first. In fact, after starting their workers on this split-shift system, production of A graded foie actually increased.

I wouldn't exactly say that the ducks were lining up to be fed, as has been suggested by some foie advocates, but they certainly didn't seem stressed. By all activists accounts, these ducks should have been so fattened that they could barely stand under their own power. I didn't see one duck vomit, nor did I see any that couldn't stand or walk due to the weight of their livers.

After the walking tour, we stopped back at the office for a tasting of a few of Bella Bella's products, as well as some straight-up fresh foie, seared on a George Foreman griddle, of all things. Of all the foie I've cooked in the world (and it's a lot), La Belle's has the unique property of being able to hold its shape well without rendering off too much fat, making it an ideal candidate for searing.

We finished the day eating our foie, talking to Herman and Bob about their business. Back home, I started doing some more research.

We'd seen the process from start to finish, and from all outward appearances, the ducks seem to live perfectly comfortable livesat least as well as you can expect for any farm animal. Certainly far better lives than the millions of cows and pigs and billions of chickens that are raised every year for our consumption. But the question I had was, why aren't they more uncomfortable? Why doesn't a duck struggle with its large liver or having a tube forced down its throat?

First off, the key to understanding this is to make a very conscious effort not to anthropomorphize the animals. As waterfowl, they are distinctly not human, and their physiology differs from ours in a few key ways. Let's take a look at the foie gras duck, shall we?

In this country, foie gras is produced exclusively from Moulard ducks. The offspring of a male Muscovy and a female Pekin duck, Moulards offer many physiological and temperamental advantages that make them ideal for producing foie, and I believe an understanding of the breed can help clear up a lot of misconceptions.

Muscovies are an incredibly hardy species. Though native to the tropical regions of South America, they are nevertheless able to adapt to temperate climates, and are even comfortable living in sub-zero conditions. As such, they are non-migratory. This is important, because it means that unlike migratory species, they don't ever have the need to gorge themselves to put on extra fat to carry them through long periods with no food. They are an aggressive species; Males attack each other with their bills and sharp claws on their feet. Despite this, they are prized for their well-flavored, lean meat. Their robust nature and tolerance of many climates make them quite easy to farm.

Pekin ducks (also known as Long Island ducks) on the other hand were originally bred in China from wild mallards, and thus have many of the characteristics of that migratory species. They are relatively petite birds who are quite gregarious. They enjoy hanging out in groups and will naturally stand together in very tight quarters, whether or not they have the space to roam around. Years of breeding have shrunk their wings and increased their breast size. Because of their plump stature, they can't jump much higher than your average womp-rat, and thus no longer migrate (which isn't to say they wouldn't waddle south for the winter, given the opportunity), but their inner organs and basic metabolism are still that of a migratory waterfowl.

When you cross a male Muscovy with a female Pekin, you get a Moulard, a hybrid that combines the more desirable behavioral features of the two species. First off, it's larger and more robust than either a Muscovy or Pekin, much in the way that a mule is bigger and stronger than either the horse or donkey it was bred from (Moulards are also sterile, like mules, and are often referred to as "mule ducks"). Like Pekins, they don't fly and are relatively gregarious, making group living and containment quite simple for farmers, and non-stressful and safe for the ducks. Their most important feature, howeverand this is importantis that like Muscovies, they don't have the urge to migrate, but like Pekins, they retain all of the interior anatomy necessary for the gorging that migration requires.

This is the real key to the safe and ethical production of foie gras.

You see, migration depends upon gorging. The rapid intake and metabolism of large quantities of food in order to store enough energy to fly south for the winter. So while during the warm summer months, a duck may be content paddling around eating weeds, bugs, and the occasional minnow, when the weather starts getting colder, it begins to eat in earnest, stuffing itself more frequently, and with larger prey. Unlike in humans where excess fat builds up mostly in large deposits just under the skin, with migratory birds, this excess fat builds up both under the skin, and in the liver.

Granted, the production of foie gras requires feeding a duck far more than it would naturally consume (though if you are to believe Dan Barber's fantastic TED talk, there are wild geese who would feed themselves to almost the same degree), but this is true of all farm animals. Cows, pigs, chickens, they all get far fatter from the rich feeds we give them than they'd ever get if left to their own devices. Does that make it cruel? I'd say no. As long as the animal shows no sign of stress or discomfortand the ducks we saw today certainly did notthen what harm is a few extra pounds?

What about the act of feeding? Surely the duck feels discomfort when a tube is slid down its throat?

Tony Bourdain likes to remind us that we see worse things committed against human beings on late night pay-per-view. And he's right: humans have a gag reflex. But ducks? Not so. I tried hard to find a good video online of a duck eating fish, but they are all too blurry or too annoying to watch. The closest I came is this video of a cormorant, another migratory waterfowl.

Watch closely as it swallows a spiky fish several times wider than its neck.

Incredible, right? And that, folks, is the reason why ducks don't struggle when a feeding tube deposits food in its throat. Its body is built for exactly the same type of stress in the wild.

Humans chew their food in their mouth until it breaks down into pieces small enough to swallow. Ducks, on the other hand, have no teeth in their mouth, and they don't chew. Instead, they swallow their food whole, storing it in the bottom of the esophagus in a stretchy pouch known as the crop. Eventually, the solid food works its way into a stomach and a sac-like organ called the gizzard. Throughout the day, a duck will swallow small rocks and pebbles, which get stored in the gizzard. Once food enters it, the muscular organ uses the pebbles as make-shift teeth, grinding up the food so the duck can digest it.

Because of this, their esophagi are custom-built for stretching. I had Bob send a few of them to the office where I tied off one end and filled it up, water-balloon style in order to see exactly how much a duck can hold in its crop. The four we tested stretched out to a little over a quart of liquid apiece, or around 950 gramsfar more than the 80 grams of meal they were fed at each serving.

Surely they'd have difficulty breathing with a tube down their throat though, right? Not so fast. Humans have a single passageway leading from their mouth down into their neck. From there, it divides into the esophagus, which leads to the stomach, and the trachea, which leads to the lungs. Separating these two passages is a little flap of muscle called the epiglottis. Try to force something past the epiglottis, and you trigger a gag reaction. It's intended to make sure that the wrong things don't end up in your stomach.

Ducks, on the other hand, have completely independent tracheas and esophagi. Their esophagus goes straight from the mouth to the crop, while the trachea runs from the lungs and out the end of the tongue. That's right: Ducks breathe through their tongues. The cartilage that surrounds their trachea (called the tracheal ring) is also a complete circle, as opposed to ours, which is C-shaped, making their trachea much sturdier and less prone to collapse. What this means is that you can place a feeding tube in a ducks throat, and it can sit there indefinitely, neither gagging, nor suffocating.

So there it is. The evidence is out there, and from what Bob and Herman tell me, they are more than happy to be transparent with their operations, to let people see what goes on inside their farm. They believe they've got nothing to hide, and so do I. So why is it that activists are so zealous about destroying foie gras operations? I've worked in restaurants that have been picketed by protestors, and they aren't a particularly friendly bunch. Threats have even been made against the lives of chefs and their families for serving it in their restaurants.

In large part, it's because foie gras is an easy target. There are only three foie farms in the country, and none of them have the money or government clout to defend themselves the way that the chicken or beef industry does. It's a food product that is marketed directly at the affluent, and the rich are always an easy target. As an occasional delicacy, it's also a food that's relatively easy for most people to give up.

Personally, I find this kind of protesting abhorrent. If you are going to protest anything, it should be the industrial production of eggs, where chickens are routinely kept in cages so small that they can't even turn around for an entire year. The problem, of course, is that you tell people to stop eating cheap eggs, and nobody will listen. The leaders of the anti-foie movement know this and use it to their advantage, using video and photographs taken from the worst of the farms (none of the ones in this country, for the record), and making it seem like all foie production is as despicable.

If you are against the confinement, slaughter, and eating of all animals, then that's a different argument to be had at a different time. But to single out foie as the worst of the worst is misguided at best, and downright manipulative at worst. Just as there are good eggs and bad eggs, good beef and bad beef, good chicken and bad chicken, so there is good foie and bad foie. We are especially lucky, because we happen to live in a country where all of the foie produced is good foie.

The only question left for me is whether to serve it hot or cold.

Read this article:
The Physiology of Foie: Why Foie Gras is Not Unethical ...

Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants.[1] Closely related fields include plant morphology (structure of plants), plant ecology (interactions with the environment), phytochemistry (biochemistry of plants), cell biology, genetics, biophysics and molecular biology.

Fundamental processes such as photosynthesis, respiration, plant nutrition, plant hormone functions, tropisms, nastic movements, photoperiodism, photomorphogenesis, circadian rhythms, environmental stress physiology, seed germination, dormancy and stomata function and transpiration, both parts of plant water relations, are studied by plant physiologists.

The field of plant physiology includes the study of all the internal activities of plantsthose chemical and physical processes associated with life as they occur in plants. This includes study at many levels of scale of size and time. At the smallest scale are molecular interactions of photosynthesis and internal diffusion of water, minerals, and nutrients. At the largest scale are the processes of plant development, seasonality, dormancy, and reproductive control. Major subdisciplines of plant physiology include phytochemistry (the study of the biochemistry of plants) and phytopathology (the study of disease in plants). The scope of plant physiology as a discipline may be divided into several major areas of research.

First, the study of phytochemistry (plant chemistry) is included within the domain of plant physiology. To function and survive, plants produce a wide array of chemical compounds not found in other organisms. Photosynthesis requires a large array of pigments, enzymes, and other compounds to function. Because they cannot move, plants must also defend themselves chemically from herbivores, pathogens and competition from other plants. They do this by producing toxins and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract pollinators or herbivores to spread ripe seeds.

Secondly, plant physiology includes the study of biological and chemical processes of individual plant cells. Plant cells have a number of features that distinguish them from cells of animals, and which lead to major differences in the way that plant life behaves and responds differently from animal life. For example, plant cells have a cell wall which restricts the shape of plant cells and thereby limits the flexibility and mobility of plants. Plant cells also contain chlorophyll, a chemical compound that interacts with light in a way that enables plants to manufacture their own nutrients rather than consuming other living things as animals do.

Thirdly, plant physiology deals with interactions between cells, tissues, and organs within a plant. Different cells and tissues are physically and chemically specialized to perform different functions. Roots and rhizoids function to anchor the plant and acquire minerals in the soil. Leaves catch light in order to manufacture nutrients. For both of these organs to remain living, minerals that the roots acquire must be transported to the leaves, and the nutrients manufactured in the leaves must be transported to the roots. Plants have developed a number of ways to achieve this transport, such as vascular tissue, and the functioning of the various modes of transport is studied by plant physiologists.

Fourthly, plant physiologists study the ways that plants control or regulate internal functions. Like animals, plants produce chemicals called hormones which are produced in one part of the plant to signal cells in another part of the plant to respond. Many flowering plants bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism. The ripening of fruit and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant.

Finally, plant physiology includes the study of plant response to environmental conditions and their variation, a field known as environmental physiology. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.

The chemical elements of which plants are constructedprincipally carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur, etc.are the same as for all other life forms animals, fungi, bacteria and even viruses. Only the details of the molecules into which they are assembled differs.

Despite this underlying similarity, plants produce a vast array of chemical compounds with unique properties which they use to cope with their environment. Pigments are used by plants to absorb or detect light, and are extracted by humans for use in dyes. Other plant products may be used for the manufacture of commercially important rubber or biofuel. Perhaps the most celebrated compounds from plants are those with pharmacological activity, such as salicylic acid from which aspirin is made, morphine, and digoxin. Drug companies spend billions of dollars each year researching plant compounds for potential medicinal benefits.

Plants require some nutrients, such as carbon and nitrogen, in large quantities to survive. Such nutrients are termed macronutrients, where the prefix macro- (large) refers to the quantity needed, not the size of the nutrient particles themselves. Other nutrients, called micronutrients, are required only in trace amounts for plants to remain healthy. Such micronutrients are usually absorbed as ions dissolved in water taken from the soil, though carnivorous plants acquire some of their micronutrients from captured prey.

The following tables list element nutrients essential to plants. Uses within plants are generalized.

Among the most important molecules for plant function are the pigments. Plant pigments include a variety of different kinds of molecules, including porphyrins, carotenoids, and anthocyanins. All biological pigments selectively absorb certain wavelengths of light while reflecting others. The light that is absorbed may be used by the plant to power chemical reactions, while the reflected wavelengths of light determine the color the pigment appears to the eye.

Chlorophyll is the primary pigment in plants; it is a porphyrin that absorbs red and blue wavelengths of light while reflecting green. It is the presence and relative abundance of chlorophyll that gives plants their green color. All land plants and green algae possess two forms of this pigment: chlorophyll a and chlorophyll b. Kelps, diatoms, and other photosynthetic heterokonts contain chlorophyll c instead of b, red algae possess chlorophyll a and " d". All chlorophylls serve as the primary means plants use to intercept light to fuel photosynthesis.

Carotenoids are red, orange, or yellow tetraterpenoids. They function as accessory pigments in plants, helping to fuel photosynthesis by gathering wavelengths of light not readily absorbed by chlorophyll. The most familiar carotenoids are carotene (an orange pigment found in carrots), lutein (a yellow pigment found in fruits and vegetables), and lycopene (the red pigment responsible for the color of tomatoes). Carotenoids have been shown to act as antioxidants and to promote healthy eyesight in humans.

Anthocyanins (literally "flower blue") are water-soluble flavonoid pigments that appear red to blue, according to pH. They occur in all tissues of higher plants, providing color in leaves, stems, roots, flowers, and fruits, though not always in sufficient quantities to be noticeable. Anthocyanins are most visible in the petals of flowers, where they may make up as much as 30% of the dry weight of the tissue.[2] They are also responsible for the purple color seen on the underside of tropical shade plants such as Tradescantia zebrina. In these plants, the anthocyanin catches light that has passed through the leaf and reflects it back towards regions bearing chlorophyll, in order to maximize the use of available light

Betalains are red or yellow pigments. Like anthocyanins they are water-soluble, but unlike anthocyanins they are indole-derived compounds synthesized from tyrosine. This class of pigments is found only in the Caryophyllales (including cactus and amaranth), and never co-occur in plants with anthocyanins. Betalains are responsible for the deep red color of beets, and are used commercially as food-coloring agents. Plant physiologists are uncertain of the function that betalains have in plants which possess them, but there is some preliminary evidence that they may have fungicidal properties.[3]

Plants produce hormones and other growth regulators which act to signal a physiological response in their tissues. They also produce compounds such as phytochrome that are sensitive to light and which serve to trigger growth or development in response to environmental signals.

Plant hormones, known as plant growth regulators (PGRs) or phytohormones, are chemicals that regulate a plant's growth. According to a standard animal definition, hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in target cells at other locations. Unlike animals, plants lack specific hormone-producing tissues or organs. Plant hormones are often not transported to other parts of the plant and production is not limited to specific locations.

Plant hormones are chemicals that in small amounts promote and influence the growth, development and differentiation of cells and tissues. Hormones are vital to plant growth; affecting processes in plants from flowering to seed development, dormancy, and germination. They regulate which tissues grow upwards and which grow downwards, leaf formation and stem growth, fruit development and ripening, as well as leaf abscission and even plant death.

The most important plant hormones are abscissic acid (ABA), auxins, ethylene, gibberellins, and cytokinins, though there are many other substances that serve to regulate plant physiology.

While most people know that light is important for photosynthesis in plants, few realize that plant sensitivity to light plays a role in the control of plant structural development (morphogenesis). The use of light to control structural development is called photomorphogenesis, and is dependent upon the presence of specialized photoreceptors, which are chemical pigments capable of absorbing specific wavelengths of light.

Plants use four kinds of photoreceptors:[1]phytochrome, cryptochrome, a UV-B photoreceptor, and protochlorophyllide a. The first two of these, phytochrome and cryptochrome, are photoreceptor proteins, complex molecular structures formed by joining a protein with a light-sensitive pigment. Cryptochrome is also known as the UV-A photoreceptor, because it absorbs ultraviolet light in the long wave "A" region. The UV-B receptor is one or more compounds not yet identified with certainty, though some evidence suggests carotene or riboflavin as candidates.[4] Protochlorophyllide a, as its name suggests, is a chemical precursor of chlorophyll.

The most studied of the photoreceptors in plants is phytochrome. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms. It also regulates other responses including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.

Many flowering plants use the pigment phytochrome to sense seasonal changes in day length, which they take as signals to flower. This sensitivity to day length is termed photoperiodism. Broadly speaking, flowering plants can be classified as long day plants, short day plants, or day neutral plants, depending on their particular response to changes in day length. Long day plants require a certain minimum length of daylight to starts flowering, so these plants flower in the spring or summer. Conversely, short day plants flower when the length of daylight falls below a certain critical level. Day neutral plants do not initiate flowering based on photoperiodism, though some may use temperature sensitivity (vernalization) instead.

Although a short day plant cannot flower during the long days of summer, it is not actually the period of light exposure that limits flowering. Rather, a short day plant requires a minimal length of uninterrupted darkness in each 24-hour period (a short daylength) before floral development can begin. It has been determined experimentally that a short day plant (long night) does not flower if a flash of phytochrome activating light is used on the plant during the night.

Plants make use of the phytochrome system to sense day length or photoperiod. This fact is utilized by florists and greenhouse gardeners to control and even induce flowering out of season, such as the Poinsettia.

Paradoxically, the subdiscipline of environmental physiology is on the one hand a recent field of study in plant ecology and on the other hand one of the oldest.[1] Environmental physiology is the preferred name of the subdiscipline among plant physiologists, but it goes by a number of other names in the applied sciences. It is roughly synonymous with ecophysiology, crop ecology, horticulture and agronomy. The particular name applied to the subdiscipline is specific to the viewpoint and goals of research. Whatever name is applied, it deals with the ways in which plants respond to their environment and so overlaps with the field of ecology.

Environmental physiologists examine plant response to physical factors such as radiation (including light and ultraviolet radiation), temperature, fire, and wind. Of particular importance are water relations (which can be measured with the Pressure bomb) and the stress of drought or inundation, exchange of gases with the atmosphere, as well as the cycling of nutrients such as nitrogen and carbon.

Environmental physiologists also examine plant response to biological factors. This includes not only negative interactions, such as competition, herbivory, disease and parasitism, but also positive interactions, such as mutualism and pollination.

Plants may respond both to directional and non-directional stimuli. A response to a directional stimulus, such as gravity or sunlight, is called a tropism. A response to a nondirectional stimulus, such as temperature or humidity, is a nastic movement.

Tropisms in plants are the result of differential cell growth, in which the cells on one side of the plant elongates more than those on the other side, causing the part to bend toward the side with less growth. Among the common tropisms seen in plants is phototropism, the bending of the plant toward a source of light. Phototropism allows the plant to maximize light exposure in plants which require additional light for photosynthesis, or to minimize it in plants subjected to intense light and heat. Geotropism allows the roots of a plant to determine the direction of gravity and grow downwards. Tropisms generally result from an interaction between the environment and production of one or more plant hormones.

Nastic movements results from differential cell growth (e.g. epinasty and hiponasty), or from changes in turgor pressure within plant tissues (e.g., nyctinasty), which may occur rapidly. A familiar example is thigmonasty (response to touch) in the Venus fly trap, a carnivorous plant. The traps consist of modified leaf blades which bear sensitive trigger hairs. When the hairs are touched by an insect or other animal, the leaf folds shut. This mechanism allows the plant to trap and digest small insects for additional nutrients. Although the trap is rapidly shut by changes in internal cell pressures, the leaf must grow slowly to reset for a second opportunity to trap insects.[6]

Economically, one of the most important areas of research in environmental physiology is that of phytopathology, the study of diseases in plants and the manner in which plants resist or cope with infection. Plant are susceptible to the same kinds of disease organisms as animals, including viruses, bacteria, and fungi, as well as physical invasion by insects and roundworms.

Because the biology of plants differs with animals, their symptoms and responses are quite different. In some cases, a plant can simply shed infected leaves or flowers to prevent the spread of disease, in a process called abscission. Most animals do not have this option as a means of controlling disease. Plant diseases organisms themselves also differ from those causing disease in animals because plants cannot usually spread infection through casual physical contact. Plant pathogens tend to spread via spores or are carried by animal vectors.

One of the most important advances in the control of plant disease was the discovery of Bordeaux mixture in the nineteenth century. The mixture is the first known fungicide and is a combination of copper sulfate and lime. Application of the mixture served to inhibit the growth of downy mildew that threatened to seriously damage the French wine industry.[7]

Sir Francis Bacon published one of the first plant physiology experiments in 1627 in the book, Sylva Sylvarum. Bacon grew several terrestrial plants, including a rose, in water and concluded that soil was only needed to keep the plant upright. Jan Baptist van Helmont published what is considered the first quantitative experiment in plant physiology in 1648. He grew a willow tree for five years in a pot containing 200 pounds of oven-dry soil. The soil lost just two ounces of dry weight and van Helmont concluded that plants get all their weight from water, not soil. In 1699, John Woodward published experiments on growth of spearmint in different sources of water. He found that plants grew much better in water with soil added than in distilled water.

Stephen Hales is considered the Father of Plant Physiology for the many experiments in the 1727 book;[8] though Julius von Sachs unified the pieces of plant physiology and put them together as a discipline. His Lehrbuch der Botanik was the plant physiology bible of its time.[9]

Researchers discovered in the 1800s that plants absorb essential mineral nutrients as inorganic ions in water. In natural conditions, soil acts as a mineral nutrient reservoir but the soil itself is not essential to plant growth. When the mineral nutrients in the soil are dissolved in water, plant roots absorb nutrients readily, soil is no longer required for the plant to thrive. This observation is the basis for hydroponics, the growing of plants in a water solution rather than soil, which has become a standard technique in biological research, teaching lab exercises, crop production and as a hobby.

One of the leading journals in the field is Plant Physiology, started in 1926. All its back issues are available online for free.[1] Many other journals often carry plant physiology articles, including Physiologia Plantarum, Journal of Experimental Botany, American Journal of Botany, Annals of Botany, Journal of Plant Nutrition and Proceedings of the National Academy of Sciences.

In horticulture and agriculture along with food science, plant physiology is an important topic relating to fruits, vegetables, and other consumable parts of plants. Topics studied include: climatic requirements, fruit drop, nutrition, ripening, fruit set. The production of food crops also hinges on the study of plant physiology covering such topics as optimal planting and harvesting times and post harvest storage of plant products for human consumption and the production of secondary products like drugs and cosmetics.

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Plant physiology - Wikipedia, the free encyclopedia

Greg Ikonnikov and Dominique Yelle

Clin Anat.2009 Jan;22(1):99-113. Can J Anaesth.1993 Nov;40(11):1053-64.

An organized rhythmic contraction of the heart requires adequate propagation of electrical impulses along the conduction pathway. Of note, the impulses in the His-Purkinje system travel in such a way that papillary muscle contraction precedes that of the ventricles, thereby preventing regurgitation of blood flow through the AV valves.

Physiol Rev.2005 Oct;85(4):1205-53.

Heart Rhythm.2010 Jan;7(1):117-26.

Note: The different types of cardiac ion channels are discussed below, throughout the description of the phases of action potentials in different cardiac cells.

Physiol Rev.2005 Oct;85(4):1205-53.

Action potential: electrical stimulation created by a sequence of ion fluxes through specialized channels in the membrane (sarcolemma) of cardiomyocytes that leads to cardiac contraction.

The action potential in typical cardiomyocytes is composed of 5 phases (0-4), beginning and ending with phase 4.

Pharmacol Ther.2005 Jul;107(1):59-79. Drugs.2007;67 Suppl 2:15-24. (The funny current)

Table 1. Cardiac cell types displaying pacemaker behavior.

Pacemaker

Location

Inherent rate (beats per minute, BPM)

Sinoatrial (SA) node

Right atrium (RA) at junction with superior vena cava (SVC)

60-100 BPM

Atrioventricular (AV) node

RA at posteroinferior area of interatrial septum

40-60 BPM

Purkinje fibers and ventricular cardiomyocytes

Throughout the ventricles

20-40 BPM

The sequence of events for pacemaker action potential:

Nature.2002 Jan 10;415(6868):198-205.

Excitation-contraction coupling represents the process by which an electrical action potential leads to contraction of cardiac muscle cells. This is achieved by converting a chemical signal into mechanical energy via the action of contractile proteins.

Calcium is the crucial mediator that couples electrical excitation to physical contraction by cycling in and out of the myocytes cytosol during each action potential.

Main contractile elements:

Regulatory elements:

The initial influx of Ca2+ into myocytes through L-type Ca2+ channels during phase 2 of the action potential is insufficient to trigger contraction of myofibrils. This signal is amplified by the CICR mechanism, which triggers much greater release of Ca2+ from the sarcoplasmic reticulum.

As with myocyte contraction, this process is synchronized with the electrical activity of the cell.

Adv Physiol Educ.2011 Mar;35(1):28-32.

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