Neurohumoral Regulation (Thermoregulatory Centers)
Cooling responses are initiated when the ambient temperature rises or when heated blood flows to the brain. This warm blood washes over the anterior hypothalamus, where the heat loss center is located. Specialized thermosensitive neurons are activated within it, and excitation is transmitted to effector cells.
Thermoregulation is based on the principle of reciprocal inhibition. Neurons of the anterior hypothalamus send inhibitory signals to the posterior hypothalamus, where the heat production center is located. As a result, heat generation mechanisms are suppressed, while heat dissipation processes (vasodilation, sweating) are activated.
Physical Mechanisms of Heat Loss
Heat dissipation into the environment obeys physical laws and is implemented through four main pathways:
- Radiation — the primary pathway, accounting for about 70% of total loss. It occurs via the emission of electromagnetic waves in the infrared spectrum (this is what thermal imagers detect).
- Conduction — the transfer of kinetic energy from body molecules to environmental molecules upon direct contact (accounts for about 15% of loss).
- Convection — heat transfer to moving volumes of air or water. The higher the velocity of the medium (wind, river current), the more intensive the cooling.
- Evaporation — the expenditure of thermal energy on the phase transition of liquid into vapor from the skin and mucous membranes. At rest, it accounts for about 14% of heat loss.
Physiological and Behavioral Responses
Physical processes of thermoregulation are complemented by vascular and visceral reactions of the body:
- Vasomotor response: during hyperthermia, cutaneous vasodilation occurs. Arterioles and venous plexuses dilate, increasing the influx of warm blood from the body core to the skin to dissipate heat via radiation and convection.
- Sudomotor response (sweating): a critically important mechanism at high temperatures. Excitation is transmitted along descending spinal cord pathways to sympathetic nerves. Postganglionic endings release acetylcholine (an exception to the sympathetic system), which acts on muscarinic acetylcholine receptors of the sweat glands. At 50 °C, a person can secrete up to 12 L of sweat per day, dissipating up to 8000 kcal.
- Piloerection state: the arrector pili muscle (m. arrector pili) relaxes. Hairs lie flat against the skin, destroying the insulating air layer.
- Respiration and excretion: heat is lost through external respiration, warming of cold food, as well as urine and feces (excreta account for about 1% of heat loss).
- Behavioral regulation: in warm environments, animals and humans tend to increase their exposed body surface area (sprawling) to maximize energy dissipation.
Factors Affecting Heat Loss Efficiency
Cooling intensity is directly proportional to the temperature gradient (the temperature difference between the body and the environment), body surface area, and air velocity. Inverse proportionality is observed with respect to the insulating properties of skin, fur, or clothing.
External conditions and the type of activity have a significant impact:
- Ambient humidity: heat loss in humid air (around 0 °C) occurs significantly faster than in dry, freezing air. This makes high humidity at low temperatures a serious risk factor for frostbite.
- Aquatic environment: water has a higher thermal conductivity than air while excluding the evaporation mechanism. Therefore, a person freezes faster in cold water.
- Physical activity: mental work practically does not stimulate sweating, whereas intense unmechanized physical labor causes a sharp increase in sweat secretion (up to 1.4–1.5 L/h at 45 °C).