Thermorégulation et Homéostasie

• Thermoregulation is the set of mechanisms that allows humans to maintain a core temperature close to 37°C.
• The purpose of thermoregulation is to maintain the actual value of the core temperature (temperature of organs located inside the thoracic, abdominal, and cranial cavities) around a reference or setpoint value of about 37°C despite any variation in the environmental temperature.
• Thermoregulation requires thermoreceptors, regulatory centers, and effector mechanisms with afferent and efferent conduction pathways connecting them.

Control of Thermoregulation

• The control center for thermoregulation is the hypothalamus, which acts like a true thermostat.
• The hypothalamus receives thermal information from two types of thermoreceptors:
  • Central receptors: deep, visceral, and especially hypothalamic, sensitive to blood temperature.
  • Peripheral receptors: superficial, cutaneous, sensitive to low temperatures (impression of cold) and high temperatures (impression of heat).

Structure of the Hypothalamus

• The hypothalamus is a small structure of the CNS (a few cubic centimeters) and consists of numerous pairs of nuclei formed of nerve cells, located below the thalamus and above the pituitary gland.
• A small stalk connects the pituitary gland to the hypothalamus.
• The neurons of the hypothalamus synthesize different hormones and release them into a primary capillary network that gathers to form a vein, the hypothalamo-hypophyseal portal system.
• This system transports hypothalamic hormones to the pituitary gland, where the hormones are released by a second capillary network to activate pituitary neurons.

Function of the Hypothalamus

• The hypothalamus is a small region located at the heart of the brain, serving as a bridge between the autonomic nervous system and the endocrine system.
• It is involved in regulating major functions such as hunger, thirst, sleep, and body temperature.
• It is also involved in sexual behavior and emotions.
• Its endocrine functions (secretion of neurohormones) regulate the endocrine function of the pituitary gland, located below.
• The hormones secreted by the hypothalamus all, or almost all, influence the production of pituitary hormones.
• Dopamine inhibits prolactin production.
• Thyrotropin-releasing hormone (TRH) stimulates the production of thyroid-stimulating hormone (TSH).
• Somatocrinin (GH-RH) allows the release of growth hormone somatotrophin (STH).
• Somatostatin (GH-RIH) inhibits the release of STH.
• Gonadotropin-releasing hormone (GnRH) is responsible for the synthesis and secretion of FSH and LH (hormones).
• Corticotropin-releasing hormone (CRH) promotes the production of adrenocorticotropic hormone (ACTH).

Thermoregulatory Centers

• The anterior hypothalamic center (center for thermolysis) ensures the thermal response to heat by triggering: thermolysis through vasodilation and increased sweating.
• The posterior hypothalamic center (center for thermogenesis) ensures the thermal response to cold by stimulating: muscle and chemical thermogenesis and cutaneous vasoconstriction to avoid heat loss.
• Information transmitted from receptors sensitive to decreases or increases in temperature travels to the thermoregulatory centers of the hypothalamus via:
  • A fibers for cold sensitivity.
  • C fibers (unmyelinated) for heat sensitivity.
• Heat production results from cellular metabolism; at rest, heat production in adults is 100K cal/H.
• Body temperature is a balance resulting from heat production and dissipation.

Adaptation Mechanisms

• Maintaining a constant temperature of 37°C requires that the inputs, i.e., heat production (thermogenesis), equal heat loss (thermolysis).

Mechanisms of Heat Exchange

• Four mechanisms of heat transfer:
  • Radiation: heat loss in the form of infrared waves (thermal energy).
  • Conduction: heat transfer between two objects in direct contact.
  • Convection: cooling of the skin by wind (fan) or water.
  • Evaporation: water evaporates because molecules absorb heat from the environment and possess enough energy to escape as a gas (water vapor) in areas where the skin is in contact with air.

Mechanisms of Thermoregulation

• There are two mechanisms of thermoregulation: thermogenesis and thermolysis.

Thermogenesis

• Definition: thermoregulation is the production of heat (combustion).
• It increases with physical activity.
• When the temperature decreases, the hypothalamic center for thermogenesis is activated.
• Thermogenesis involves chemical thermogenesis at rest, where heat production is dominated by the metabolic activity of tissues (heart, liver, endocrine glands).
• Muscle thermogenesis predominates during exercise.

Mechanisms of Thermogenesis

• Vasoconstriction of cutaneous blood vessels through activation of sympathetic nervous system fibers, which stimulates the smooth muscles of the skin's arterioles, resulting in vasoconstriction.
• Increased metabolism rate: if the temperature decreases (cold), it triggers sympathetic nerve stimulation, liberating noradrenaline by the adrenal glands. This increases the rate of cellular metabolism, increasing glycogen usage (O2 consumption increases), and therefore increasing heat production = chemical thermogenesis.
• Thermal shivers (TREMOR): activation of brain centers regulating muscle tone, involuntary contraction of skeletal muscles = shivering. The extrapyramidal system is responsible for the shivers. An increase in heat equals an increase in body temperature because muscle activity generates heat production.
• Increased liberation of thyroxine: any decrease in external temperature triggers activation of the hypothalamus and liberation of thyreoliberin (TRH), then activation of the adenohypophysis, which secretes thyrostimulin (TSH), then stimulation of the thyroid gland (target organ), which releases more thyroxine into the blood, thus increasing metabolism of target cells and increasing heat production.

Thermolysis

• Definition and Generalities: thermolysis is the elimination of heat (excess heat) either by radiation, conduction (15-20% of heat loss), convection, or evaporation.
  • Cutaneous evaporation: occurs either passively (insensible respiration) or actively through sweating (sudoriparous glands).
  • Pulmonary evaporation: passive insensible loss of water and heat (condensation when the ambient air is cold).

Mechanisms of Thermolysis

• Vasodilation of cutaneous arterioles: any increase in temperature will trigger a modulation of sympathetic nervous system fibers or inhibition of this system (SNS). Stimulation of smooth muscles of skin arterioles decreases, causing vasodilation. Warm blood invades skin vessels, and heat dissipates to the skin's surface by radiation, conduction, and convection.
• Increased perspiration: any significant increase in temperature will stimulate nerve fibers of the parasympathetic nervous system (the cholinergic system), which stimulate sweat glands, increasing sweat (transpiration), followed by evaporation of sweat, thus losing heat.

Heat Transport

• The calories produced by metabolic activity are distributed throughout the body by the blood and are brought to the outer integuments, where exchanges take place.
• Blood is the heat transfer agent between the inside of the body and its surface.
• Cutaneous circulation functions as a thermal exchanger under the control of the autonomic nervous system:
  • Sympathetic (cold) induces vasoconstriction to prevent heat loss (decreasing thermolysis).
  • Parasympathetic (stimulated during heat) induces vasodilation to increase thermolysis.

Conclusion

• Thermoregulation is under hormonal control by stimulation of the hypothalamo-pituitary axis, the adrenal medulla, and the thyroid, and under nervous control (autonomic nervous system).
• For a constant temperature: thermogenesis = thermolysis.
• For any temperature variation:
  • If temperature decreases: thermogenesis increases, with increased physical activity, increased hormonal secretion, and vasoconstriction.
  • If temperature increases: thermolysis increases, with vasodilation, perspiration, and polypnea.