BIO 3520 Notes, 12/09/05
THERMOREGULATION
I. Introduction. [Widmaiera, pp. 640-641]
A. Importance of homeostasis.
B. Birds and mammals maintain body temperature through internal
mechanisms -- homeotherms.
C. Other vertebrates (fishes, reptiles, amphibians) cannot maintain body
temperature through internal mechanisms -- poikilotherms.
D. Body temperature depends on a balance of heat gained and heat lost.
ΔT = Heat gain - heat loss
E. The goal of thermoregulation is to balance heat gain and heat loss.
a Widmaier, E.P., Raff, H., and Strang, K.T. Vander, Sherman, and Luciano's, Human
Physiology: The Mechanisms of Body Function, 10th ed., McGraw-Hill, New York, 2006.
II. Heat Transfer. [pp. 641-642]
A. Sources of heat gain (fig. 16-17, figure).
1. External environment.
a. Radiation = Emission of heat from an object through the
atmosphere.
1. Thermal radiation from sun.
b. Conduction = Transfer of heat through direct contact between
objects.
1. Conduction from ground or water.
2. Internal heat production -- cell metabolism.
a. 60% of energy in fuel molecules is lost as heat during cell
metabolism.
b. Fat has twice the kcal of carbohydrates or protein.
B. Sources of heat loss.
1. Radiation of heat into atmosphere.
2. Conduction (ex. heat loss in swimming pool).
3. Convection = Transfer of heat by movement of air or water.
a. Heat lost through respiration.
4. Evaporation = Loss of heat through conversion of liquid to gas.
a. Evaporation of sweat from skin surface.
b. Large amt of energy is required to vaporize water.
c. Energy is derived from the skin surface
> cooling.
C. Factors affecting heat transfer.
1. Temperature gradient.
2. Surface area.
3. Insulation.
III. Regulation of Body Temperature. [pp. 642-645]
A. Sensors -- thermoreceptors (fig. 16-18).
1. Warm and cold thermoreceptors on skin surface.
a. Sensitive -- can detect a change in skin temp of 0.01o C.
b. Adaptable -- can adapt within 10 sec.
2. Central thermoreceptors in hypothalamus monitor body temp.
B. Control center -- thermostat is in hypothalamus.
1. Set point = 37o C in human.
C. Effectors -- involve a wide range of organ systems.
IV. Physiological Response to Cold. [pp. 642-644]
A. Increase heat gain (fig. 1-6, table 16-9).
1. Thermogenesis = Internal generation of heat.
2. Nonshivering thermogenesis.
a. Increased BMR.
b. Thyroid gland.
1. Increased TRH
> increased TSH>increased thyroxine
> increased cell metabolism.
c. Adrenal medulla.
1. Increased sympathetic activity
> increased epinephrine
> increased cell metabolism.
3. Shivering thermogenesis.
a. Repetitive, asynchronous contraction of skeletal muscle fibers.
B. Reduce heat loss.
1. Vasoconstriction of arterioles in skin
> reduces blood flownear skin surface (figure).
2. Piloerection -- improves insulating properties of fur.
V. Physiological Response to Heat. [pp. 644-645]
A. Increase heat loss (table 16-9).
1. Vasodilation of cutaneous arterioles -----> increases blood flow
near skin surface.
2. Sweating -- evaporative heat loss.
3. Panting -- convection and evaporation.
VI. Thermoregulatory Adaptations. [pp. 645-646]
A. Exercise.
1. Body temperature rises during exercise (fig. 16-20).
2. Increased muscular activity generates rapid increase in heat
production.
3. Compensation -- vasodilation in skin and sweating.
B. Torpor.
1. Small animals with high metabolic rates (ex. small bats and birds)
could starve if not feeding constantly.
2. Torpor = Temporary state of inactivity accompanied by lowering of
body temperature.
3. Involves temporary resetting of thermostat to a few degrees lower temp.
4. Sleep is mild example of torpor.
C. Hibernation.
1. Small mammals (ex. hamster, ground squirrel).
2. Decrease in all physiological functions for weeks or months during
winter.
3. Resetting of thermostat to lower temp.
4. No eating or drinking -- use stored fat to meet metabolic needs.
