Human Thermoregulation


Thermoregulation

(v.) Regulate temperature, especially one’s own body temperature. (Source:  Oxford Dictionaries)

Normothermia

(n.) Normal body temperature; the condition of having a normal body temperature. (Source:  Oxford Dictionaries)

Hypothermia

(n.) The condition of having an abnormally low body temperature, typically one that is dangerously low. (Source: Oxford Dictionaries)

Hyperthermia

(n.) The condition of having a body temperature greatly above normal(Source: Oxford Dictionaries)

Set Point

(n.) The target value of the controlled variable that is maintained by an automatic control system, e.g., the point at which body temperature iscontrolled by the hypothalamic thermostat. Written also set point. (Source:  Miller-Keane Encyclopedia and Dictionary of Medicine)

Classic Heatstroke

(n.) This type of heatstroke typically occurs after exposure to hot, humid weather, especially for prolonged periods, such as two or three days. It occurs most often in older adults and in people with chronic illness. (Source: Mayo Clinic)

Exertional Heatstroke

(n.) Exertional heatstroke is caused by an increase in body temperature brought on by intense physical activity in hot weather. (Source: Mayo Clinic)


In the human body, heat is produced as a bi-product of metabolic reactions. For the average adult, a temperature of 97.5°F – 99.0°F is considered to be a normal temperature range (normothermia). The process of retaining or dissipating heat to warm or cool the body in an effort to maintain a consistent, normothermic temperature is called thermoregulation.

Thermoregulation is a homeostatic process controlled by the hypothalamus. In concert with numerous temperature receptors located throughout the body, the hypothalamus acts as the body’s “thermostat.” Given that the body cannot produce cold, temperature is controlled by retaining or dissipating heat. The dissipation of heat is achieved in one of four ways: (Source: UNM)

  • Radiation: Heat loss to the environment (when the environment is cooler than the skin).  
  • Convection: Heat loss from the movement of air or water over the surface of the skin.
  • Conduction: Heat loss from direct contact with a cooler surface.
  • Evaporation: Heat loss as a result of kinetic energy (heat) transferring into a liquid, the transition of the liquid into a gas (vapor), and the subsequent loss of kinetic energy (heat) in the remaining liquid.

When the hypothalamus senses that the body is cold, physiological responses such as vasoconstriction (the tightening of blood vessels) and/or shivering are instigated to respectively retain and/or generate additional heat. Conversely, when the body senses that is is warm, the hypothalamus triggers physiological responses such as vasodilation (relaxing of blood vessels) and amplified perspiration to dissipate additional heat. Scientifically, thermoregulation is expressed as:

M ± W = ± R ± C ± E ± S [W/m²]  (Source: Harvard University)

Where

M=Production of heat through metabolic processes
W=Work by or on the body
R=Radiant heat exchange
C=Convective heat exchange
E=Evaporative heat loss
S=Rate of heat storage

Homeostasis Chart (Thermoregulation)2

Ultimately, S (the rate of heat storage) should be approximately zero (0) to maintain a constant body temperature. Sometimes, however, due to physiological or environmental factors, the body loses heat faster than it can be produced, or produces heat faster than it can be dissipated. This can lead to hypothermia and hyperthermia.

Hypothermia occurs when the body cools to an unsafe temperature. As a result of this, organs (to include the heart) and the nervous system cannot function ordinarily. Symptoms of mild hypothermia include shivering, increased heart rate and respiration, excessive urine production, mental confusion, and liver dysfunction. As symptoms progress, respiration and pulse rates decrease, blood pressure drops, and the patient experiences motor skill dysfunction, difficulty speaking, and may display irrational behavior. If left untreated, major organs will fail ultimately resulting in death.

Hyperthermia occurs when the body warms to an unsafe temperature. Hyperthermia differs from fever insomuch as there is no change to the hypothalamic set point. Hyperthermic conditions range from painful, yet somewhat benign, heat cramps, to the more severe heat exhaustion and heatstroke. Classic heatstroke can result from prolonged exposure to heat in conjunction with various underlying physiological conditions. However, in otherwise healthy individuals, heatstroke is typically brought on by conducting physical activity in high heat and/or humidity; this is known as exertional heatstroke. Heatstroke requires immediate treatment to prevent death or to mitigate numerous sequela (a condition resulting from another injury or illness) such as behavioral dysfunction (confusion, inappropriate behavior, and/or delirium), Rhabdomyolysis (release of muscle tissue into the blood), hepatic dysfunction (liver damage), and/or renal impairment (kidney failure).

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