Principles of Body Temperature Distribution
Thermoregulation operates as a complex functional system working to achieve a useful adaptive result. This result is the maintenance of a tissue temperature level optimal for all metabolic processes. Because the metabolic intensity in various tissues and organs differs significantly, their heat values are also non-uniform.
Thus, the output of the thermoregulation system is multiparametric, consisting of multiple local values. A continuous temperature gradient exists throughout the body. The body temperature profile is the individual distribution of temperatures across the skin surface and internal organs. Under normal conditions, it remains relatively constant, strictly depending on two factors: the functional state of the body and the ambient temperature (which heats or cools tissues).
Normal temperature values at standard sites:
- Axillary (armpit): 36.6–36.8 °C.
- Oral (mouth) and rectal (rectum): 36.8–37.2 °C.
- Fingers: 28–32 °C.
- Toes: 24–28 °C.
Characteristics of the Core and Shell
The thermal structure of the body can be divided into two key functional zones.
1. Body Core (Homeothermic Zone) Includes deeply located internal organs and the brain. The temperature here is higher than at the surface and is maintained at a stable level by heat generated through internal metabolism. The "hottest" organ is the liver; venous blood draining from it has a temperature of 37.8–38.0 °C. The universally accepted standard for estimating average core temperature is the blood in the right ventricle of the heart (37.5 °C).
2. Body Shell (Poikilothermic Zone) Represented by superficial tissues: skin, subcutaneous adipose tissue, and extremities. The shell temperature is always lower than the core temperature and exhibits high variability. For example, during severe cooling or warming, the tissues of the upper and lower extremities can change their temperature within a range of 16 to 35 °C without causing discomfort.
Temperature Distribution in the Cold
Exposure to low temperatures triggers a centralization of heat conservation. Peripheral blood vessels sharply constrict (vasoconstriction) to reliably isolate heat within vital organs.
Under these conditions, the thermal core (37 °C) shrinks to a minimal volume, encompassing only the deep trunk and head. Conversely, the thermal shell thickens significantly.
A pronounced proximo-distal gradient forms (a sharp temperature drop from the body center toward the periphery of the limbs). The difference between the core and the foot under such conditions reaches 21 °C.
Exact temperature values in the cold:
- Superficial layers of the chest and trunk: 36 °C.
- Upper extremity: shoulder cools to 34 °C, forearm to 32 °C, hand to 28 °C.
- Lower extremity: thigh 34 °C, leg 31 °C, ankle 29 °C, foot 16 °C (maximum shell hypothermia is observed in the foot area).
Temperature Distribution in Warm and Comfortable Conditions
Upon exposure to high ambient temperatures or in comfortable conditions, the body activates heat dissipation mechanisms. Peripheral vessels dilate (vasodilation), transporting excess heat via the blood to the body surface.
Here, the thermal core expands maximally, encompassing not only the trunk but also the proximal limb segments. The thermal shell thins considerably.
The temperature gradient flattens: the skin warms more uniformly, and its values approach core temperatures. The temperature difference between the core and the foot drops to 13 °C.
Exact temperature values in warmth:
- Upper extremity: shoulder and forearm warm uniformly to 36.9 °C, practically making them part of the homeothermic core.
- Lower extremity: thigh fully enters the core zone (37 °C), leg temperature is 36 °C, ankle 30 °C, foot 24 °C.