Neural regulation: the role of the hypothalamus
Temperature control is based on the activity of brain structures that receive signals from internal environment and skin receptors.
- Temperature decrease: cooled blood and tissue thermoreceptors activate the heat production center in the posterior hypothalamus. Heat generation is triggered and its loss is inhibited.
- Temperature increase: the heat loss center of the anterior hypothalamus is activated. Heat conservation mechanisms are suppressed, and physical heat loss is enhanced (e.g., via sweating).
Skin receptors can trigger reflex regulation: exposure to heat turns on heat loss, while cold turns on heat production.
Local mechanisms and vascular reactions
The skin and individual body parts have their own ways of maintaining balance:
- Vascular reactions: in the cold, skin arterioles constrict, blood is shunted via arteriovenous anastomoses and directed to internal organs (centralization of blood flow). In the heat, conversely, skin capillaries engorge, bypassing the subcutaneous heat exchanger, which increases heat loss.
- Countercurrent heat exchanger: cool venous blood takes up some heat from the adjacent arterial blood, minimizing overall losses.
- Change in thermal insulation (piloerection): contraction of the hair-raising muscles (m. arrector pili) causes "goosebumps." In humans, this is a vestigial sign of sympathetic activation, while in animals, it is a way to increase the air layer.
- Regulation of the scrotum (scrotum): muscles relax in the warmth and contract in the cold, maintaining the temperature required for spermatogenesis.
The role of hormones and behavior
Humoral support is provided by the pituitary gland, thyroid gland, and adrenal glands.
- In the cold: GH, TSH, thyroid hormones, and adrenaline are released. Adrenaline stimulates oxidation in tissues (especially muscle) and constricts skin vessels.
- In the heat: TSH secretion drops, and adrenaline changes its targets — it acts on $\beta_2$-adrenergic receptors of arterioles, dilating them for better heat loss.
The external behavioral component also plays a significant role. Humans are capable of purposefully changing the microclimate or the number of layers of clothing. In addition, conditioned-reflex regulation is possible — the body can trigger thermoregulatory mechanisms in response to signals that were previously paired with a temperature change.
Briefly: answers to topical questions
Through what specific pathways does physical heat loss occur from the surface of the human body?
Physical heat loss occurs via four main pathways:
- Radiation — heat loss in the form of infrared rays; at rest and in thermal comfort, it accounts for 45% of heat loss.
- Convection — movement of ambient media volumes (air or water) around the body; at rest and in thermal comfort, it accounts for 30% of heat loss.
- Evaporation (perspiration) — evaporation of water from the surface of the skin and upper respiratory tract mucosa; involves the expenditure of thermal energy for the phase transition of liquid to vapor. At rest and in thermal comfort, perspiration accounts for 25% of heat loss.
- Conduction — transfer of heat upon direct contact with cold surfaces of a solid medium.
What is the mechanism of non-shivering thermogenesis?
The mechanism of non-shivering thermogenesis consists in active heat production by uncoupling tissue respiration and oxidative phosphorylation in brown adipose tissue.
- Upon cooling, the sympathetic nervous system releases norepinephrine.
- Norepinephrine activates TAG lipase, increasing the concentration of free fatty acids in cells.
- Fatty acids activate uncoupling proteins — thermogenin (UCP-1) in the inner mitochondrial membrane.
- Thermogenin returns protons to the matrix bypassing ATP synthase.
As a result, the energy of electron transport is not used for ATP synthesis, but is dissipated as heat.
What is the normal temperature of the human body "core" and "shell"?
Normal temperature differs for the body "core" and "shell".
| Zone or region | Temperature values |
|---|---|
| Core: average temperature, accepted standard — blood in the right ventricle of the heart | 37.5 °C |
| Core: venous blood from the liver | 37.8–38.0 °C |
| Shell: superficial tissues, skin | temperature is lower than in the core; characterized by high variability |
| Limbs skin during heating or cooling | can vary from 16 to 35 °C without discomfort |
| Fingers | 28–32 °C |
| Toes | 24–28 °C |
Additionally, sources cite normal temperature indicators by site: axillary — 36.6–36.8 °C; oral and rectal — 36.8–37.2 °C.
Class and exam questions
Why does cold feel more intense in humid weather?
Humid air has a higher thermal conductivity compared to dry air, which is why it removes heat from the body surface faster.
How are piloerection and sweating linked in the cold?
During cooling, piloerection ("goosebumps") occurs to retain heat, and sweating stops completely to eliminate heat loss via evaporation.
How does adrenaline help in the heat?
Under high temperature conditions, adrenaline binds to $\beta_2$-adrenergic receptors of skin arterioles, leading to their dilation and increased heat loss.
What else to study on this topic
- Mechanisms of heat production (muscular shivering and non-shivering thermogenesis)
- Physical heat loss: evaporation, convection, radiation
- The effect of food intake and physical activity on heat balance
- Mechanisms of formation of conditioned temperature reflexes
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