The biological systems of our planet, from single-celled creatures like bacteria all the way to complex mammals, are orders of magnitude more intricate and sophisticated, in their design and function, than the most advanced man-made technology. It is simply astounding that in a world where billions upon billions of variables are constantly in flux, life not only exists but thrives, in millions of complex forms. Each of these forms have numerous specialised adaptations that allow them to survive the harshness and unpredictability of the external environment and ensure their species’ continuity.
This ability of an organism to maintain its internal state relatively constant in the face of fluctuations in the external environment, is called homeostasis. Homeostasis is an umbrella term that pertains to several independent processes, aimed at maintaining different variables. Temperature is one such variable and a very important one at that.
Thermoregulation refers to an organism’s ability to maintain and regulate its internal temperature, irrespective of the surrounding thermal conditions. Organisms that have internal thermoregulatory mechanisms are called endotherms; Humans, along with mammals and birds, as examples of endothermic creatures. On the other hand, creatures than rely on the external environment for heat are called ectotherms; Amphibians and reptiles, like frogs, snakes and crocodiles are a few common examples of ectothermic organisms.
From an evolutionary point of view, so many organisms have such developed and advanced thermoregulatory mechanisms because it is physiologically desirable to maintain a high and stable internal temperature, in order to convert food to energy more efficiently. Nevertheless, there is a limit to this advantage, because if the body temperature gets too high, it leads to rapid depletion of energy reserves, because of the increased metabolic rate. it could even be fatal to the organism if it gets close to the upper lethal temperature.
Thermoregulation is an absolutely indispensable part of human life, without which we wouldn’t be able to survive the perpetual changes in external temperature. In fact, thermoregulation is just one component homeostasis, which is a crucial aspect of any organisms survival; homeostasis is the ability of an organism to maintain a state of internal equilibrium despite changes in the external environment.
Down the millennia, humans have evolved an extremely advanced and precise thermoregulatory apparatus that allows us to survive across a wide range of temperature conditions - be it the bitter cold of the tundra, or the scorching heat of the tropics. This amazing ability to maintain our body temperature so effectively is a major contributing factor to our success as a species and is one of the key reasons that has enabled us to survive several natural disasters, throughout our history.
In humans, body temperature is regulated through a system that consists of a centralised control unit and several mechanisms operating under its command. To understand how thermoregulation works in humans, it is important to understand how heat is generated and dissipated.
About 60% of all the energy generated by the body, is in the form of heat, in order to maintain the body’s core temperature steady around 36.5–37.5 °C [ 97.7–99.5 °F]. Although core temperature varies slightly from individual to individual and also experiences constant ups and downs, within any given day, it is tightly regulated and is predominantly maintained within this range.
When the body isn’t able to adequately regulate its temperature, it could be very harmful to the person and cause a variety of pathologies. Therefore, the body responds to changes in the external temperature by either increasing heat production and conserving heat or alternatively, promoting heat loss, depending on the conditions.
In the following sections, we’ll examine, in depth, the components, mechanisms and pathways involved in human thermoregulation. In essence, thermoregulation is a coordinated effort between the heat-producing and heat-dissipating systems of the body, which work together to make sure that the core temperature is stable.
The body has several means of generating heat, a process which is called thermogenesis.
When the ambient temperature is low, the body is challenged to produce excess heat in order to counter the cold and maintain its core temperature. To this end, it cranks up the metabolic rate, which increases ATP production in the mitochondria of all cells. This process is humorally mediated (i.e through endocrine hormones).
In humans, heat is primarily generated in the visceral organs - i.e the brain, liver, heart and the skeletal muscles. As mentioned previously, most of the energy produced by the body, though ATP production, is in the form of heat. The body expends a huge chunk of its energy budget to maintain a steady core temperature. This only serves to show how essential thermoregulation is for human survival and health.
The body’s heat producing mechanisms can broadly be separated into three categories - Exercise associated thermogenesis (EAT), non-exercise associated thermogenesis (NEAT) and diet induced thermogenesis (DIT).
One of the mechanisms that the body uses to generate heat and increase its temperature is shivering. When skeletal muscles shiver, the chemical energy of the body (ATP) is transformed into kinetic energy, causing almost all of it to be made available as heat.
In addition to shivering, the body also generates heat through a mechanism called “non-shivering thermogenesis” wherein triglycerides from brown adipose tissue is broken down, generating heat. This type of thermogenesis is regulated through the sympathetic nervous system and the endocrine system.
The body loses heat to the surroundings through four distinct pathways - evaporation, radiation, conduction and convection.
Evaporation refers to the loss of heat through the conversion of water to vapour. Water has a very high specific heat capacity, which is to say that it takes a lot of heat energy to convert each gram of water to water vapour. When we sweat, a lot of heat energy is released in order to evaporate the sweat; this huge loss of heat cools us down. Normally, about 20% of the body’s heat loss is through evaporation. During intense exercise, this number goes up to about 85%.
Radiation is the transfer of heat between objects as infrared waves. For example, we receive heat from the sun through radiation. 65% of the body’s heat loss occurs through radiation.
Conduction is the transfer of heat between two objects that are directly in contact with one another. For example, the cooling effect we experience when we jump into a swimming pool in hot weather is because of conduction. Usually, conduction accounts for not more than 2% of the body’s heat loss.
Convection is the loss of heat to the air surrounding the skin. This happens because hot air rises up and is replaced by cooler air, which them warms up and continues the cycle. Convection accounts for about 10-15% of the heat lost by the body.
In cold conditions, the body loses heat to the environment through conduction and radiation. However, In hot conditions, when the ambient temperature is high, the body gains heat through the same processes. Therefore, in hot weather, the body primarily relies on evaporation, through sweating, to lose heat. Especially during strenuous exercise, when tremendous amounts of heat are generated, the body relies, to a great extent, on heat loss through sweating.
When the ambient temperature is low, the body is challenged to produce excess heat in order to counter the cold and maintain the core temperature. To this end, it cranks up the metabolic rate, which results in more energy production and therefore, more heat.
Let’s try and understand the basic structure of the human thermoregulatory system. There are three basic components involved in this system: Afferent sensing, central control and efferent response.
1. Afferent Sensing - Afferent sensing refers to the sensory, data-collecting part of the thermoregulatory system. There are cutaneous thermoreceptors located all over the body. There are separate receptors to detect heat and cold. Information is gathered about the body’s thermal state, with respect to the surroundings, through these receptors. This information is essential in order to maintain thermal equilibrium and keep the core temperature stable.
2. Central Control - The hypothalamus is the centre for thermoregulatory control in humans. The hypothalamus (more specifically, the anterior hypothalamic nucleus and the pre optic area) is responsible for regulating the body’s temperature. The hypothalamus is located in the brain and can be understood as a link between the nervous and endocrine systems. It receives signals about the body’s thermal state and if the body’s temperature is either lower or higher than the hypothalamic “set-point”, it mediates hormonal responses that trigger adaptive changes aimed at normalising the body’s temperature.
3. Efferent Control - Efferent control refers to the body’s adaptive changes directed against thermal stress, either cold or hot. Efferent control is of two types - behavioural and autonomic. Behavioural control essentially refers to conscious actions taken in order to escape or mitigate thermal stress - eg. putting on a sweater, going into a warm room, turning on the air conditioning etc. In addition to this, the body uses a wide repertoire of autonomic (i.e involuntary/not-conscious) responses, such as sweating, vasomotor changes, shivering etc.
The process of thermoregulation involves a wide range of systems and structures in the body. Although the body’s temperature setting is centrally regulated, the actual changes that help the body adapt to changing temperatures happen all throughout, across different organ systems. In this section, we examine some of the key components that are involved the human thermoregulatory apparatus.
Not many may know it as such, but the skin is the human body’s largest organ. Measuring about 2m², the skin serves as the body’s outermost barrier against the outside world. In addition to being the body’s first line of defence against external threats, the skin also serves as the body’s sensor-unit, collecting data that is used in feedback loops. These feedback loops are integral to the process of homeostasis, as they constantly relay information to and from the environment, so that the body can maintain its internal state constant in the face of external changes.
The skin contains numerous cutaneous receptors that collect various kinds of information from the environment. Receptors are essentially nerve cells that are specialised to pick up a certain kind of information - e.g photoreceptors that are sensitive to light, mechanoreceptors that are sensitive to pressure etc. Thermal information is gathered by thermoreceptors. There are two kinds of thermoreceptors - heat receptors and cold receptors. Cold receptors are located, predominantly, on the surface of the skin. Heat receptors, on the other hand, are located further below, in the dermis. The information collected by these receptors is relayed to the brain, for central processing.
The hypothalamus is a small region located at the base of the brain which is responsible for connecting the nervous system with the endocrine system. It’s about the size of an almond in human beings. It is an integral part of the limbic system, which roughly speaking, is one of the more primitive parts of the human brain. It is responsible for various functions below the level of consciousness and outside executive control.
The hypothalamus is responsible for a wide range of functions, one of which is thermoregulation. The hypothalamus, as mentioned above, is the part of the human thermoregulatory system that is responsible for central control. It can be imagined as performing the role of a thermostat. It can also be seen as the CPU of the computer that is the body’s thermoregulatory system.
The pre optic area (POA) of the hypothalamus receives sensory input (i.e temperature data) and if the data indicates that the temperature of the body is going to be higher or lower than the set-point, due to environmental changes, it launches a series of adaptive changes aimed at normalising the core temperature. These changes are usually carried out by endocrine glands which function under the command of the hypothalamus, through it’s arsenal of releasing hormones (hormones which trigger the release of other hormones).
The thyroid gland is one of the main players in the endocrine system of the body. It is a key component of the neuro-endocrine complex that controls the overall functioning of the body, across the organ systems of the body. The thyroid gland produces a hormone called thyroxine (T4), which in its active form, triiodothyronine (T3), has a cascade of metabolism-spiking effects on the body.
Broadly speaking, T3 increases the body’s metabolic rate which is to say that it triggers more energy production, resulting in increased heat production, among other effects. The thyroid gland is one of the many avenues through which the hypothalamus directs its thermoregulatory activity. When a person suffers from hypothyroidism, they tend to feel very cold, especially in their hands and feet. Likewise, in people with hyperthyroidism, heat intolerance is one of the major symptoms.
The release of thyroid hormones is mediated by a pituitary hormone called TSH which in turn is regulated by a hormone called TRH that is secreted by the hypothalamus.
Both the Central Nervous System (CNS) and the Autonomic Nervous System (ANS) are involved in thermoregulation. For the most part, one of the earlier sections about the hypothalamus covers the role of the CNS in thermoregulation.
The ANS plays a huge role in the third arm of thermoregulation, i.e efferent control. The autonomic nervous system has two components - sympathetic and parasympathetic; In cold conditions, the sympathetic nervous system is activated; It responds by increasing the heart rate and constricting the blood vessels - changes which reduce heat loss. Conversely, in hot conditions, the parasympathetic nervous system dilates peripheral blood vessels, in a bid to increase the volume of blood at the periphery, which in turn promotes heat loss and decreases the temperature.
The organs of the body also play a significant role in thermogenesis. Organs such as the liver and the brain are very metabolically active; i.e cellular processes are carried out at a very high rate; This makes them vital seats of heat production in the body. For instance, when the body experiences cold, adrenaline is released which increases the production of heat in the liver. The liver being an incredibly large and metabolically active organ, is especially vital when it comes to thermogenesis.
In addition to the liver and the brain, the heart, along with the circulatory system, also plays a key role in thermoregulation.
Blood plays an integral role in maintaining body temperature. It is responsible for distributing heat throughout the body. When body temperature increases sharply, during vigorous exercise for example, the heat generated is transferred to the blood which dissipates it outside the body, as it flows through the peripheral vessels. On the other hand, when the body is aiming for heat conservation (eg. in cold weather), blood is redirected away from the periphery and to the visceral organs in order to keep them warm and maintain core temperature.