Thermoregulation

The thermoregulation, thermal regulation or temperature regulation is the ability of a biological organism to modify its temperature within certain limits, even when the surrounding temperature is quite different from the target-temperature range. The term is used to describe the processes that maintain the balance between heat gain and heat loss. If a certain amount of heat is added to or removed from an object, its temperature increases or decreases, respectively, by an amount that depends on its specific heat capacity with a complete environment.

In the steady state, the rate at which heat is produced (thermogenesis) is balanced by the rate at which heat is dissipated to the environment (thermolysis). In the event of an imbalance between thermogenesis and thermolysis, there is a change in the rate of body heat storage and consequently a change in the body's heat content and body temperature.

Thermoregulatory or homeothermic organisms maintain essentially constant body temperature over a wide range of environmental conditions. On the other hand, ectotherms or poikilotherms are organisms whose body temperature varies with environmental conditions. The way of obtaining heat can be by endothermy or by ectothermy. Endothermic or homeothermic organisms control body temperature through internal heat production, and habitually maintain this temperature above ambient temperature. Ectothermic or poikilothermic organisms depend mainly on an external heat source to regulate their body temperature.

Thermoregulation in humans

Three different recent studies suggest that the average temperature in healthy adults is 36.7°C. The simplest human thermoregulatory model divides the body into two compartments: the central or core zone that produces heat and the superficial or peripheral zone that regulates heat loss.

Human beings are both homeothermic and endothermic, which implies that, despite large variations in environmental temperature, internal heat production balances heat loss, resulting in a stable body temperature. This balance is known as heat balance or heat flow. Its control is effectively effected through behavioral modulation (such as changing clothes) and physiological mechanisms (such as sweating, shivering).

The temperature considered regular in the human body varies depending on their sex, their recent activity, the consumption of food and liquids, the time of day and, in women, the phase of the menstrual cycle in which they are. Medicine traditionally considers that normal body temperature —taken orally— oscillates between 36.5 and 37.5 °C in a healthy adult; the average value is 37 °C.

The simplest human thermoregulatory model divides the body into two compartments: the central or core zone that produces heat and the superficial or peripheral zone that regulates heat loss. Under resting conditions, heat production is especially dependent on the metabolic activity of internal organs such as the brain and organs in the abdominal and thoracic cavities such as the liver, intestines, kidney and heart. Blood, propelled and distributed by the cardiovascular system, is the main medium that transports heat (by convection) from the core to the skin region. The temperature of the core, especially that of the brain, is regulated.

The human body has an internal temperature of 37 °C, while the skin temperature is 33.5 °C. The heat gained and lost by the body depends on multiple factors. The temperature with which the blood reaches the hypothalamus will be the main determinant of the body's response to climatic changes. Since the hypothalamus is the integrating center that works as a thermostat and maintains the balance between heat production and loss. If the temperature decreases, thermogenesis and heat conservation mechanisms increase: The maintenance of body temperature also depends on the heat produced by metabolic activity and the heat lost by body mechanisms, as well as environmental conditions.

Thermogenesis, or generation of temperature, is carried out in two ways:

• Quick: physical thermogenesis, produced largely by tremor and the decrease in peripheral blood flow

• Lenta: chemical thermogenesis, hormonal origin and substrate mobilization from cell metabolism.

Heat transfer processes

There are two mechanisms of heat exchange between the body of an animal, including humans, and the environment: evaporative heat loss and non-evaporative heat exchange. Non-evaporative heat exchange represents the sum of heat fluxes due to radiation, convection and conduction. Since heat flows down the temperature gradient, heat from the body is dissipated to the environment as long as the environment is cooler than the body. The body temperature of endotherms, such as humans, is generally higher than ambient temperature, so most of the heat produced by these organisms is lost by radiation, conduction, or convection. When the environmental temperature is higher than the body temperature, evaporation is the only form of heat loss, constituting an essential mechanism for the maintenance of homeothermy. It is important to note that the relative effectiveness of these heat exchange routes depends on environmental conditions.

Radiation

Like any body with a temperature greater than 0 K, living beings also radiate heat to the environment through electromagnetic waves. It is the process in which the most heat is lost: 68%.

Radiation is the propagation of energy through empty space, without requiring the presence of matter. balance is homeostasis

Driving

Conduction is the transfer of heat by contact with air, clothing, water, or other objects (a chair, for example). This transfer process occurs due to the interaction between the molecules that make up the bodies, thus those molecules that are at a higher temperature vibrate more quickly colliding with those less energetic (with lower temperatures) transferring part of their energy. If the temperature of the surrounding medium is lower than that of the body, the transfer occurs from the body to the environment (loss), otherwise the transfer is reversed (gain). In this process, 3% of the heat is lost, if the surrounding medium is air at normal temperature. If the surrounding medium is water, the transfer increases considerably because the heat transfer coefficient of water is greater than that of air.

It is the heat flux per gradient. The physical foundation is the transfer of heat energy between molecules.

Convection

This process, which occurs in all fluids, causes warm air to rise and be replaced by cooler air. Thus, 12% of the heat is lost. Clothing decreases loss. If there is an air current (wind or mechanical fan) a forced convection is produced and the transfer is greater. If there is no fresher air to make the replacement, the process stops. This happens, for example, in a small room with many people.

Evaporation

To go from the liquid to the gaseous phase of water, energy is needed. When this occurs on the surface of the body, energy is lost in the form of heat. Evaporation occurs by two mechanisms: by insensible evaporation, or perspiration, and by perceptible perspiration, or sweating. To some extent, insensible evaporation occurs continuously from the skin and respiratory surfaces. Respiratory heat loss occurs through convection and evaporation. Convective heat loss occurs when inhaled cold air is warmed to body temperature in the lungs and upper respiratory tract, and is subsequently exhaled to the environment. The evaporative component originates when inhaled air, heated and saturated with water, is released into the environment during expiration. Therefore, respiratory heat loss depends on the physical properties of the inspired air (temperature, vapor pressure) and the individual's respiratory rate. Evaporation of sweat, produced by the sweat glands, can be an important contribution to heat loss. Through the evaporation of sweat, 27% of body heat is lost, because water has a high specific heat, and to evaporate it needs to absorb heat, and it takes it from the body, which cools. A draft that replaces moist air with dry air increases evaporation.

For 1 g of sweat to evaporate from the surface of the skin, approximately 0.58 kcal are required, which are obtained from the skin tissue, thereby cooling the skin and consequently the body.

When the temperature of the hypothalamic thermostat falls below normal body temperature, sweating is completely suppressed. This response eliminates evaporative cooling except for insensible evaporation.

Mechanisms of body temperature regulation

Body temperature is regulated almost exclusively by negative feedback neural mechanisms that operate mostly through thermoregulatory centers located in the hypothalamus. In addition to neural control, hormones affect thermoregulation, but are generally associated with long-term acclimatization. Three models have been proposed to explain the mechanism of thermal homeostasis in humans. The first two propose that temperature is the regulated variable. These models consider that the thermoregulatory mechanisms try, at all times, to bring the body temperature to the set point. The third model is fundamentally different from the first two in that it proposes that the regulated variable is heat content rather than temperature per se, in this model body temperature is considered to be a byproduct of regulation.

The most recent and apparently most widely accepted models are the "balanced point" theory and the "proportional control" theory. Both theories postulate that body temperature is controlled by a "multi-sensor", "multi-processor", "multi-effector" proportional feedback control system.

Two sources of heat alter body temperature: internal heat generation and environmental heating or cooling. Due to exothermic chemical reactions, all organs produce metabolic heat, even when the body is at rest. During exercise the muscles produce several times more heat than that produced at rest. Heat is dissipated from the skin to the environment if the skin surface temperature is higher than the ambient temperature, otherwise the heat is absorbed by the skin. To maintain temperature homeostasis, humans use two mechanisms: behavioral thermoregulation and autonomous thermoregulation. Behavioral thermoregulation consists of the conscious adjustment of the thermal environment in order to maintain comfort. It is achieved by altering the degree of insulation of the body (clothing) or the ambient temperature. Autonomic thermoregulation is the process by which, through the autonomic nervous system, internal mechanisms control body temperature subconsciously and precisely. This control involves two mechanisms, one associated with the dissipation of heat, and the other with its production and conservation. Elevated ambient temperature causes heat loss through skin vasodilation, sweating, and decreased heat production. When the ambient temperature drops, additional heat is produced by shivering thermogenesis and non-shivering thermogenesis, and heat loss by constriction of cutaneous blood vessels is decreased. Long-term exposure to cold increases the release of thyroxine, which increases body heat by stimulating tissue metabolism. Technical thermoregulation constitutes a third mechanism, which can be considered part of behavioral thermoregulation. It involves the use of a system that maintains a constant ambient temperature. An example is the air conditioner that monitors the temperature of a room and adjusts the flow of heat keeping the temperature constant. It is noteworthy that both autonomous, behavioral and technical thermoregulation constitute negative feedback control systems.

The thermoneutral zone or, referring to humans, the thermal comfort zone, is the environmental temperature range in which metabolic expenditure is kept to a minimum, and temperature regulation is carried out by non-evaporative physical mechanisms, maintaining core body temperature in normal ranges. This means that thermoregulation in the thermoneutral zone occurs only by vasomotor control. The lower and upper limits of the thermoneutral zone are called the lower critical temperature and upper critical temperature, respectively. Due to differences in thermal properties, the thermoneutral zone in water is skewed upwards compared to that of air (33 to 35.5.o C in water vs. 28.5 to 32.o C in air).

The thermoregulatory functions are divided according to their purpose and physiological mechanism into two categories. The first includes thermoregulation that counteracts changes in temperature that would produce serious disturbances in thermal homeostasis, posing a threat to life. The second comprises a special type of thermoregulation, its function being to even out comparatively small but continuously occurring thermal fluctuations. These temperature fluctuations that occur even in the thermoneutral zone are an inherent part of normal life for animals and humans. In the absence of abrupt changes in temperature, the latter is the main function of the thermoregulation system.

Mechanisms of heat loss

Overheating of the thermostatic area of the hypothalamus increases the rate of heat loss by two essential processes:

Sweating

When the body overheats, information is sent to the preoptic area, located in the brain, in front of the hypothalamus. This triggers the production of sweat. Humans can lose up to 1.5 liters of sweat per hour. Through it, the loss of water is produced, which leads to a decrease in the temperature of our body.

Vasodilation

When body temperature rises, peripheral vessels dilate and blood flows in greater quantity near the skin, favoring heat transfer to the environment. For this reason, after an exercise the skin turns red, since it is more irrigated.

Heat preservation mechanisms

When the body cools below normal temperature, the following mechanisms reduce heat loss:

Vasoconstriction

Vasoconstriction of epidermal vessels is one of the first processes that improves heat conservation. When the temperature decreases, the posterior hypothalamus is activated and, through the sympathetic nervous system, the diameter of the cutaneous blood vessels decreases; this is the reason why people turn pale in the cold. This effect decreases heat conduction from the inner core to the skin. Consequently, skin temperature decreases and approaches ambient temperature, thus reducing the gradient that favors heat loss. Vasoconstriction can decrease heat loss by eight times.

Countercurrent heat exchange

Many animals, including humans, have a mechanism called a countercurrent exchanger to conserve heat. The arteries of the arms and legs run parallel to a set of deep veins but their flow is opposite. So the heat of the arterial blood (which circulates from the core to the periphery) diffuses towards the venous blood (which flows from the periphery to the core). In this way the heat is returned to the central region of the body.

Piloerection

Stimulation of the sympathetic nervous system causes the erector muscles, located at the base of the hair follicles, to contract, causing the hair to stand up. The erection of the hair expands the layer of air in contact with the skin, decreasing the convective movements of the air and, therefore, reducing heat loss. In humans, lacking fur, this mechanism is not important and produces what is commonly called goosebumps.

Mechanisms of heat production

Broadly speaking, energy expenditure can be subdivided into two categories of thermogenesis: obligatory thermogenesis and facultative thermogenesis. Mandatory thermogenic processes are essential to the life of all cells in the body and include those processes that maintain a constant and normal body temperature. The largest component of obligatory thermogenesis is provided by the basal metabolic rate. Food-induced thermogenesis derived from the digestion, absorption, and metabolism of dietary nutrients is also considered a mandatory thermogenic process. Unlike obligatory thermogenesis that occurs continuously in all organs of the body, facultative thermogenesis can be rapidly turned on or off and occurs primarily in two tissues, skeletal muscle and brown fat. Body temperature, which in animals homeotherms, as in humans, is generally several degrees higher than that of the environment, for its maintenance it requires the activation of heat production and conservation mechanisms that compensate for its constant loss by dissipation to the external environment. At thermoneutral temperature, the thyroid is the main regulator of energy expenditure through mechanisms that modulate oxygen consumption in the mitochondria of various tissues, particularly skeletal muscle and the liver. The thyroid is also involved in the regulation of adaptive thermogenesis or facultative, acting synergistically with norepinephrine (noradrenaline) in situations where the body requires additional heat to maintain normothermia during cold exposure.

When the ambient temperature is below the lower critical temperature, endothermic organisms produce heat in skeletal muscle and brown fat by two mechanisms:

Shivering Thermogenesis

The primary motor center of shivering thermogenesis is located in the posterior hypothalamus. Cold stress stimulates and heat inhibits this nerve center. When, in response to cold stress, muscle tone increases up to 5 times over normal production. Shivering thermogenesis consists of the involuntary, synchronous and rhythmic contraction of the motor units of the opposing muscles and, consequently, large movements are avoided and no external work is performed. Since no external work is done, all the energy released by shivering appears as heat.

Non-shivering thermogenesis

In small mammals and neonatal humans, non-shivering thermogenesis occurs primarily by mitochondrial uncoupling in brown adipose tissue or brown fat and is regulated by the sympathetic nervous system.

After a few hours of cold exposure, heat production in brown fat plays a dominant role in replacing shivering thermogenesis with non-shivering thermogenesis as the main source of additional heat to prevent hypothermia.

The ability of brown fat to generate heat is due to the existence of a unique protein in the mitochondria of brown fat cells: the uncoupling protein UCP1. This protein has the ability to permeabilize the mitochondrial membrane to protons. In this way, the oxidation of metabolites in mitochondrial respiration and the proton pump it generates are not reversed in ATP generation, as in normal mitochondria, but are dissipated as heat. Non-shivering thermogenesis is facultative, it is only activated when the organism needs additional heat, and it is adaptive, in the sense that weeks are required to recruit thermogenic tissue. The cold adaptation process is under the control of the hypothalamus, which activates the sympathetic nervous system and the secretion of norepinephrine and promotes the expression of UCP1. Uncoupling does not occur without sympathetic stimulation, but neither does it occur in the absence of thyroid hormone. Other hormones, such as leptin and insulin, are potent stimulators of UCP1 expression and thermogenesis in brown fat. The distinction between adrenergic and non-shivering thermogenesis is important. Although all mammals respond to norepinephrine by increasing metabolism, in non-cold adapted animals this increase primarily represents the response of organs not involved in non-shivering thermogenesis. Only the increased metabolism after cold adaptation represents thermoregulatory non-shivering thermogenesis.

As shivering thermogenesis is poorly developed in neonates, the main mechanism of heat production in these infants is non-shivering thermogenesis. In neonates, brown fat is located in the subcutaneous tissue, adjacent to the major vessels of the neck, abdomen, and thorax, around the scapula, and in large amounts in the adrenal areas.

Traditionally, brown fat in humans was thought to be found only in the neonatal stage. It was considered that brown fat involutes with age and that the adult human is practically devoid of it. However, since the 1970s, several independent works have demonstrated the presence of active brown fat in adult humans, its activity is regulated by thermogenic stimuli, and it is found in amounts that could have a considerable effect on thermogenesis. Brown fat tissue activity decreases with age, from 50% activity in 20-year-old subjects to 10% in 50-60-year-old subjects. In this sense, it was also found that brown fat is more prevalent in children than in adults, and that its activity increases in adolescence where it could have a specific metabolic function. On the other hand, recent work suggests that mitochondrial uncoupling is not only occurs in brown fat, but also in skeletal muscle tissue. Both tissues would be involved in cold-induced non-shivering thermogenesis regulated by the sympathetic nervous system.

Although activation of the shivering and non-shivering thermogenesis reactions does not require the expression of thermogenic genes, chronic cold exposure activates the expression of several genes important in the thermoregulatory process.

Fever

Homeothermic animals have developed physiological mechanisms that allow them to maintain a constant body temperature. However, the caloric balance of an organism can be lost very easily and cause alterations such as fever.

Fever is a disturbance of the body's "thermostat," located in the hypothalamus, which leads to an increase in body temperature above the normal value.

These can be caused by:

• bacterial infectious diseases
• cerebral lesions
• Heat blows.

Bacterial Infectious Diseases

This is the case of bacteria that generate toxins that affect the hypothalamus, raising the thermostat. This affects the heat gain mechanisms, which are activated by chemical compounds called pyrogens.

Brain Injuries

Practicing brain surgery can inadvertently damage the hypothalamus, which controls body temperature. Sometimes the hypothalamus during pregnancy may not fully develop which contributes to a total or partial loss of sensitivity to temperature changes in the skin, these cases usually occur in 1 in 16,000 people and can be moderate to notorious.

This alteration also occurs due to tumors that grow in the brain, specifically in the hypothalamus, so that the body's thermostat is damaged, triggering serious feverish states. Any injury to this important structure can alter the control of body temperature, causing permanent fever.

Heat strokes

The limit of heat that human beings can tolerate is related to the environmental humidity. Thus, if the environment is dry and windy, convection currents can be generated that cool the body.

On the contrary, if the environmental humidity is high, convection currents are not produced and sweating decreases, the body begins to absorb heat and a state of hyperthermia is generated. This situation is further exacerbated if the body is submerged in hot water. In humans, acclimatization to high temperatures occurs, so our body temperature can equalize that of the environment without danger of death. The physical changes that lead to this acclimatization are: increased sweating, increased plasma volume, and decreased salt loss through sweat.

Reactions in humans to different body temperatures

Calor

• 37 °C: normal body temperature (taken in oral cavity). It can range from 36.5 to 37.5 °C
• 38 °C: a light sweat is produced with unpleasant sensation and mild dizziness.
• 39 °C (pyrexia): there is abundant sweat accompanied by blond, with tachycardias and dyspnea. Exhaustion may arise. Epileptics and children may suffer seizures at this point.
• 40 °C: dizziness, dizziness, dehydration, weakness, nausea, vomiting, headache and deep sweat.
• 41 °C (medical uprising): all of the above, there can also be confusion, hallucinations, delusions and drowsiness.
• 42 °C: In addition to the above, the subject may have paleness or blond. It can reach the coma, with hyper or hypotension and a large tachycardia.
• 43 °C: usually here death occurs or leaves various brain damages as sequelae, accompanied by continuous seizures and shock. There may be a cardiorespiratory strike.
• 44 °C: Death is almost safe; however, there are people who have come to endure 46 °C.
• 47 °C or higher: there is no data from people who have experienced this temperature.

Cold

• 35 °C: is called hypothermia when it is below 35 °C. There is intense tremor, numbness and bluish/grey coloring of the skin.
• 34 °C: severe tremor, loss of movement capacity in fingers, cyanosis and confusion. There may be changes in behavior.
• 33 °C: moderate confusion, numbness, repflexia, progressive loss of tremor, bradycardia, dyspnea. The subject does not react to certain stimuli.
• 32 °C (medical emergency): hallucinations, delirium, great confusion, very adored could even reach the coma. The tremor disappears, the subject can even believe his temperature is normal. There is snatch, or the reflexes are very weak.
• 31 °C: There is a coma, it is very rare that you are aware. Absence of reflexes, severe bradycardia. There is a possibility that serious heart problems arise.
• 28 °C: severe heart alterations, can be accompanied by apnea and even appear to be dead.
• 26-24 °C or lower: here death usually occurs by cardiorespiratory alterations, however, some patients have survived at low temperatures appearing to be dead at temperatures below 14 °C.
• 13 °C or lower: There is no data from people who have endured these temperatures.

This process of heat loss is normal in some people to the point of appearing dead, cold skin, cold body, and pale skin is normal and is known as winter colds; the same characteristics but with darker skin is known as summer colds or whiter skin is known as winter colds.

Normal temperature values:

Age	Celsius Grades
Newborn	36.1-37.7
Lactante	37.2
Children 2-8 years	37
Adult	36-37

Thermoregulation in animals

The biochemical and physiological processes of an animal depend to a greater or lesser degree of body temperature. In fact, an increase of 10 ° C duplicates or triples the rate of most enzyme chemical reactions. This effect is known as factor Q _{10 Q 10}

is a ratio that is calculated by dividing the rate of a reaction or physiological process at a certain temperature (r <subs lower 10 ° C temperature (r _t-10 ). On the other hand, the exothermic chemical reactions of the cell produce heat. One of the most important factors in determining numerous physiological and ecological processes is undoubtedly the body temperature. That is why the regulation of body temperature or thermoregulation is a vital aspect for the homeostasis of an animal, from the molecular level to that of organism and many species have adaptations to regulate body temperature in the optimal range.

Factors involved in thermoregulation

An organism is in thermal equilibrium with the environment when the heat gain ratio = heat loss is maintained. The heat gain rate must balance the heat loss rate to reach a steady state, otherwise the body temperature will increase or decrease based on the magnitude of the gain or loss of heat. The mechanisms of gain and loss of heat include the production of internal heat (metabolism: only heat gain) and the exchange of heat between the body and the environment that takes place by the three means described above, conduction (including convection), radiation, radiation and evaporation (the latter always produces heat loss). Body temperature can be described by equation:

and - e _ev

where

• S is the thermal energy retained by the organism,
• M is the heat produced in metabolic reactions,
• CD is the heat driving between the body and the substrate,
• CV is heat convection with air
• EV is the evaporation of water.

Positive signs represent heat gain by the body and negative signs represent heat loss. Animals can gain or lose heat through radiation, conduction and convection and the flow of energy will go from higher to lower energy. The division of the thermal budget in several components allows to quantify and compare the different strategies that the organisms use to deal with their thermal environment.

Animal Thermoregulatory Strategies

Two dichotomies have been proposed, one is based on the type of temperature regulation and the other on the heat source. Thus, the terms poikilotherm and homeotherm were applied to animals according to the constancy of their body temperature. Poikilotherms (thermoconformists) are animals whose body temperature is variable depending on the ambient temperature. On the other hand, homeothermic animals (thermoregulators) maintain constant body temperature despite large variations in environmental temperature. Currently, biologists prefer to use the terms endothermic and ectothermic, proposed by Cowles in 1962, which refer to an animal's heat-generating source. An ectotherm is an animal whose temperature is controlled by an external source of heat and its ability to generate metabolic heat is negligible. In contrast, in endotherms the main source of heat production is internal and depends on metabolic activity. Endothermy contrasted with ectothermy is an energetically expensive strategy. The categories endothermic-ectothermic and homeothermic-poikilothermic are independent. Homeothermic organisms can be endothermic or ectothermic. In the first case there are birds and mammals and in the second a great variety of species from invertebrates to reptiles. Thus, some ectotherms under certain circumstances can maintain a constant body temperature. For example, deep-sea fish, since they inhabit a very stable thermal environment, maintain a fairly constant body temperature. Many insects and lizards control body temperature through behavioral strategies.

On the other hand, the subterranean rodent Heterocephalus glaber (naked mole rat) is a poikilothermic endothermic mammal. Another thermoregulatory strategy is that of heterothermal thermoregulators, some birds and mammals can maintain a body temperature different from the ambient temperature in a specific region of their body (regional heterotherms) or during a certain period of time (temporal heterotherms).

Thermoregulation mechanisms

It is common to classify temperature regulation mechanisms into behavioral and physiological, it should be noted, however, that the classification is somewhat arbitrary, and that some mechanisms can fit into both categories:

• Behavioral mechanisms: changes in body position, of daily and seasonal activity patterns, selection of microclimates.
• physiological mechanisms: changes in the metabolic generation of heat, modifications in vascular behavior.

<Il

In animals ectoterms the regulation of body temperature depends on the ability to regulate heat exchange with the environment. This capacity is related to the thermal conductance of the tegument, that is, with the capacity of the tegument to transfer heat between the animal and the environment. These organisms acquire and maintain body temperature through both behavioral and physiological mechanisms.

Thus, for example, insects belonging to a wide variety of taxa resort to behavior to regulate their temperature. Among vertebrates, in reptiles behavior plays a preponderant role in thermoregulation.

Some ectoterms respond to the temporal and spatial variability of ambient temperature, controlling direct and indirect solar radiation, convection and driving, through movements between areas with high and low temperatures, they also adjust the heat exchange to heat to through driving and convection by postural changes of the body. Thus, through the behavior they maintain the body temperature close to the optimal metabolic performance temperature (ecritic or preferred temperature). For example, the Galapago del Bosque (Glyptemys insulpta) moves daily to the lights of the jungle to sunbathe and raise the body temperature but at night it returns to the water currents that remain warmer. It has been documented that the Matuasto (Phymaturus Flallifer) , through the day its posture changes, combining two modes of heat transmission, conduction with the substrate (tigmothermia) and gain by solar radiation (Heliothermia).

Posture changes are also very common in insects. For example in butterflies the surface of the wings is used as an energy receiver and as a caloric conductor. In these butterflies, the opening angle of the wings with respect to the body regulates heat absorption. In addition to solar energy, there are species that use other mechanisms to heat, such as perching on hot stones or substrates, or depending on the intensity of radiation exposing the obverse or the reverse of the wings according to the degree of melanism they present.

Among the physiological mechanisms, the dark-color change that controls the absorption of solar radiation can be cited. In this sense, melanic polymorphism between related species of lizards and insects could be related to thermoregulation. Thus, in the lizard, cordylus the melanic species are heated faster than the clear species that have the highest thermal reflectance. On the other hand, it has been documented that the individuals of the iguana of the desert, dipsosourus dorsalis , absorb 73% of the visible incident light when they have dark colorations (at low temperature), which decreases to a 58% when animals acquire lighter colorations.

<p The populations of colias differ in the degree of solar absorbance of the rear wing obverse and in the thickness of the scales of the ventral part of the thorax that correlate with the altitude. The species c. Philodice of low areas and with warmer temperatures has an absorbance of the wings and thickness of the scales less than the species c. Meadii of higher altitudes and colder temperatures. However, the most important thermoregulator mechanism is the regulation of peripheral blood flow. Many animals control the diameter of peripheral blood vessels contracting or relaxing the smooth muscles of the walls of these vessels. Through vasoconstriction the diameter of the blood vessel is reduced by decreasing heat exchange with the environment, on the contrary, through vasodilation, the diameter and heat exchange is increased with the environment.

The importance of physiological thermoregulation is very evident in the marine iguana of the Galapagos Islands, amblyrhynchus cristatus a species that lives in a warm equatorial habitat but that seeks its food in the cold waters of the Humboldt current. It can remain submerged for half an hour, reducing heart rate (bradycardia) and blood flow to surface tissues, which minimizes heat loss. When it comes out of the water, it heats up exposing its dark body to the tropical sun and supporting the ventral part of the body on the rocks warmed by the sun. Simultaneously increases heart rate and peripheral blood flow, circulating cold blood from the center of the body to the periphery. The increase in cutaneous blood flow increases conductance and accelerates heat absorption from the environment to the animal.

Exceptions to the general thermal response of ectotherms

Regional endothermy

Endothermy has evolved in two groups of fish, in teleost fish of the suborder Scombroidei (tuna, mackerel, swordfish) and in sharks of the family Lamnidae. The evolution of endothermy in these fish is remarkable since it is more difficult to maintain a temperature differential with the environment for an aquatic organism than for a terrestrial one, particularly if respiration occurs through gills. Since the temperature of the heart and gills fluctuate with changes in environmental temperature, they are called regional endotherms.

To reduce convective and conductive heat exchange requires a large body size coupled with countercurrent heat exchange, this adaptation allows heat to pass from venous blood that has been heated by muscle metabolism into the blood cold blood. Since heat is exchanged between blood vessels carrying blood in opposite directions, heat is maintained within the muscle mass allowing the fish to be considerably warmer than the water.

For the generation of heat, a tissue with high oxidative capacity is required, in tuna and basking sharks the red muscles responsible for swimming fulfill this function. Barbara Block and collaborators have suggested that the main selection pressure in the evolution of heat retention mechanisms in tunas, swordfish, and lamnid sharks has been a greater efficiency in exploiting cold environments (deeper bodies of water or found at higher latitudes).

Facultative endothermy

Some species of various orders of insects, such as Odonata (dragonflies), Diptera (flies), Hymenoptera (bees), Coleoptera (beetles) and Lepidoptera (butterflies) have the ability to raise their body temperature for certain periods of time. Before flight, these insects produce heat by isometric contraction, often called tremor, of the thoracic flight muscles. In relation to the conservation of heat in the thoracic region, they have a countercurrent heat exchanger system between the cold hemolymph that enters the thorax from the abdomen and the warm hemolymph that leaves the thorax towards the abdomen. Using this mechanism, the moth of the genus Eupsilia maintains its thorax temperature at 30 °C during flight when the ambient temperature is below 0 °C. Active heat loss from the thorax to the abdomen prevents overheating during flight when high muscle temperatures are generated.

Thermoregulation is a key factor in the foraging energetics of flower-visiting bees and bumblebees. The higher the muscle temperature, the more flowers a bee or bumblebee can visit per unit time. Among reptiles, facultative endothermy from muscle tremor has been well documented in female Indian pythons, Python molurus. .

After oviposition, the female python wraps itself around the eggs and performs rhythmic contractile movements of the musculature, increasing its temperature and therefore that of the egg mass.

Inertial homeothermy

Body size and shape affect thermoregulation. A large animal has a smaller surface area to volume ratio than a small one. Therefore, it has higher thermal inertia and will heat up and cool down more slowly. Some large ectotherms, especially in hot climates, maintain relatively constant body temperatures.

Among the inertial homeothermic ectotherms are the giant tortoises of the Galápagos (Chelonoides nigra) and Aldabra (Geochelone gigantea).

Medium and large dinosaur species (over 500 kg) would have been inertial homeotherms little affected by diurnal temperature fluctuations

Thermoregulation in Endotherms

In endothermic homeothermy, the maintenance of constant temperature implies variations in the metabolic rate, in the temperature differential between the organism and the environment, and in thermal conductance.

Metabolic responses to temperature

In endotherms, the mechanisms of regulation of body temperature and metabolic rate vary specifically with environmental temperature, although the ranges vary between different taxonomic groups, the basic pattern is maintained:

• Thermoonutral zone: range of environmental temperatures in which metabolic heat production is not affected by ambient temperature. In this range the body temperature is kept constant by passive changes in thermal conduct through feather/pel adjustments, posture adjustments, and peripheral vasoconstriction or vasodilation.
• Lower critical temperature: below this temperature, to keep the body temperature constant an endotermo should increase the production of metabolic heat, the production of heat originates from the non-tyrant thermogenesis and from the tiritant thermogenesis.
• Higher Critical Temperature: Temperature above which an endotermo regulates body temperature by dissipating heat by evaporation (e.g. jade, breath)

Thermal regulation mechanisms

Mechanisms of regulation of thermal conductance

The most prominent mechanisms are:

• Postal adjustments: alter the thermal conductance (Conductance = 1/Insulation) of the animal's surface by modifying the surface-volume ratio (S/V). For example, by curling an animal decreases the S/V ratio by reducing heat loss. On the other hand, exposing large areas of the body to the air increases body heat dissipation.
• Adjustments of the fur or the plumage: Pyloerección de pelo (erección del pelo) and ptiloerección (erección de las feathers) increase the air layer between the tegumento and the environment by increasing isolation. On the contrary, pylodepression and ptilodepression facilitate heat dissipation.
• Adjustments of the peripheral circulation: the vasoconstriction of the peripheral vessels reduces the loss of heat by the tegument while vasodilation increases it. The insulating value of the thick layer of fat that the aquatic mammals have can be regulated by the capillary blood circulation. When the blood circulates on the fat layer it reduces insulation and increases heat dissipation, instead when circulating below the fat layer increases insulation.

Heat production mechanisms

The mechanisms that produce heat are:

• Activity: The heat produced as a by-product of the activity represents a potentially useful energy source for animals that thermorrhage in cold environments. For example, the heat produced during physical activity provides much of the thermogulatory requirements of small wild birds. In this sense, the heat produced during the foraging activity contributes 42% to thermoregulation of a small passiform, the ball (Auriparus flaviceps).
• Postpandrial thermogenesis: When an animal in fasts consumes food, the metabolic rate rapidly increases over the rest level. Postpandrial heat production is a real substitute for thermogenesis of regulation in many species, both birds and mammals.

On the other hand, birds and mammals have evolved thermogenic mechanisms specialized in heat production for thermoregulation. These mechanisms include shivering thermogenesis and non -revritant thermogenesis:

• Thermogenesis tiritante: It is called thermogenesis tiritante to involuntary isometric contraction of skeletal muscles, does not involve voluntary movements or external work. To tiritar, virtually all chemical energy is transformed into heat that is transferred to the center of the body while the vasoconstriction of the peripheral vessels reduces heat loss through the tegument.
• Non-tyritant thermogenesis: includes heat production by processes that do not involve skeletal muscle contractions. In small body-sized endoters, and in general in calves, in response to exposure to cold below the lower critical point, heat is produced by non-tyrant thermogenesis. In placental mammals, the main effector organ is the parda fat that is catabolized to produce heat and not ATP. This fat is located in the cervical and chest region, near the central nervous system and the heart.

Heat dissipation mechanisms

Evaporative cooling

To lose heat, above the upper critical temperature, endothermic homeotherms use evaporative cooling (540 calories of energy are dissipated for every gram of water evaporated) and vasodilation of peripheral blood vessels. the animal increases the metabolic rate to increase the rate of evaporation, the amount of heat removed by evaporation exceeds the amount of heat produced by physiological processes that accelerate evaporation. Five active evaporative cooling mechanisms are known:

• Jadeo: In birds and mammals, it includes the evaporation of water from the wet and hot membranes that tap the respiratory tract.
• Gutural movements: in birds, understands the rapid vibration of the floor of the mouth cavity, often used together with the jade.
• Breathing: in mammals, although some species do not sweat, it includes the skin evaporation of the fluid from the sweat glands.
• Saliva spread: some rodents and marsupials spread saliva over their limbs, chest or other body surfaces, evaporation takes place on the surface of the coat.
  

  Kangaroo spreading saliva over its previous members.
• Urohidrosis: some birds (storm, vulture), water evaporation of the urine and feces that the bird downloaded on the squamous part of the legs.

Tolerance to hyperthermia

Large ungulates native to the desert allow their body temperatures to rise during the hot part of the day by losing stored heat at night by non-evaporative methods. Fluctuations in body temperature exceeding 2 °C are considered to be used by desert animals to minimize water loss. The brain is kept cooler by a countercurrent exchange system known as the carotid rete mirabile. For example, the dehydrated camel in summer allows its body temperature to drop overnight to 34-35°C and then rise above 40°C during the day. The beisa oryx (Oryx beisa) and Grant's gazelle (Gazella granti) are able to withstand body temperatures of up to 45°C. Ratite birds (ostrich, rhea, emu, cassowary) have a similar strategy depending on adaptive hyperthermia and countercurrent cooling.

Facultative hypothermia

At low ambient temperatures, endothermia is associated with large energy costs, particularly in mammals and small birds with high surface area-to-volume ratios. During facultative hypothermia, basal metabolic rate and body temperature are reduced. It is a regulated state and allows great energy savings. These heterothermic responses include daily torpor and hibernation, which should not be confused with compelled hypothermia resulting from the inability to maintain physiologic thermoregulation. Drowsiness is a physiological state of little activity, with decreased metabolic rate and generally on a daily basis. For example, hummingbirds are among the smallest endothermic vertebrates with high specific weight metabolic rates. As an energy-saving strategy, they present nocturnal torpor. Hibernation is a state of regulated hypothermia for days, weeks, or months, which allows animals to conserve their energy in the face of low environmental temperatures and/or food shortages. Examples of animals that hibernate are marmots, bats, dormouse, and hamsters.

Thermoregulation in plants

More than two hundred years ago Jean-Baptiste Lamarck observed that, during the flowering sequence, the flowers of the European ring, probably Arum italicum, became hot. Since then, botanists have recorded endothermic heating and even thermoregulation in the flowers, inflorescences, or strobila of various families of primitive spermatophyte plants. These plants have large, fleshy flowers that are often pollinated by beetles, bees, or flies. Examples occur in monocots (for example, Araceae), dicots (for example, Nelumbonaceae), and gymnosperms (for example, Araceae). Cycadaceae). Endogenous heat generation is usually associated with mammals and birds, however, it also occurs in some flowers and is an adaptation that increases the rate of pollination through the release of chemical attractants, which provides a reward of heat to pollinating insects, or that may be associated with floral development or protection from low temperatures.

Based on the temperature transition patterns of their tissues, thermogenic activities have been categorized into two groups: transient thermogenesis and homeothermic thermogenesis. Transient heat production has been observed in most thermogenic plants studied, in these plants the temperature of the thermogenic organ increases discontinuously during one or two events. This type of thermogenesis has been well studied in species of the genus Arum. In plants belonging to the second group, the thermogenic organ maintains, with remarkable precision, constant temperature for several days. This type of thermogenic heat production has been reported for a limited number of species, such as the sacred lotus (Nelumbo nucifera) and skunk cabbage (Symplocarpus sp). For example, it has been observed that at the peak of thermogenesis transiently the spadix temperature of Arum maculatum can be higher than 30 °C when the ambient temperature is 7 to 22 °C. On the other hand, it has been observed that skunk cabbage, Symplocarpus foetidus, can thermoregulate for two or more weeks. When the ambient temperature varies between -15 °C to 15 °C, the temperature of the spadix increases by 15 to 35 °C above ambient temperature. In its natural environment the heat produced is sufficient to melt the surrounding snow. The inflorescence of the philodendron, Philodendron selloum, can heat up to above 40 °C at ambient temperatures close to freezing. Since most Araceae species are tropical, escape from freezing by thermogenesis would be a physiological exaptation of a process that originally evolved in response to selective pressures other than freezing stress.

The heat production of thermogenic plants has been attributed to a large increase in alternative oxidase (AOX) expression. The AOX enzyme acts as an alternative terminal oxidase in the mitochondrial respiratory chain, where it reduces molecular oxygen to water, bypassing two energy conservation centers and thus not generating the proton gradient that would lead to ATP synthesis.. Consequently, the energy that is conserved as ATP in the classical pathway is lost as heat when electrons pass through the alternative pathway. It has been suggested that, in general, heat production begins in the male flowers and then subsides. propagates throughout the inflorescence. This pattern would reflect the movement of a chemical signal, "calorigen", which could be salicylic acid. The physiological control mechanism of thermoregulation in plants is not yet known, but unlike what occurs in mammals and birds, the control would take place at a strictly biochemical or molecular level. In this regard, it has been found that in the sacred lotus, Nelumbo nucifera, there is a marked delay between the change in flower temperature and the compensatory response, this result would indicate that the temperature is regulated through a biochemical feedback mechanism and not by structural changes of enzymes or membranes.