This Chapter is written following discussions with my colleague, Leif Vanggaard, MD, Arctic Institute, Copenhagen.

Study Objectives

To define body core and body shell, heat balance, heat exchange (conduction, conversion, evaporation and radiation), hyperthermia, hypothermia, mean body temperature, heat capacity, and thermal steady state.

To describe fever (pyrogens), benignant and malignant hyperthermia, heat exhaustion, heat syncope, heat stroke, sun stroke, and hypothermia.

To describe radiation sickness.

To calculate one thermal variable, when relevant variables are given.

To explain the concepts heat exchange, thermogenesis by food and shivering, the human temperature control system and its function at different environmental temperatures.

To use the above concepts in problem solving and case histories.

Principles

Newtons law of cooling: The dry heat loss is proportional to the temperature difference between the human body (shell) and the surroundings.

The total energy of a system is conserved in an interaction, not the kinetic energy or the mass (Einstein). If the mass changes during an interaction, there is a resultant change in kinetic energy, so that the total energy remains constant. Heat energy is proportional to molecular movement rates heat energy equals movement.

Stefan-Boltzmanns rule: The higher the temperature of an object, the more it radiates. The energy radiated from an object is proportional to the fourth power of its Kelvin temperature. The energy radiating from an object and received by the human body is proportional to the temperature difference between the object and the skin (see Eq. 21-4). This is because human life implies relatively small temperature gradients.

Definitions

Body core consists of the thermoregulated deeper parts of the body and the proximal extremity portions of warm-blooded animals including man.

Body shell refers to those outer parts of the body (skin and subcutaneous tissue) that change temperature at cold exposure.

Conductance changes of the shell are used as a measure of skin bloodflow.

Conductive heat loss describes a direct transfer of heat energy by contact between two bodies of different temperature (eg, skin and objects).

Convective heat loss is defined as the heat loss by contact between the surface (skin) and a moving medium (air or water).

Evaporative heat loss is defined as the heat loss by evaporation from the body surface or lungs.

Fever occurs when the core temperature of the body is raised above normal steady state levels. The body reacts as if it is too cold. Fever implies a disorder resulting in shivering combined with vasoconstriction, headache, dedolation, and general discomfort (eg, malaria).

Heat flow is defined as energy exchanged due to a temperature difference. Heat flow is transmitted along a temperature gradient.

Heat capacity is the amount of heat required to produce a temperature increase for a given amount of substance.

Heat energy balance in a resting person is a condition, where the heat production is equal to the heat loss. Thus the body temperature is constant and the heat storage is zero (thermal steady state). Usually, there is no internal heat energy flux between body core and shell.

Hyperthermia is an increase in core temperature above normal.

Hypothermia refers to a clinical condition with a lowered core temperature (below 35 oC).

Mean body temperature is defined according to Eq. 21-1 (see end of Chapter).

Non-shivering thermogenesis is a rise in metabolism, which is not related to muscular activity (shivering or exercise).

Insensible perspiration (leakage of the skin) is the small cutaneous evaporation loss, which is unrelated to sweat gland function.

Insulation refers to resistance to heat transfer.

Radiative heat loss is a transfer of heat energy between 2 separate objects at different temperature. Heat energy is transferred via electromagnetic waves (photons). This heat transfer does not require a medium, and the temperature of any intervening medium is immaterial.

Shell temperature is the temperature of the outer parts of the body (measured on the skin surface) and related to cold environments.

Shivering is a reflex myogenic response to cold with asynchronous or balanced muscle contractions performing no external work.

Specific heat capacity is the relationship between heat energy exchanged per weight unit of a substance and the corresponding temperature change. The specific heat capacity of water is 4.18 and of the human body (blood and tissues) 3.49 kJ kg-1 oC-1, respectively. The specific heat capacity of atmospheric air is 1.3 kJ (m3)-1 oC-1.

Temperature is the measurement of heat energy content.

Essentials

This paragraph deals with

1. The temperatures of the body, 2. Body responses to cold, 3. Body responses to heat, 4. Emotional sweating, 5. Metabolic Rate and environmental temperature, 6. Temperature control, 7. The human thermo-control system, and 8. Thermoregulatory effectors.

1. The temperatures of the body

The human body consists of a peripheral shell and a central core (Fig. 21-1). The heat content (H or enthalpy) of the human body is reflected by its temperature. By definition a thermometer only measures the temperature of the thermometer, so its location is essential. The mean core temperature is 37 oC in healthy adults at rest, but small children have larger diurnal variations.

The skin is the main heat exchanger of the body. The skin temperature is determined by the core temperature and by the environment (temperature, humidity, air velocity). Thus the shell temperature is governed by the needs of the body to exchange heat energy.

Fig. 21-1: Heat transfers, body cores and shells temperatures of a naked person standing in cold and warm air, respectively.

The shell temperature is measured on the skin surface and at the hands and feet to approach the room temperature of 19oC in a person standing in a cold room for hours (Fig. 21-1, left). The shell temperature is several degrees lower than the temperature in the central core. The limbs have both a longitudinal and a radial temperature gradient. The shell temperature and the size of the shell vary with the environmental temperature and the termal state of the person. A naked person, standing on a cold floor in 19oC air has a small core and a thick shell compared to the same person in a warm environment (Fig. 21-1). The shell temperature of the skin and distal extremities is difficult to evaluate. The best estimate is measurement of the infrared heat radiation flux with a radiometer.

The core temperature is the rather constant temperature in the deeper parts of the body and in the proximal extremity portions (see the red stippled lines of Fig. 21-1). However, the core temperature may vary several Centigrades between different regions depending on the cellular activity. The brain has a radial temperature gradient between its deep and superficial parts. In a sense, the temperature of the mixed venous blood represents an essential core temperature.

The rectal temperature

A high core temperature is found to be constant in the rectum about 10-15 cm from the anus. When measuring the rectal temperature a standard depth of 5-10 cm is used clinically. The venous plexus around the rectum communicate with the cutaneous blood in the anal region. The rectal temperature falls when the feet are cold, because cold blood passes the rectum in the veins from the legs for the same reason. The rectal temperature rises during heavy work involving the legs.

Parents should be advised to measure the rectal temperature in disease suspect children. The rectal temperature is a reliable estimate of the core temperature in resting persons.

Sublingual (oral) or axillary temperatures are unreliable measures of the core temperature - often more than half a degree lower than the rectal temperature.

The cranial temperature (tympanic and nasal)

The main control of temperature is performed by the anterior hypothalamus, which has a high bloodflow. Within the cranium the hypothalamus lies over the Circle of Willis, which supplies it with blood, and close to the cavernous sinus which drains it. Hypothalamus elicits heat loss responses when stimulated by heat. The tympanic membrane and areas in the nasal cavity (the anterior ethmoidal region, part of the sphenoid sinus) are supplied with blood from the internal carotid artery just like the hypothalamus. These cranial locations then serve as a substitute for the measurement of the inaccessible hypothalamic temperature.

Intake of 250 g of ice releases an abrupt fall in the nasal temperature in a warm person, whereas the change in rectal temperature is smaller and delayed (Fig. 21-2). The cranial core temperature is more dynamic than the rectal.

Fig. 21-2: Intake of ice reduces the temperature in a warm person resting at 45oC.

In sports and in surgical hypothermia dynamic measurements of core temperature are essential. The cranial temperature is often preferred. During forceful movements the thermistor may be displaced. In such situations an oesophageal location is applied at heart level. This is an approximative measure of the temperature of the mixed venous blood of the right heart located close to the thermistor.

The mean body temperature is defined according to Eq. 21-1. The storage of heat energy in the body can be calculated according to its heat capacity (3.49 kJ* kg-1*oC-1), the body weight (kg) and the change in mean body temperature in the period (Eq. 21-2).

According to the first law of thermodynamics, the storage of heat energy equals the metabolic energy change minus the heat loss (Eq. 21-3). Quantification of thermodynamics in humans is possible using equations 21-1 to 21-7 (later in this chapter).

The body is in heat energy balance, when the storage is zero. However, the core temperature may change with internal fluxes of heat energy between core and shell without storage or loss of heat energy at a constant activity.

Venous blood draining active muscles and the liver is likely to be warmer than pulmonary venous blood, since this has undergone evaporative cooling in the alveoli. A patient with high fever can be in thermal steady state, with a high constant heat production, if both core- and shell-temperatures are constant, and no internal energy flux occurs.

Warm-blooded animals, homeotherms such as humans, can change their metabolism in order to keep their heat production equal to the heat loss. Such animals have a temperature control system and thereby maintain a rather constant core temperature. Warm-blooded animals live with the advantage of an unchanged cell activity and temperature in their core. However, the human core temperature falls during the oestrogen phase of the menstrual cycle and during sleep (circadian rhythm). The lowest temperature is between 18 at night and 6 oclock in the morning (Fig. 21-3). The temperature cycle is part of the circadian periodicity. Our biological clock seems to be synchronised with the rotation of the globe. Also meals, light and temperature plays a role.

Ovulation releases a sharp rise in morning temperature. Progesterone effects seem to explain the higher temperature in the last phase of the menstrual cycle (Fig. 21-3).

Fig. 21-3: Variations of the core temperature during 24 hour (above), and variations related to phases of the menstrual cycle (below).

Cold-blooded animals (poikilotherms) live with a behavioural temperature rhythm, but have no autonomic temperature control. The core- and shell-temperatures vary with the environment and the cellular activity. Reptiles, premature and low weight-premature newborn babies are cold-blooded. These babies have no thermoregulation (see later). However, their capacity for heat production is 5-10 times as great per unit weight as that of adults.

Humans have a warm-blooded (homeothermic) coreand a cold-blooded (poikilothermic) shell in a cold environment.

Persons exposed to general anaesthesia, alcohol, and certain drugs lose the autonomic thermoregulation. Cold-blooded animals must live with varying core and shell temperature, whereby the rate of their cellular activities varies with the surrounding temperature (Fig. 21-4).

Fig. 21-4: The body core temperature and the environmental body temperature for a warm-blooded animal (cat) and a cold-blooded animal (lizard).

a) Convection. The convective heat loss is calculated by Eq. 21-7. A healthy person in sports clothes experiences thermal comfort at three times the resting metabolic rate (3 MET), when the surrounding temperature is 20oC, the humidity is 50% and the wind velocity is 0.5 m*s-1.

Diving (water has a high thermal conductivity) illustrates the importance of conduction andconvection in heat energy transfer.

The dry diving suit excludes water from contact with the skin and traps low-conductance air in insulating clothing worn inside the watertight sealing.

The wet suit traps water next to the skin but prevents its circulation. The water is warmed through contact with the skin, and the high insulation of the foam rubber wet diving suit, with its many pockets of trapped air, minimises the rate of heat energy loss to the surrounding water. Air is a poor heat conductor and thus a good insulator. During deep diving high pressures compress these air pockets and thus reduce the insulation properties of wet diving suits.

b) Radiation describes a transfer of energy between objects in the form of electromagnetic waves (photons). This includes ultraviolet and visible (sun light) radiation from the outside and from the body infrared or warm heat radiation.

Radiative heat transfer can be calculated for a naked person according to Eq. 21-4.

When the skin temperature (Tskin) is less than the temperature of the surrounding objects, heat is gained by radiation.

At wintertime, heat can be lost through a window glass by radiation from the body to the cold environment irrespective of the room temperature. This is because the skin temperature is higher than the outside temperature.

c) Conduction. Sitting on a cold stone is a typical example of conduction loss, just as standing on a cold floor (Fig. 21-1). Conduction heat can also be gained, although it is really possible to walk on glowing coals with speed and a thick epidermal horn layer.

d) Evaporative heat loss- see sweat secretion below.

2. Body-responses to cold

Cutaneous vasoconstriction lowers skin temperature, and thereby reduces the conductive-convective heat loss that is determined by the temperature gradient from the skin surface to the environment. Cutaneous vasoconstriction directs the peripheral venous blood back to the body core through the deep veins and the commitant veins. These veins are located around the arteries with warm blood, so that the venous blood receives part of the heat energy from the arterial blood - so-called counter current heat exchange (Fig. 21-5). The vasoconstriction is so effective, that the bloodflow through the arterio-venous anastomoses in the fingers and toes can fall to below one percent of the flow at normal temperature. The cooling of the shell is immediate, and the size of the shell increases (Fig. 21-1). Obviously, the shell is large for a naked person in cold air. The resistance vessels of the hands may open periodically to nourish the tissues, but the high viscosity of the cold blood can endanger the tissue nutrition and result in trench foot.

The arterio-venous shunts of the hands and feet are closed, so the bloodflow to the limbs is a nutritive minimum.

The deep arteries and veins of the limbs lie in parallel, so the arterial bloodflow loses heat to the incoming venous blood partially surrounding the arteries (Fig. 21-5). This is a typical counter-current heat exchange. In a cold environment, where vasoconstriction and heat exchange produces cold extremities, the total insulation is increased at the expense of reduced neuromuscular efficiency.

Fig. 21-5: Counter-current exchange in a human arm conserving heat energy in a cold climate (left). Superficial venous cooling ribs eliminate heat energy in a warm climate (right).

In a warm climate the high bloodflow of the extremities ensures an optimal temperature of the deeper structures (eg, the neuromuscular system). The temperature of the arterial blood is maintained (Fig. 21-5, right) and the arterio-venous anastomoses are wide open conveying warm blood to the superficial veins. The superficial veins also act as cooling ribs and transfer large amounts of heat to the skin surface, where it is eliminated from the body by convection, conduction and evaporation (Fig. 21-5, right).

Shivering is a reflex myogenic response to cold with asynchronous or balanced muscle contractions elicited from the hypothalamus via cutaneous receptors. The activity in agonist and antagonist muscles balance, so there is no external work. Without outside work, all energy is liberated as metabolic heat energy. Heat production is also increased by thyroid gland activity and by release of catecholamines from the adrenal medulla.

External work, such as running, is helpful in maintaining body temperature when feeling cold. Cold increases the motivation for warm-up exercises and illustrates the voluntary, cortical (feedforward influence) on temperature homeostasis. The core temperature increases proportionally to the work intensity during prolonged steady state work (Fig. 21-6). The mean skin temperature falls with increasing work intensity at 20oC, because the sweat evaporation cools the skin.

Fig. 21-6: Muscular and oesophageal temperature during steady state exercise. The levels of exercise range from zero to 100% of the maximum oxygen uptake.

The temperature in the active muscles determines the level of the rectal temperature. Following marathon rectal temperatures of more than 41oC have been measured and heat strokes have occurred. A marathon is even more difficult to accomplish in warm, humid environments and strong sun may cause sunstroke (see later).

People may adapt to prolonged exposure to cold by increasing their basal metabolic rate up to 50% higher than normal. This metabolic adaptation is found in Inuits (Eskimos) and other people continuously subject to cold.

The environmental temperature, where we maintain our autonomic temperature control, is in the range of zero to 45oC. Below and above this range we adapt to the environment by behaviour (adding or removing clothing, warm or cold bath, sun or shadow). A core temperature above 44oC starts protein denaturation in all cells and is incompatible with life. Below 32oC humans lose consciousness and below 28oC the frequency of malignant cardiac arrhythmias is increasing, ending with ventricular fibrillation and death at a core temperature below 23 oC (Fig. 21-7).

Fig. 21-7: Environmental temperature variations and temperature control. Lack of vital signs in the clinic (respiration, heart rate, EEG) must not be taken as death. Treatment must be instituted until death signs are developed.

3. Body-responses to heat

Sweat secretion. Three million sweat glands produce sweat at a rate of up to 2 litres per hour or more during exercise in extreme warm conditions. If not compensated by drinking, such high sweat rates lead to circulatory failure and shock. Sweat resembles a dilute ultrafiltrate of plasma. Healthy humans cannot maintain their body temperature, if the environmental air reaches body temperature and the air is saturated with water vapour. Primary sweat is secreted as an isosmotic fluid into the sweat duct, and subsequent NaCl reabsorption results in the final hypo-osmotic sweat. Thermal sweating is abolished by atropine, proving that the postganglionic fibres are cholinergic. Cholinergic drugs provoke sweating just as adrenergic agonists do. Evaporation of water on the body surface eliminates 2428-2436 J g-1 at mean shell temperatures of 30-32oC. Evaporation of a large volume of sweat per time unit (Vsweat) implies a substantial loss of heat according to Eq. 21-5.

Normally, the skin temperature falls with increasing work intensity, because the sweat evaporation cools the skin (Fig. 21-6). Danger occurs when the average skin temperature and the body core temperature converge towards the same value.

Condensation of water on the skin gains heat energy, which is stored in the body. This is what happens in a Sauna.

Vasodilatation of skin vessels in warm environments results in increased cardiac output. The arterio-venous anastomoses in the hands and feet are open, and the bloodflow can rise up to at least 10 folds. The shell is minimal, when a naked person is in warm air (Fig. 21-1, right). The skin bloodflow, mainly in the extremities, determines the amount of heat energy, which is carried from the body core to be lost on the surface. The heat energy is transported from the large body core to the skin by convection in the blood. A substantial part of the heat energy is lost through the superficial veins of the extremities acting as cooling ribs (Fig. 21-5). The blood of the superficial veins is thus arterialized, when the person is warm.

A piece of steak has the same composition as human skin but of course no blood flow and no sweat evaporation. Thus the steak will be cooked at an air temperature that humans can survive. A person can stay in a room with dry air at 128oC for up to 10 min during which time the steak is partially cooked.

Read more from the original source:
New Human Physiology Ch 21 - zuniv.net