Thermoregulation - Pathophysiology, Mechanism, Functions

Thermoregulation is a mechanism by which mammals maintain body temperature with tightly controlled self-regulation independent of external temperatures. Temperature regulation is a type of homeostasis and a means of preserving a stable internal temperature in order to survive. Ectotherms are animals that depend on their external environment for body heat, while endotherms are animals that use thermoregulation to maintain a somewhat consistent internal body temperature even when their external environment changes. Humans and other mammals and birds are endotherms. Human beings have a normal core internal temperature of around 37 degrees Celsius (98.6 degrees Fahrenheit) measured most accurately via a rectal probe thermometer. This is the optimal temperature at which the human body’s systems function. Thermoregulation is crucial to human life; without thermoregulation, the human body would cease to function. Thermoregulation also plays an adaptive role in the body’s response to infectious pathogens. [rx][rx]

Mechanism

Thermoregulation has three mechanisms: afferent sensing, central control, and efferent responses. There are receptors for both heat and cold throughout the human body. Afferent sensing works through these receptors to determine if the body core temperature is too hold or cold. The hypothalamus is the central controller of thermoregulation. There is also an efferent behavioral component that responds to fluctuations in body temperature. For example, if a person is feeling too warm, the normal response is to remove an outer article of clothing. If a person is feeling too cold, they choose to wear more layers of clothing. Efferent responses also consist of automatic responses by the body to protect itself from extreme changes in temperature, such as sweating, vasodilation, vasoconstriction, and shivering.[rx][rx][rx]

Pathophysiology

When external environments are exceedingly warm, or a person is engaging in strenuous physical activity, the heat that is produced inside his or her body is typically transported to the blood. The blood then carries the heat through numerous capillaries that are located directly under the skin. Near the surface, the blood can lose heat. This cooled blood can then be transported back through the body to prevent the body temperature from becoming too high. Sweat is also a means by which the body cools itself down. Sweat is produced by glands and evaporation at the topmost skin layer (the epidermis) can release heat. This describes vaporization, one of the four mechanisms used to maintain core body temperature. Radiation occurs when the heat that is released from the body’s surface is moved into the surrounding air; convection occurs when cooler air surrounds the body’s surface, and conduction is when heat is transferred by direct contact with a cooler object (such as an ice pack). Hydration is paramount while exposed to environmental heat or during physical activity—not only to maintain adequate circulating intravascular fluid volume but also, to aid in conduction processes that cool the body down. When cold fluids are ingested, the heat is released into the fluid and excreted out of the body as sweat or urine.

While the infection is a central mechanism for raising the core body temperature, several peripheral mechanisms can also result in elevated body temperature. As previously discussed, multiple diseases with dysfunctional thermoregulatory mechanisms including small fiber and autonomic neuropathies, radiculopathies, and central autonomic disorders such as multiple system atrophy, Parkinson’s disease with autonomic dysfunction, and pure autonomic failure. Decreased cardiac function is also a notable risk factor for dysfunctional thermoregulation as the body depends on the heart to efficiently pump blood to the surface as a cooling mechanism. Without this mechanism, patients with impaired cardiac function are at risk of having heat-related illnesses, including those whose medications exert therapeutic effects through negative inotropic and chronotropic properties. [rx]

Volume depletion in conditions such as dehydration is another risk factor for dysfunctional thermoregulation. Without sufficient intravascular fluid, the body loses a mechanism for cooling as well as increased blood viscosity and the resultant strain on the cardiovascular system.

In contrast, hypothermia is defined as low internal body temperature, or a temperature less than 35 degrees Celsius (95 degrees Fahrenheit). It is usually caused by too much heat loss from cold weather exposure or cold water immersion. During cold water immersion, the diving reflex causes vasoconstriction in the visceral muscles as a mechanism to keep a person’s essential organs, like their heart and brain, supplied with blood and protected from hypoxia and ischemia.

There are two different types of hypothermia: primary and secondary. Primary hypothermia is when the cold environment is the direct cause and secondary hypothermia is when a patient’s illness causes hypothermia. Conduction, convection, and radiation also come into play with hypothermia; this is how the rate of heat loss is determined. Hypothermia decelerates all physiologic roles include metabolic rate, mental awareness, nerve conduction, neuromuscular reaction times, and both the cardiovascular and respiratory systems. As previously mentioned, the vasoconstriction caused by hypothermia induces renal dysfunction and cold diuresis due to the decreased levels of ADH. These decreased levels of antidiuretic hormone result in dilute urine. The vasoconstriction during hypothermia can mask concomitant hypovolemia. During rewarming, the subsequent vasodilation results in a redistribution of fluid which can cause cardiac arrest or abrupt shock, known as rewarming collapse. [rx][rx]

Homeostatic Process

Homeostatic processes ensure a constant internal environment by various mechanisms working in combination to maintain setpoints.

Key Points

Homeostasis is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change.

Homeostatic regulation is monitored and adjusted by the receptor, the command center, and the effector.

The receptor receives information based on the internal environment; the command center receives and processes the information; and the effector responds to the command center, opposing or enhancing the stimulus.

Key Terms

homeostasis: the ability of a system or living organism to adjust its internal environment to maintain a stable equilibrium
effector: any muscle, organ, etc. that can respond to a stimulus from a nerve

Homeostatic Process

The human organism consists of trillions of cells working together for the maintenance of the entire organism. While cells may perform very different functions, the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with everything that the cells need to survive (oxygen, glucose, mineral ions, waste removal, etc.) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis.

Homeostasis

Homeostasis, in a general sense, refers to stability, balance, or equilibrium. Physiologically, it is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change. Adjustment of physiological systems within the body is called homeostatic regulation, which involves three parts or mechanisms: (1) the receptor, (2) the control center, and (3) the effector.

The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. The effector responds to the commands of the control center by either opposing or enhancing the stimulus. This ongoing process continually works to restore and maintain homeostasis. For example, during body temperature regulation, temperature receptors in the skin communicate information to the brain (the control center) which signals the effectors: blood vessels and sweat glands in the skin. As the internal and external environment of the body is constantly changing, adjustments must be made continuously to stay at or near a specific value: the set point.

Purpose of Homeostasis

The ultimate goal of homeostasis is the maintenance of equilibrium around the set point. While there are normal fluctuations from the setpoint, the body’s systems will usually attempt to revert to it. A change in the internal or external environment (a stimulus) is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood glucose rises after a meal, adjustments are made to lower the blood glucose level by moving the nutrient into tissues in the command center that require it, or storing it for later use.

Blood glucose homeostasis: An example of how homeostasis is achieved by controlling blood sugar levels after a meal.

Homeostasis: Thermoregulation

Animals use different modes of thermoregulation processes to maintain homeostatic internal body temperatures.

Key Points

In response to varying body temperatures, processes such as enzyme production can be modified to acclimate to changes in the temperature.

Endotherms regulate their own internal body temperature, regardless of fluctuating external temperatures, while ectotherms rely on the external environment to regulate their internal body temperature.

Homeotherms maintain their body temperature within a narrow range, while poikilotherms can tolerate a wide variation in internal body temperature, usually because of environmental variation.

Heat can be exchanged between the environment and animals via radiation, evaporation, convection, or conduction processes.

Key Terms

ectotherm: An animal that relies on the external environment to regulate its internal body temperature.
endotherm: An animal that regulates its own internal body temperature through metabolic processes.
homeotherm: An animal that maintains a constant internal body temperature, usually within a narrow range of temperatures.
poikilotherm: An animal that varies its internal body temperature within a wide range of temperatures, usually as a result of variation in the environmental temperature.

Thermoregulation to Maintain Homeostasis

Internal thermoregulation contributes to an animal’s ability to maintain homeostasis within a certain range of temperatures. As internal body temperature rises, physiological processes are affected, such as enzyme activity. Although enzyme activity initially increases with temperature, enzymes begin to denature and lose their function at higher temperatures (around 40-50 C for mammals). As internal body temperature decreases below normal levels, hypothermia occurs and other physiological processes are affected. There are various thermoregulation mechanisms that animals use to regulate their internal body temperature.

Types of Thermoregulation (Ectothermy vs. Endothermy)

Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms create most of their heat via metabolic processes, and are colloquially referred to as “warm-blooded.” Ectotherms use external sources of temperature to regulate their body temperatures. Ectotherms are colloquially referred to as “cold-blooded” even though their body temperatures often stay within the same temperature ranges as warm-blooded animals.

Ectotherm

Ectotherm: The Common frog is an exotherm and regulates its body based on the temperature of the external environment.

An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Since ectotherms rely on environmental heat sources, they can operate at economical metabolic rates. Ectotherms usually live in environments in which temperatures are constant, such as the tropics or the ocean. Ectotherms have developed several behavioral thermoregulation mechanisms, such as basking in the sun to increase body temperature or seeking shade to decrease body temperature.

Endotherms

In contrast to ectotherms, endotherms regulate their own body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Heat is usually generated from the animal’s normal metabolism, but under conditions of excessive cold or low activity, an endotherm generates additional heat by shivering. Many endotherms have a larger number of mitochondria per cell than ectotherms. These mitochondria enable them to generate heat by increasing the rate at which they metabolize fats and sugars. However, endothermic animals must sustain their higher metabolism by eating more food more often. For example, a mouse (endotherm) must consume food every day to sustain high its metabolism, while a snake (ectotherm) may only eat once a month because its metabolism is much lower.

Homeothermy vs. Poikilothermy

Homeotherm vs. Poikilotherm: Sustained energy output of an endothermic animal (mammal) and an ectothermic animal (reptile) as a function of core temperature. In this scenario, the mammal is also a homeotherm because it maintains its internal body temperature in a very narrow range. The reptile is also a poikilotherm because it can withstand a large range of temperatures.

A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism that maintains thermal homeostasis. Poikilotherm’s internal temperature usually varies with the ambient environmental temperature, and many terrestrial ectotherms are poikilothermic. Poikilothermic animals include many species of fish, amphibians, and reptiles, as well as birds and mammals that lower their metabolism and body temperature as part of hibernation or torpor. Some ectotherms can also be homeotherms. For example, some species of tropical fish inhabit coral reefs that have such stable ambient temperatures that their internal temperature remains constant.

Means of Heat Transfer

Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction. Radiation is the emission of electromagnetic “heat” waves. Heat radiates from the sun and from dry skin the same manner. When a mammal sweats, evaporation removes heat from a surface with a liquid. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat can be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock.

Mechanisms for heat exchange: Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction.

Heat Conservation and Dissipation

Animals have processes that allow for heat conservation and dissipation in order to maintain a homeostatic internal body temperature.

Key Points

Heat conservation is characterized by the ability to ensure blood remains in the core by undergoing vasoconstriction, reducing blood flow to the periphery (also known as peripheral vasoconstriction).

Heat dissipation is characterized by the ability to undergo vasodilation which increases blood flow to the periphery, resulting in evaporative heat loss.

Endothermic animals are defined by their ability to utilize both vasoconstriction and vasodilation to maintain internal body temperature.

Ectothermic animals are defined by their change in behavior (lying in sunlight to warm up, hiding in shade to cool down) to regulate body temperature.

Key Terms

endotherm: a warm-blooded animal that maintains a constant body temperature
ectotherm: a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings

Heat Conservation and Dissipation

Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment, yet they maintain a constant, warm, body temperature. The arctic fox uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles create “goosebumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end; the loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.

Endotherms use their circulatory systems to help maintain body temperature. For example, vasodilation brings more blood and heat to the body’s surface, facilitating radiation and evaporative heat loss, which helps to cool the body. However, vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, thus, warming blood that returns to the heart. This is called a countercurrent heat exchange; it prevents cold venous blood from cooling the heart and other internal organs. This adaption, which can be shut down in some animals to prevent overheating the internal organs, is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations (as in dolphin flukes and elephant ears) can help cool endotherms when needed.

Control of body temperature: In endotherms, the circulatory system is used to help maintain body temperature, either by vasodilation or vasoconstriction.

Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. In cases of severe cold, a shivering reflex is activated that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.

Exothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from becoming too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity, such as the activity of bees to warm a hive to survive winter.

Organ Systems Involved

Multiple organs and body systems are affected when thermoregulation is impaired. During a heat-related illness, insufficient thermoregulation can result in multiple organ and system impairments. (Notice that many of these issues are interconnected.)

The heart experiences increased work as it increases both heart rate and cardiac output.
The circulatory system can experience intravascular volume depletion.
The brain can experience ischemia and/or edema.
The gastrointestinal tract is vulnerable to hemorrhage and infection as the intestinal mucosa becomes increasingly permeable.
The lungs become impaired if sustained hyperventilation, hyperpnea, and pulmonary vasodilation lead to ARDS.
Acute renal failure is an effect of intravascular volume depletion and impaired circulation.
Liver cells suffer because of fever, ischemia, and cytokine increase in the intestinal tract.
Various organs can become ischemic from microthrombi or DIC.
Electrolyte abnormalities are likely as well as hypoglycemia, metabolic acidosis, and respiratory alkalosis.

When body temperatures are severely decreased in hypothermia, the body’s systems are also adversely affected. The cardiovascular system is susceptible to dysrhythmias such as ventricular fibrillation. The central nervous system’s (CNS) electrical activity is noticeably diminished. Noncardiogenic pulmonary edema can occur as well as cold diuresis. Additionally, hypothermia causes preglomerular vasoconstriction which leads to decreased glomerular filtration rate (GFR) and decreased renal blood flow (RBF). [rx]

Function

The core body temperature is tightly controlled in a narrow range although slight changes in core body temperature occur every day, depending upon variables such as circadian rhythm and menses. When a person is unable to regulate his or her body temperature, various pathologies ensue. The human body has four different methods for maintaining core temperature: vaporization, radiation, convection, and conduction. To keep the body functioning, it must be at its ideal temperature. This requires sufficient intravascular volume and cardiovascular function as the body must be able to transport the rising internal heat to its surface for release. Elderly people are at increased risk for disorders of thermoregulation due to a generally decreased intravascular volume and decreased cardiac function.[rx][rx]

References