The thermoregulation is an integrated system of biological mechanisms, designed to maintain an almost constant internal temperature regardless of the climatic conditions outside the organism. These mechanisms - particularly effective in birds and mammals (all homeothermic animals), less so in fish, amphibians and reptiles (poultry moths) - include processes of production, conservation and dispersion of heat.
Since frequently the obese subject does not eat abnormally if compared to other normopeso individuals, who sometimes eat even more, it is likely that - with equal physical activity - alterations of the thermoregulatory processes can lead to reduced energy consumption, with accumulation excess energy in the form of fat. The thin subjects, unlike the obese, would therefore be better at disposing of the food excesses (see brown adipose tissue) in the form of heat.
The thermoregulation can be first of all voluntary or involuntary. In the first case it is the animal itself that voluntarily sets in motion adequate behavioral strategies, such as the search for a shelter sheltered from the weather or migration to the most suitable places for maintaining its body temperature.
Even involuntary thermoregulatory responses can be evoked by exposure to cold environments or warm environments. In any case, they foresee the intervention of the hypothalamic thermoregulatory center, capable of picking up and processing the signals coming from the cutaneous and central thermoreceptors (located in the brain, spinal cord and central organs), coordinating the most suitable physiological response to maintain body temperature.
Thermoregulation in cold environments
The thermoregulatory adaptations to the cold are intended to conserve and / or produce heat.
An organism's ability to produce heat is called thermogenesis; it is largely mandatory and linked to the physiological and metabolic processes involved in the movement, digestion, absorption and processing of the nutrients introduced with the diet.
Mammals have the ability to increase heat production (optional thermogenesis), involving or not the thrill mechanism. In the first case we speak of shivering thermogenesis (shivering). This mechanism leads to the production of heat through a rhythmic and isometric contraction of muscle tissue, not aimed at movement. The alternation of contractions and relaxations leads to a characteristic tremor called shiver, which appears when the body temperature tends to decrease "noticeably". The shiver generates a share of heat even 6-8 times greater than that produced by the muscle at rest. Typically, it occurs only when maximal vasoconstriction (see below) has not been able to maintain body temperature.
Thermogenesis without shivering, also called chemical thermogenesis, involves the production of heat through exothermic biochemical reactions (which generate heat). These reactions take place in particular organs, such as brown adipose tissue (BAT), liver and muscle.
Brown adipose tissue, typical of hibernating animals and scarce in humans (greater in newborns), is thus defined by the characteristic brown pigmentation (visible to the naked eye) given by the carotenoids present at the mitochondrial level. These power plants of the brown fat cell are distinguished by a further characteristic, the presence of the mitochondrial protein UCP1. This protein, located at the level of the mitochondrial membrane, has the characteristic of decoupling oxidative phosphorylation, thus favoring the production of heat to the detriment of the formation of ATP molecules. Simply put, brown adipose tissue is intended to burn nutrients (mainly fat) in order to increase heat production. The activation of the brown adipose tissue, stimulated by the cold, is mainly linked to the release of noradrenaline and its interaction with the β3 receptors, but also guaranteed by endocrine mechanisms such as the release of T3 and T4 from the thyroid. The largest deposits of brown adipose tissue are recorded in the interscapular, periaortic and perirenal areas; at these levels, they are placed near blood vessels, to which they give heat so that it is transported with the blood flow to the peripheral areas of the body.
It is currently believed that the liver also participates in thermoregulation, increasing its metabolic activity - with consequent production of heat - when the human body is exposed to low temperatures. Another recent discovery was the finding of isoforms of the UCP1 protein in muscle, which suggests a presumed thermogenetic role of metabolic origin (in addition to the ability to produce heat through the shiver). Finally, exposure to low temperatures increases cardiac activity, necessary to support the metabolic demands of active tissues in these circumstances (such as BAT) and to increase the transport of heat produced therein in all anatomical districts. In addition to guaranteeing all this, the increase in cardiac activity is in itself capable of producing a non-negligible quantity of heat.
The control of heat losses is governed by the physical laws of conduction, convection, radiation and evaporation.
CONDUCTION : heat transfer between two objects at different temperatures, in contact with each other through a surface.
RADIATION or IRRADIATION : heat transfer between two objects at different temperatures, which are NOT in contact. The loss or purchase of heat occurs in the form of radiation with wavelengths in the visible or infrared range; to be clear, it is the same way that the sun warms the earth through space. Heat loss by radiation constitutes more than half of the amount of heat lost by the human body.
CONVECTION : transfer of heat from a body to a source that moves through it (currents of air or water). The movement of water or cold air through the warmer skin causes the continuous elimination of heat.
EVAPORATION : transfer of heat by passing from the liquid to the gaseous state of fluids lost through sweating, insensitive losses through the skin and respiratory tract.
The reduction of thermal dispersion in the environment occurs essentially through the containment of the cutaneous blood flow (vasoconstriction) and the piloerection (in fur animals, between the warm skin and the cold environment, an air cushion is created that works by thermal insulation).
The increase in appetite, for its part, increases the production of heat through the thermogenetic mechanisms induced by the diet, and supports the energy demands of the thermogenetic organs.
Thermoregulation in hot environments
During the stay in warm environments the organism reacts through a series of thermodispersive mechanisms, in many ways contrary to those just illustrated; moreover, the metabolic processes underlying the optional thermogenesis are suspended. These include cutaneous vasodilation and increased sweating, frequency and depth of breath (polypnea), all processes that aim to increase heat dispersion by evaporation. In these circumstances the appetite and heart rate also decrease, in response to a lower demand for oxygen by the thermogenetic organs.
Among the long-term adaptation processes we can also appreciate a decrease in the pituitary secretion of a thyroid stimulating hormone, with a consequent slowing down of metabolism, and therefore of heat production.
As mentioned in the previous chapter, the process of vasoconstriction is largely controlled by the sympathetic nervous system. The smooth muscles at the level of the precapillary sphincters and arterioles receive afferents from postganglionic sympathetic (adrenergic) neurons. If the deep temperature drops (exposure to cold), the hypothalamus selectively activates these neurons, which through the release of norepinephrine determine the contraction of the arteriolar smooth muscle, reducing the cutaneous blood flow. This thermoregulatory response keeps the blood warmer on the internal organs, minimizing the blood flow on the skin surface made cold by the weather. While vasoconstriction is an active process, vasodilation is a predominantly passive process, which depends on the suspension of vasoconstrictor activity by inhibition of sympathetic activity. If this process is typical of bodily extremities, vasodilation is favored in other parts of the body by specialized neurons that secrete acetylcholine. Particular cases are also represented by the local expansion of some vascular districts following the release of nitrogen monoxide (NO), or other vasodilating paracrine substances.
In the context of thermoregulation, the cutaneous blood flow varies from values close to zero, when it is necessary to conserve heat, up to almost 1/3 of the cardiac range when the heat must be released to the environment.
