True or false: Nomral internal temp of the human body is only a few degrees below lethal limits.
To keep body temperature constant, there must be a balance btwn ___ ___ and ___ ____.
heat production and heat loss
Physiological methods of conserving and dissipating heat are necessary to sustain the thermal balance necessary for human life. Aside from behavioral regulation (voluntary muscular activity), what system does the body use to regulatin core temperature?
Aside from behavioral regulation (e.g., voluntary muscular activity), the human body utilizes a system of thermoreceptors, a CNS “integrating” network, and effector mechanisms for shivering, sweating, and vasomotor actions to regulate core temperature.
What is heat exchange? What is temperature a reflection of?
1. Heat exchange is the transfer of heat from hotter regions to colder regions. Heat always moves from higher to lower temperatures and in the process, high-temp regions become colder and low-temp regions becom warmer.
2. Temperature is a reflection of how fast molecules are moving.
• High temperature means molecules are (on average) moving fast.
• Low temperature means molecules are (on average) moving slowly.
What are the 4 mechanisms of heat transfer?
What is conduction? What 3 factors does the amount of heat flowing depend on?
Conduction involves molecule to molecule transfer of heat where the fast (high-temperature) molecules lose some of their speed to the slow (low-temperature) molecules when they collide.
The amount of heat flowing depends on:
• the magnitude of the temperature gradient (the difference in temperature between the high- and low-temperature regions); the greater the temperature difference, the more heat flows.
• the surface area of the body involved; the more surface area, the more heat flows.
• how well the materials conduct heat (thermal conductivity). Thermal conductivity of air is very low; thermal conductivity of water is about 25 times higher than that of air.
Is conduction involved in heat exchange within the body? The environment?
Conduction is involved in heat exchange within the body (i.e., between the core and the shell of the body) and in heat exchange between the body and the environment.
What is convection? What is the difference when the low temp material is moving vs the high temp material?
Convection is also a molecule to molecule transfer of heat, like conduction. Unlike conduction, where high- and low-temperature materials do not move and the low-temperature material gradually becomes warmer, convection occurs where one material is flowing past the other:
If the low-temperature material is moving, then the slow (cold) molecules gain speed from the fast (hot) molecules like in conduction, but then these warmed-up molecules are moved away and replaced by new slow (cold) molecules. Examples include cold wind blowing on you (“wind chill”) and standing in a cold, flowing river. Because the conductivity of water is 25 times higher than air, you lose heat much faster in the flowing river than in a cold wind of the same temperature and speed.
If the high-temperature material is moving, then the fast (hot) molecules lose speed to the slow (cold) molecules like in conduction, but then these cooled-down molecules are moved away and replaced by new fast (hot) molecules. Examples include a hot wind blowing on you.
Is convection involved in heat exchange within the body? The environment?
Heat exchange due to convection can occur between the body surface and the environment (e.g., “wind chill”). Convection is also involved in heat exchange within the body. This is forced convection of the blood by the circulatory system (“Forced” refers to the heart actively pumping the material (i.e., the blood) that transfers the heat.)
What is radiation?
Is it involved in heat exchange in the environment? In the body?
Radiation: transfer of heat energy by electromagnetic waves. For heat transfer the most important wavelengths are in the infrared (IR) spectrum. Infrared vision cameras and tympanic thermometers measure the amount of IR radiation and convert that to a temperature. Examples include the sun radiating heat to warm you. Thermal radiation is only involved in heat loss from or heat gain from the environment and is not involved in heat transfer within the body.
What is evaporation? What are the 2 ways in which evaporation is important in heat loss from the body?
Is evaporation involved in heat loss from the environment? Within the body?
Evaporation: Change of state from liquid to gas. Heat is required for this phase transition and thus evaporation always results in heat loss. Evaporation is not involved in heat exchange within the body.
2 ways in which evaporation is important to heat loss from the body:
- Insensible evaporation heat losses arise from respiration and a slow transduction of water through the skin. Recall that exhaled breath is at 100% humidity and that water constantly leaves the body through the skin. When these two sources of water enter the gas phase, they evaporate and the body loses heat. Under resting conditions this amounts to the evaporation of about 25–30 ml of H2O per hour (which eliminates about 15–20 kcal/hr of heat).
- Sweating can reach levels as high as 2 liters per hour (4L/hr in heat acclimatized individuals). The evaporation of 2 liters of sweat would eliminate 1160 kcal of heat from the body. But, note that sweat must evaporate in order to be effective in removing heat. Sweat can only evaporate if the relative humidity of the air is <100%; at 100% the air is saturated with water and water cannot evaporate then. The lower the humidity, the faster sweat evaporates and the more efficient the cooling. Also, the faster the wind is blowing, the faster sweat evaporates (this is why fans are used).
Core-Shell (Periphery) model of the human body:
What parts of the body are considered the core? How is temperature sensed within the core?
What parts of the body are considered teh periphery (shell)? How is temperature sensed within the periphery?
Btwn the core and periphery, where is the site of the most heat production? Which is the site of heat exchange? Which of the two temperatures is regulated?
• Core consists of the brain, viscera, skeletal muscle
• Core is the site of almost all heat production.
• Core contains internal temperature sensors and the neural regulatory mechanisms.
• It is the core temperature that is regulated. The core temperature is denoted Tc.
Shell (or Periphery)
• Periphery consists of skin and subcutaneous fat.
• Periphery is the site of heat exchange with the environment.
• Periphery also contains some important temperature sensors.
• The temperature of the periphery is not regulated or constant.
In the core-shell model, the core is at a constant temp, Tc. How does heat flow btwn the cor and the skin?
What is heat exchange btwn the core and skin primarily due to?
What effect does modifying cutaneous blood flow have?
How does heat flow btwn the skin and the environment?
The periphery is at Tc at its inner boundary and Ts (the skin temperature) at its outer boundary. Heat flows between the core to skin driven by the temperature gradient Ts – Tc
Heat exchange between the core and the skin is primarily due to forced convection of the blood. Varying cutaneous blood flow can greatly modify the effective thermal conductivity of the periphery.
Heat flows between the skin and the environment (at ambient temperature TA) driven by the temp gradient TA – TS.
True or false: For example, not all parts of the core are at TC, but only approximately so. Also, the core is not fixed. In extremely cold situations, the core shrinks to include only the vital organs and brain.
What are the 3 most common sites of temperature measurement? Which one is most often measured and why?
The most common sites
• Oral. under the tongue
• Rectal. Rectal temperature is typically about 0.6 degrees C higher than oral temperature;
there are spatial variations of temperature within the core.
• Tympanic temperature is often measured since it more closely approximates the temperature of the brain and therefore of the hypothalamic control center of the thermoregulatory system. Today, tympanic thermometers are readily available and easier to use than other kinds.
True or false: Core temp can vary from person to person and response to activity pattern and environment can vary from person to person.
What are the only 3 mechanisms by which the body can regulate its core temperature?
• vary the rate of metabolic heat production
• vary the rate of heat transfer between surface and environment
• vary the rate of heat transfer between core and surface.
In what wasy can metabolic rate be increased? How can metabolic heat production be reduced?
Metabolic rate can readily be increased by muscular activity (increases in muscle tone, shivering, voluntary activity).
However, in man the ability to reduce metabolic heat production is very limited.
Conduction, convection, and radiation can either cause the body to lose heat to the environment or gain heat from the environment.
TA > TS heat gain
TA < TS heat loss
TA << TS excessive heat loss
In each situation, what mechanisms does the body employ to combat heat loss or gain?
For TA << TS (i.e., a very cold environment) the only regulatory response in terms of the body surface is to reduce TS and thus reduce the temperature gradient between the skin and the environment, thereby reducing heat loss. TS is increased by increasing blood flow to the skin; TS is decreased by decreasing blood flow to the skin.
For TA > TS evaporation (sweating) is the only mechanism by which the body can lose heat. In this case, the body only gains heat from the environment by conduction, convection, and radiation.
What glands secrete sweat? How are those glands innervated?
What is sweat? Where does the water in sweat come from?
How does humidity and wind velocity impact the evaporation of sweat?
What is the other source of evaporative heat loss?
Sweat is secreted by eccrine and apocrine glands. Only eccrine glands are involved in thermoregulatory response. The innervation of the glands is sympathetic and cholinergic (unlike other sympathetic end organs).
Sweat is primarily a dilute salt solution (about 50 mM NaCl). The water in sweat comes from the blood volume and, if not replaced by drinking, is replaced by water movement from the ICF (but there is a lag time for this).
For water to evaporate from sweat (which must occur for sweat to cool the body) humidity must be less than 100%. Wind velocity increases the rate of evaporation of sweat.
There is another source of evaporative heat loss that is different from sweating and is not physiologically controlled. Insensible evaporation from skin and respiratory tract amounts to about 25–30 ml/hr (15–18 kcal/hr). It is unregulated and reasonably constant (although, of course, it varies with breathing rate, ambient temperature, humidity).
What are the 2 mechanism by which heat may be transferred from core to the surface? Which method is more important?
There are two mechanisms to conduct heat from the core to the surface: conduction and forced convection by blood. Normally forced convection by the circulatory system is more important.
How does conduction occur in the body?
Why is the circulation needed for heat transport?
Conduction: Simple cell-cell transfer of heat. Normally much less important than circulation, but it does determine the minimum amount of heat transfer in extreme cold when there is extensive peripheral vasoconstriction.
Circulation: Transport of heat by circulatory system needed to:
• counter non-uniformity of heat production within the body; different organs produce different amounts of heat because they have different metabolic rates.
• deal with varying levels of heat production (e.g.,removing excess heat during exercise).
• overcome insulating properties of subcutaneous fat layer; fat has a low thermal conductivity and does not transfer heat well.
True or false: The cutaneous circulation suplies the metabolic needs of the skin as well as transfers heat to the body surface by moving warm blood to the skin where it cools. Conversely, keeping blood out of the skin conserves heat. Under normal resting conditions, the skin is overperfused (i.e. blood flow is high relative to metabolic needs of the skin). Under neural control, blood flow the entire skin as a tissue can be varied from about 100 ml/min or less (cold environment) to as much as about 4000 ml/min (exercise in a warm environment)
What 2 effects does transferring blood (and therefore heat) to the surface of the skin have?
- Overcomes the insulating properties of the subcutaneous fat layer
- changes effective thermal conductivity of skin
- effective thermal conductivity of the periphery of the body is defined by the equation
- H=Keff(Ts-Tc) where H is the rate of heat exchange btwn the core and surface, Keff is the effective thermal conductivity, Ts is skin temp, and Tc is core temp
How can blood flow to the skin be varied (name specific vessel)? Under what system of the ANS are these vessels contolled?
Blood flow to the skin can be varied by vasodilation/constriction and through venous plexuses and arteriovenous (AV) shunts (anastomoses). Dilation of AV shunts causes warm arterial blood to be poured directly into venous plexuses in dermis and epidermis. This greatly augments heat flow to the body surface. The short, muscularly-walled vessels of the AV shunts are heavy innervated by sympathetic nerve fibers.
Vasoconstriction of arterioles, metarterioles, and AV shunts are under control of a sympathetic center located in the ____ ____.
How are cutaneous blood vessels locally controlled? What other systems control cutaneous circulation? Which dominates?
Cutaneous blood vessels also respond directly to heating (leading to vasodilation). Such local responses are in part dependent on hypothalamic control. In general, control of cutaneous circulation involves the CNS, spinal cord reflexes, and local control. Under normal conditions hypothalamic control dominates.
Circulatory control over temp regulation is not just limited to the skin. How are veins implicated in temp regulation?
In response to cold, venous blood from the extremities is shifted from superficial to deeper veins. The blood in these deeper veins is warmed by adjacent arteries (countercurrent heat exchange). Arterial blood is cooled on the way to the surface of the extremities to reduce Ts and lower the temperature difference TA – Ts so that less heat is lost to the environment by conduction, convection, and radiation. At the same time, venous blood is warmed while returning to the core so that Tc is reduced less (as compared to having cold venous blood return).
The response patterns to varying environmental conditions and levels of metabolic heat production are finely graded and coordinated. In a reasonably comfortable environment and provided that you are not vigorously exercising, thermal equilibrium is achieved primarily through ____ ____.
vasomotor regulation (i.e., fine tuning of sympathetic nervous system control of cutaneous blood flow)
In response to cold, thermoregulatory system initates responses to conserve heat and if neccessary, augment heat production. What are these mechanims? (have been previously stated-just put them all together)
Heat conserving mechanisms
• Peripheral vasoconstriction occurs and AV shunts close. This has a dual effect: heat transfer from core to surface is reduced and Ts is reduced thereby decreasing heat losses from skin to environment due to conduction, convection, and radiation.
• Counter-current exchange mechanism is activated. Conserves core heat, but continues to supply metabolic needs of extremities.
• Sweating is obviously inhibited, although insensible evaporation continues.
Increased heat production
• voluntary activity
Recall that the body's ability to reduce metabolic rate in response to warm is very limited. In response to warm environment or increased heat production (e.g., exercise), the thermoregulatory system activates heat loss mechanisms. What are these mechanisms? (have been previously discussed, just put them all together)
• Peripheral vasodilation, AV shunts open. In moderate situations (e.g. moderate exercise in comfortable environment, moderately warm temp) these responses may be all that is needed. Peripheral vasodilation results in increased Ts and thus increased heat loss to environment. Despite the reduction in the temperature gradient from core to surface, heat transfer from core to surface is augmented due to greatly increased
peripheral blood flow.
• Increased evaporative heat loss (sweating). If peripheral vasodilation cannot by itself achieve thermal balance, sweating will result. If TA > Ts, only evaporation can remove heat from the body, since in this case the body will actually gain heat via conduction, convection, and radiation. In this situation where TA > Ts and TA > Tc, evaporation of sweat serves a dual role it not only removes heat from the body, but also cools the skin so that Ts
True or false: The general scheme of thermoregulatory control system includes:
• detection of temperature. Thermoreceptors are located in hypothalamus, skin, and elsewhere.
• temperature information transmitted to CNS integrating network in hypothalamus and neighboring preoptic area of brain.
• set-point (preferred temperature)
• comparison of set-point and afferent temperature signals.
• production of “error” signals
• transmission of error signals to appropriate effector mechanisms controlling shivering, sweating, and vasomotor action.
Temp info from the core and periphery is transmitted to the hypothalamic preoptic "control center". This incoming info is integrated (core temp is of the greatest importance) and compared to the set-point (preferred temp).
The set-point is more than just an abstraction, there clearly is an internal reference for preferred body temperature, although its precise physical nature remains unknown. A number of things can modify the set point. What are they?
• In monkeys (and therefore probably in man) increased serum Na+ at the hypothalamus increases the set-point; decreased serum Na+ decreases the set point.
• Increased serum Ca2+ at the hypothalamus decreases the set-point; decreased serum Ca2+ increases the set-point.
• Pyrogens increased the set-point
• Dehydration increases the set-point.
• Exercise may also increase the set-point.
• Skin temperature may exert its influence on the thermoregulatory system by modifying the set-point (e.g., cold skin may slightly increase the set-point and warm skin may slightly decrease the set point). This remains controversial.
• Various neurological disorders can also modify the set-point.
What are thermoreceptors composed of? What are the 2 classes of thermoreceptors?
In general, thermoreceptors are not specialized anatomical structures, but rather neurons or naked nerve endings whose firing rate changes with temperature. There are both cold sensors (firing rate increases as temp decreases) and warm sensors (firing rate increase as temp increases).
Where are skin thermoreceptors located? Where are internal thermoreceptors located?
Skin Thermoreceptors are distributed over entire body surface, but their distribution is not uniform. Internal thermoreceptors are located in the hypothalamus itself, the spinal cord, respiratory tract, viscera, and tongue. Hypothalamic receptors are the most important of the internal thermoreceptors.
True or false: Some generalizations:
• Sweating, shivering, and vasomotor responses are primarily controlled by the core temperature.
• Skin temperature can modify (and in some cases initiate) thermoregulatory responses, but in general much larger changes in Ts than in Tc are required for a given thermoregulatory response.
• The existence of temperature thresholds is generally accepted.
• The rate of change of skin temperature and perhaps of core temperature can also effect thermoregulatory response, with rapid changes producing larger responses.
Sweating and shivering are both controlled info from
a. internal thermoreceptors
b. skin thermoreceptors
At what values of Ts and Tc does sweating occur?
Sweating begins when core temperature rises above about 37 degrees C; rate of sweat production rises as Tc rises above this level. Skin temperature (Ts) does not affect the rate of sweat production unless Ts is less than about 32 degrees C. Lower values of Ts (i.e., below about 32 degrees C) progressively inhibit sweat production for a given value of Tc. However, much larger changes in Ts than in Tc are required for a given thermoregulatory response (i.e., Tc is dominant).
At what values of Ts and Tc does shivering occur?
Shivering occurs when Tc < 37 degrees C. For Tc < 37 degrees C, Ts influences the thermoregulatory response so that higher Ts reduces the increase in metabolism for given value of Tc. However, once again larger changes in Ts than in Tc are required for a given response. For core temperature greater than about 37 degrees C, no elevation of metabolic rate (shivering) occurs as skin temperature is reduced at least as low as 20 degrees
True or false: Lastly, vasomotor responses through CNS centers, spinal cord reflexes, and local control are all involved in the control of peripheral circulation. However, within the normal range of temperature variation, the hypothalamic control system determines the general response pattern.
Define the following conditions:
anesthetic hyperthermia (malignant hyperthermia)
Describe the events that occur in fever.
When bacteria and viruses invade the body, toxins are released that stimulate phagocytic cells (monocytes, granulocytes, Kupffer cells) to produce and release proteins called pyrogens. Pyrogens (via prostaglandins and cyclic AMP) raise the hypothalamic set-point resulting in fever.
When the set-point is elevated the body responds as if it is cold: cutaneous vasoconstriction, shivering, etc. until the core temperature is elevated to the new set-point temperature. When the fever “breaks” the set-point returns to normal, the patient feels hot and heat loss mechanisms are activated until core temperature returns to normal. (attached figure)
Fever is not a temperature regulation disorder, but part of the integrated homeostatic response to pathogenic agents.