Cold critters: Understanding hypothermia
There are four basic mechanisms of heat loss5:
Receptors for cold and warm are distributed throughout the body. Cold signals traverse A-delta fibers, and signals from warmth receptors are relayed through C fibers.7 Processing thermoregulatory information occurs through three pathways7: afferent thermal sensing from the periphery, central regulation in the hypothalamus, and efferent responses.
Given these three pathways, peripheral body temperatures are constantly fluctuating while the posterior hypothalamic thermoregulatory center maintains a relatively constant core temperature.8 Cellular metabolism results in heat production by the body, and heat is lost from the body when core heat is transferred through variably conductive tissues to the skin and is subsequently lost to the environment.6 Specifically, heat is transferred from the body's core to the skin through a multitude of blood vessels, including venous plexuses and capillaries, with arteriovenous connections that are under the control of the autonomic nervous system.9,10 The rate of blood flow through these arteriovenous anastomoses varies depending on the degree of vasoconstriction or vasodilation desired.9,10 Increased blood flow leads to increased heat loss, whereas decreased blood flow results in core heat conservation.5
As core body temperature dips below 94 F (34.4 C), thermoregulation is impaired, and animals characteristically cease to shiver or seek heat.11 Peripheral vasodilation rather than vasoconstriction predominates, leading to continued core heat loss.12 Additionally, heat production decreases because of the decreased metabolic rate.4,6 Concurrently, severe hypothermia depresses the central nervous system, ultimately resulting in a hypothalamus that is less responsive to hypothermia.6 Indeed, when the body core temperature drops below 88 F (31.1 C), thermoregulation ceases.5