A. Transfer of Body Heat. (p. 315)    Mechanisms:
1. Conduction, transfer of heat from one material to another. Conduction occurs from deep body tissues to superficial tissues, and to clothing, or to air in contact with your skin.
2.  Convection, transfer of heat from one area to another by moving fluid (air or water).
Note: both conduction and convection can either transfer heat to the body. The body will gain heat when the environment is hot.  When the environment is cool, the body will lose heat via conduction and convection.  This heat loss accounts for about 10 to 20 percent of heat loss when in an air environment.  When the body is submerged in water, heat loss via conduction is 26 times greater compared to a similar air temperature.
3.  Radiation, heat is dissipated through electromagnetic waves (form of infrared rays) and is the primary way in which heat is dissipated.  In a room which is around 70 to 77 degrees, the body will lose about 60% of its excess heat via radiation.
4. Evaporation, the vaporization of sweat at the skin results in heat loss. This is the primary way the body dissipates heat during exercise (about 80% of heat loss), and accounts for about 20% of heat loss at rest.

B.  Understand the interaction of the body’s mechanisms for heat balance and environmental conditions. It is important to consider that humidity, and water vapor content, is a major factor in heat loss, especially via evaporation.

C.  Control of Heat Exchange.
1.Human are homeotherms, which means that the body can maintain a constant core temperature despite changes in temperature in the environment. Normal body temperature is 36.1 to 37.8 Celsius or 97 to 100 Fahrenheit (according to your authors).
2. The hypothalamus, located deep within the brain, acts as a thermostat by monitoring core temperature. The hypothalamus will accelerate either heat loss or increase heat production.

3. Thermoreceptors.  Peripheral receptors in the skin relay information to the hypothalamus regarding environmental temperature while central receptors in the hypothalamus monitor core temperature.

D.  Effectors that alter core temperature.  (Ways in which the body can alter internal temperature)
  1. Sweat glands.  Increase sweat gland activity, such as occurs during exercise in a hot environment, will increase heat loss through the process of evaporation. The hypothalamus increases sweat gland activity.  Sweat is a filtrate of blood plasma.
  2. Smooth muscle around small arteries (arterioles).  The hypothalamus will send signals to the smooth muscles of arterioles, consequently resulting in vasodilation of the arterioles and greater blood flow to the skin.  The latter promotes heat loss through convection, conduction, radiation and evaporation.
  3. Skeletal muscle.  Skeletal muscle activity increases thereby increasing the production of heat.  This would occur in a cold environment where you would find the skin thermoreceptors sending neural signals to the hypothalamus which in turn sends signals to brain centers that control muscle tone. Also, if blood temperature drops, central thermoreceptors in the hypothalamus will ultimately result in increased muscle activity.  The increased muscle activity under these condition is called shivering (rapid, involuntary muscular contraction and relaxation of muscles).
  4. Endocrine glands.  The hormone thyroxine (from thyroid gland) and catecholamines (epinephrine and norepinephrine) increase metabolic heat production through increasing the metabolic activity of body cells.

Note: Review figure 10.5 (p.317).  This figure provides an overview of the role of the hypothalamus in controlling core temperature.

E.  Methods of measuring core temperature.
1. Oral temperature.  Taken at the mouth. Limitations, thermometer may fall out and plus this measurement would be difficult to administer during exercise.
2.  Rectal temperature.  A rectal probe is inserted into the rectum. Is considered second most accurate method.  Limitations included temperature varies within rectum and probe may fall out.
3.  Tympanic temperature.  Measure the temperature of the tympanic membrane within the ear.  Proximity of membrane to the hypothalamus makes this test attractive.  Limitation, difficult to administer while exercising.
4. Stomach temperature.  Telemetry method in which a radio transmitter is swallowed. The transmitter sends signals to a receiver regarding temperature.  Limitation, a large difference between stomach temperature and temperature of the hypothalamus. Also, any food in the stomach will affect temperature reading.
5. Esophageal temperature.  A temperature probe is fed through the nose, down the back of the throat, into the esophagus, and placed at the level of the heart.  This is considered the most accurate.

F. Physiological responses to exercise in the heat.  Heat loss mechanisms compete with the active muscle during exercise.  Because of increased sweat loss, blood plasma levels will drop when exercising in a hot environment. Consequently, stroke volume will decrease and result in an increased heart rate during submaximal exercise (cardiac drift=increased heart rate during submaximal exercise).  Also, during maximal exercise, a decrease in plasma volume will result in a decreased maximal stroke volume and therefore a decreased cardiac output.  Consequently, blood flow and oxygen supply to the working muscle is compromised, which results in increased glycogen use and lactic acid production.   Also, exercise in the heat results in a greater oxygen consumption during submaximal exercise.  In addition, aldosterone and antidiuretic hormone are released because of the increased water loss during exercise, especially exercise in a warm to hot environment.  Aldosterone (from adrenal cortex) limits sodium excretion by the kidneys which results in the conservation of body water.  Antidiuretic hormone (from posterior pituitary gland) stimulates water reabsorption from the kidneys.  Both of the above hormones work to increase plasma volume of the blood.
G.  Heat disorders in athletes. (Great presentation of info. on p. 326).
1. Heat cramps.  Characterized by muscle spasms and usually occurs in unacclimatized individuals.
2. Heat syncope.  General weakness, fatigue, low blood pressure, brief loss of consciousness, elevated skin and core temperature.
3. Heat exhaustion.  Reduced sweating, large weight loss (water depletion), thirst, elevated skin and core temperature, urine is concentrated and orange color.
4. Heat Stroke.  Elevated skin and core temperature. May exceed 105O F, involuntary limb movement, seizures, coma, vomiting, tachycardia (elevated HR), irrational, hallucinating.
Note: Heat disorders can be prevented by consuming adequate amounts of water and electrolyte drinks, acclimatization to the heat, and awareness of the environmental temperature. Wear light colored clothes, rest in shade, decrease work rate, plan activities for appropriate times of day (early morning or evening).  Athletes can do better in the heat if they are heat acclimatized.  Athletes should periodically train in a warm to hot environment if they anticipate competing in such conditions.  Heat acclimatization allows the athlete to compete better in the heat by increasing ability sweat rate which promotes heat loss.  Also, stroke volume increases with heat acclimatization, which aids in delivery of blood to both skin and muscle.   Heat acclimatization results in reduced rate of muscle glycogen use, which delays the onset of fatigue.

H.  Exercise in a Cold Environment.
Primary ways the body avoids excessive cooling.
A. Shivering: uncontrollable muscle contraction and relaxation.
B. Nonshivering thermogenesis: stimulation of metabolism via the sympathetic nervous system
B. Peripheral vasoconstriction: Sympathetic nervous system causes smooth muscle arteriole contraction, which results in constriction of the arterioles and therefore reduced blood flow to the skin.  This prevents the loss of heat.

Body size and composition, windchill and exercise on land or in water affects heat loss.  An increased body surface area to mass ratio (i.e., in children), and low levels of subcutaneous fat
 increases heat loss and changes of developing hypothermia.  High levels of subcutaneous fat insulates the body, thereby resulting in less heat loss.  Exercising when it is windy increases
heat loss by convection and conduction (windchill).   Immersion in cold water increases heat loss through conduction.  Fatigue occurs more rapidly in  cool muscle. Interestingly, fat mobilization may be impaired while exercising in a cold environment. It appears that the circulation of catecholamines (epinephrine and norepinephrine) to fat tissue is impaired while exercising in a cold environment. Usually, appropriate clothing enables the body to tolerate a cold environment.