THERMOREGULATION: PHYSIOLOGY, MECHANISMS, TYPES AND ALTERATIONS - BIOLOGY - 2025

The thermoregulation is the process that allows organisms regulate their body temperature, modulating heat loss and gain. In the animal kingdom there are different mechanisms for regulating temperature, both physiological and ethological.

Regulating body temperature is a basic activity for any living being, because the parameter is critical for the body's homeostasis and influences the functionality of enzymes and other proteins, the fluidity of the membrane, the flow of ions, among others..

Mammals are homeothermic and endothermic. Source: Alan Wilson

In its simplest form, thermoregulation networks are activated by means of a circuit that integrates the inputs of thermoreceptors located in the skin, in the viscera, in the brain, among others.

The main mechanisms for these hot or cold stimuli include cutaneous vasoconstriction, vasodilation, heat production (thermogenesis) and sweating. Other mechanisms include behaviors to promote or reduce heat loss.

Basics: heat and temperature

To talk about thermoregulation in animals, it is necessary to know the exact definition of terms that are often confusing among students.

Understanding the difference between heat and temperature is essential to understand the thermal regulation of animals. We will use inanimate bodies to illustrate the difference: let's think of two cubes of a metal, one is 10 times bigger than the other.

Each of these cubes is in a room at a temperature of 25 ° C. If we measure the temperature of each block, both will be at 25 ° C, although one is large and the other small.

Now, if we measure the amount of heat in each block, the result between the two will be different. To carry out this task we must move the blocks to a room with a temperature of absolute zero and quantify the amount of heat they give off. In this case, the heat content will be 10 times higher in the largest metal cube.

Temperature

Thanks to the previous example, we can conclude that the temperature is the same for both and independent of the amount of matter in each block. Temperature is measured as the speed or intensity of movement of the molecules.

In the biological literature, when the authors mention “body temperature” they refer to the temperature of the central and peripheral regions of the body. The temperature of the core regions reflects the temperature of the "deep" tissues of the body - brain, heart and liver.

The temperature of the peripheral regions, for its part, is influenced by the passage of blood to the skin and is measured on the skin of the hands and feet.

Hot

In contrast - and going back to the example of blocks - heat is different in both inert bodies and directly proportional to the amount of matter. It is a form of energy and depends on the number of atoms and molecules of the substance in question.

Types: thermal relationships between animals

In animal physiology, there are a number of terms and categories used to describe the thermal relationships between organisms. Each of these animal groups has special adaptations - physiological, anatomical, or anatomical - that help them keep their body temperature in a suitable range.

In everyday life, we call endothermic and homeothermic animals as "warm-blooded" and poikilothermic and ectothermic animals as "cold-blooded."

Endotherm and ectotherm

The first term is endothermy, used when the animal manages to warm itself by mediating the metabolic production of heat. The opposite concept is ectothermy, where the animal's temperature is dictated by the surrounding environment.

Some animals are incapable of being endothermic, because although they produce heat, they do not do it fast enough to retain it.

Poikilothermic and homeothermic

Another way to classify them is according to the thermoregulation of the animal. The term poikilotherm is used to refer to animals with varying body temperatures. In these cases, the body temperature is high in hot environments and low in cold environments.

A poikilothermic animal can self-regulate its temperature through behaviors. That is, by locating in areas with high solar radiation to increase the temperature or hide from said radiation to decrease it.

The terms poikilotherm and ectotherm refer to basically the same phenomenon. However, poikilotherm emphasizes the variability of body temperature, while ectotherm refers to the importance of environmental temperature in determining body temperature.

The opposite term for poikilotherm is homeothermic: thermoregulation by physiological means - and not only thanks to the display of behaviors. Most endothermic animals are capable of regulating their temperature.

Examples

Fishes

Fish are the perfect example of ectothermic and poikilothermic animals. In the case of these swimming vertebrates, their tissues do not produce heat through metabolic pathways and, furthermore, the temperature of the fish is determined by the temperature of the body of water where they swim.

Reptiles

Reptiles exhibit very marked behaviors that allow them to regulate (ethologically) their temperature. These animals seek out warm regions - such as perching on a hot stone - to increase the temperature. Otherwise, where they want to reduce it, they will seek to hide from radiation.

Birds and mammals

Mammals and birds are examples of endothermic and homeothermic animals. They metabolically produce your body temperature and regulate it physiologically. Some insects also exhibit this physiological pattern.

The ability to regulate their temperature gave these two animal lineages an advantage over their poikilothermic counterparts, as they can establish thermal equilibrium in their cells and organs. This led to the processes of nutrition, metabolism and excretion being more robust and efficient.

Human beings, for example, maintain their temperature at 37 ° C, within a fairly narrow range - between 33.2 and 38.2 ° C. The maintenance of this parameter is totally critical for the survival of the species and mediates a series of physiological processes in the body.

Spatial and temporal alternation of endothermy and ectothermy

The distinction between these four categories is often confused when we examine cases of animals that are able to alternate between the categories, either spatially or temporally.

Temporal variation in thermal regulation can be exemplified with mammals experiencing periods of hibernation. These animals are generally homeothermic during the times of the year when they are not hibernating and during hibernation they are not able to regulate their body temperature.

Spatial variation occurs when the animal differentially regulates temperature in body regions. Bumblebees and other insects can regulate the temperature of their thoracic segments and are unable to regulate the rest of the regions. This differential regulation condition is called heterothermy.

Physiology of thermoregulation

Like any system, the physiological regulation of body temperature requires the presence of an afferent system, a control center, and an efferent system.

The first system, the afferent, is responsible for capturing information through skin receptors. Subsequently, the information is transmitted to the thermoregulatory center through the neural pathway through the blood.

Under normal conditions, the body's organs that generate heat are the heart and liver. When the body is doing physical work (exercise), skeletal muscle is also a heat-generating structure.

The hypothalamus is the thermoregulatory center and the tasks are divided into heat loss and heat gain. The functional zone to mediate the maintenance of heat is located in the posterior zone of the hypothalamus, while the loss is mediated by the anterior region. This organ works like a thermostat.

Control of the system occurs in two ways: positive and negative, mediated by the cortex of the brain. Effector responses are of the behavioral type or mediated by the autonomic nervous system. These two mechanisms will be studied later.

Thermoregulatory mechanisms

Physiological mechanisms

The mechanisms for regulating temperature vary between the type of stimulus received, that is, whether it is an increase or a decrease in temperature. So we will use this parameter to establish a classification of the mechanisms:

Regulation for high temperatures

To achieve the regulation of body temperature in the face of heat stimuli, the body must promote its loss. There are several mechanisms:

Vasodilation

In humans, one of the most striking characteristics of skin circulation is the wide range of blood vessels it has. Blood circulation through the skin has the property of varying enormously depending on environmental conditions and modifying from high to low blood flow.

The ability of vasodilation is crucial in the thermoregulation of individuals. The elevated blood flow during periods of increased temperature allows the body to increase the transmission of heat, from the core of the body to the surface of the skin, to be finally dissipated.

When blood flow is increased, the cutaneous blood volume in turn increases. Thus, a greater amount of blood is transferred from the core of the body to the surface of the skin, where heat transfer occurs. The now colder blood is transferred back to the core or center of the body.

Sweat

Along with vasodilation, sweat production is crucial for thermoregulation as it helps dissipate excess heat. In fact, the production and subsequent evaporation of sweat are the body's main mechanisms for losing heat. They also work during physical activity.

Sweat is a fluid produced by sweat glands called eccrine, distributed throughout the body in a significant density. The evaporation of sweat manages to transfer heat from the body to the environment as water vapor.

Regulation for low temperatures

In contrast to the mechanisms mentioned in the previous section, in situations of temperature drop, the body must promote the conservation and production of heat in the following way:

Vasoconstriction

This system follows the opposite logic described in vasodilation, so we will not elaborate much on the explanation. The cold stimulates contraction of the cutaneous vessels, thus avoiding the dissipation of heat.

Piloerection

Have you ever wondered why “goose bumps” appear when we are in front of low temperatures? It is a mechanism to prevent heat loss called piloerection. However, as humans have relatively little hair on our bodies, it is considered an ineffective and rudimentary system.

When the elevation of each hair occurs, the layer of air that comes into contact with the skin is increased, which decreases the convection of the air. This reduces heat loss.

Heat production

The most intuitive way to counteract low temperatures is by producing heat. This can occur in two ways: by shivering and non-shivering thermogenesis.

In the first case, the body produces rapid and involuntary muscle contractions (that's why you shiver when you're cold) that lead to the production of heat. Shivering production is expensive - energetically speaking - so the body will fall back on it if the aforementioned systems fail.

The second mechanism is led by a tissue called brown fat (or brown adipose tissue, in English literature it is usually summarized under the acronym of BAT for brown adipose tissue).

This system is responsible for decoupling energy production in metabolism: instead of forming ATP, it leads to the production of heat. It is a particularly important mechanism in children and small mammals, although more recent evidence has noted that it is also relevant in adults.

Ethological mechanisms

Ethological mechanisms consist of all the behaviors that animals exhibit to regulate their temperature. As we mentioned in the example of reptiles, organisms can be placed in the right environment to promote or prevent heat loss.

Different parts of the brain are involved in processing this response. In humans these behaviors are effective, although they are not finely regulated like the physiological ones.

Thermoregulation disorders

The body experiences small and delicate temperature changes throughout the day, depending on some variables, such as the circadian rhythm, the hormonal cycle, among other physiological aspects.

As we mentioned, body temperature orchestrates a huge range of physiological processes and its loss of regulation can lead to devastating conditions within the affected organism.

Both thermal extremes - both high and low - negatively affect organisms. Very high temperatures, above 42 ° C in humans, have a very marked effect on proteins, promoting their denaturation. Also, DNA synthesis is affected. Organs and neurons are also damaged.

Similarly, temperatures below 27 ° C lead to severe hypothermia. Changes in neuromuscular, cardiovascular and respiratory activity have fatal consequences.

Multiple organs are affected when thermoregulation does not work the right way. These include the heart, brain, gastrointestinal tract, lungs, kidneys, and liver.

References

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