The body tightly regulates the body temperature through a process called thermoregulation, in which the body can maintain its temperature within certain boundaries, even when the surrounding temperature is very different. The core temperature of the body (i.e., at the head and trunk) remains steady at around 36.5 °C–37.5 °C (or 97.7 °F–99.5 °F). As you have learned, in the process of ATP production by cells throughout the body, approximately 60% of the energy produced is in the form of heat used to maintain body temperature.
EXAMPLE
Thermoregulation is an example of negative feedback.
Recall that the hypothalamus in the brain is the master switch that works as a thermostat to regulate the body’s core temperature. If the temperature is too high, the hypothalamus can initiate several processes to lower it. These include increasing the circulation of the blood to the surface of the body to allow for the dissipation of heat through the skin and initiation of sweating to allow the evaporation of water on the skin to cool its surface. Conversely, if the temperature falls below the set core temperature, the hypothalamus can initiate shivering to generate heat. The body uses more energy and generates more heat.
did you know
Why do hands and feet feel cold first when the temperature drops?
Superficial blood vessels in the limbs vasoconstrict to decrease blood flow and reduce heat loss. This is why your hands and feet are the first to feel cold and numb.
In addition, thyroid hormone will stimulate more energy use and heat production by cells throughout the body.
An environment is said to be thermoneutral when the body does not expend or release energy to maintain its core temperature. For a naked human, this is an ambient air temperature of around 84 °F. If the temperature is higher, for example, when wearing clothes, the body compensates with cooling mechanisms. The body loses heat through the mechanisms of heat exchange.
Hypothalamus Control of Thermoregulation - How the hypothalamus controls thermoregulation.
IN CONTEXT
Non-Shivering Thermogenesis: A Mechanism In Newborn Babies
Birth exposes newborns to a cooler environment in which they have to regulate their own body temperature. Newborns have a higher ratio of surface area to volume than adults. This means that their body has less volume throughout which to produce heat, and more surface area from which to lose heat. As a result, newborns produce heat more slowly and lose it more quickly. Newborns also have immature musculature that limits their ability to generate heat by shivering. Moreover, their nervous systems are underdeveloped, so they cannot quickly constrict superficial blood vessels in response to cold. They also have little subcutaneous fat for insulation. All of these factors make it harder for newborns to maintain their body temperature.
Newborns, however, do have a special method for generating heat: nonshivering thermogenesis, which involves brown adipose tissue, or brown fat, which is distributed over the back, chest, and shoulders. Brown fat differs from the more familiar white fat in several ways:
It is highly vascularized. This allows for faster delivery of oxygen, which leads to faster cellular respiration.
This type of fat contains large numbers of mitochondria and many small globules of fat instead of one large fat globule.
Brown fat mitochondria are able to uncouple oxidative phosphorylation so that as electrons move down the electron transport chain, the energy is not used to make ATP, but rather produces heat. This is done by a special protein in the inner mitochondrial membrane called thermogenin (or uncoupling protein, UCP). This protein opens a pore that allows an alternate path for protons ejected from the mitochondrial matrix during electron transport to flow back through the inner membrane. This basically “short circuits” the ATP synthase in the mitochondrial inner membrane.
step by step
The process is under the control of the sympathetic nervous system. Here’s how it works (please follow with the image below):
The newborn’s hypothalamus receives cold signals from thermoreceptors on its skin.
The hypothalamus sends signals to the brown fat through the sympathetic nervous system.
Norepinephrine from the sympathetic nerve binds to the membrane receptor on the surface of the cell.
This binding activates a cyclic adenosine monophosphate (cAMP) second messenger.
The cAMP activates a lipase inside the brown fat cell.
The lipase breaks down triglycerides into fatty acids, which cross into the mitochondrion.
The fatty acids open the UCP. In addition, the fatty acids are oxidized as fuel.
Electrons from the fatty acid oxidation move down the electron transport chain.
Protons are pumped from the matrix to the outside of the inner mitochondrial membrane and energy is stored in the proton difference across that membrane.
Most of the protons re-entering the matrix do so through the UCP rather than the ATP synthase, thereby short circuiting the ATP synthase. Therefore, very little ATP is made.
Consequently, most of the energy of the proton gradient across the inner membrane gets dissipated as heat.
Sympathetic nervous system control of nonshivering thermogenesis.
reflect
As the newborn grows, the brown fat breaks down. Therefore, children and adults do not have this mechanism to generate heat.
terms to know
Thermoregulation
The process of regulating the temperature of the body.
Thermoneutral
The external temperature at which the body does not expend any energy for thermoregulation, about 84 °F.
2. Mechanisms of Heat Exchange
When the environment is not thermoneutral, the body uses four biological mechanisms of heat exchange to maintain homeostasis: conduction, convection, radiation, and evaporation. Each of these mechanisms relies on the property of heat to flow from a higher concentration to a lower concentration; therefore, each of the mechanisms of heat exchange varies in rate according to the temperature and conditions of the environment.
Conduction is the transfer of heat by two objects that are in direct contact with one another. It occurs when the skin comes in contact with a cold or warm object.
EXAMPLE
When holding a glass of ice water, the heat from your skin will warm the glass and in turn melt the ice. Alternatively, on a cold day, you might warm up by wrapping your cold hands around a hot mug of coffee. Only about 3% of the body’s heat is lost through conduction.
Convection is the transfer of heat to the air surrounding the skin. The warmed air rises away from the body and is replaced by cooler air that is subsequently heated. Convection can also occur in water. When the water temperature is lower than the body’s temperature, the body loses heat by warming the water closest to the skin, which moves away to be replaced by cooler water. The convection currents created by the temperature changes continue to draw heat away from the body more quickly than the body can replace it, resulting in hypothermia. About 15% of the body’s heat is lost through convection.
reflect
Have you ever experienced “goosebumps”?
When it is cold, the muscles attached to the hairs on your skin contract and the hairs stand on end, which causes what are often referred to as goosebumps. This reduces convection and maintains a slightly warmer layer of air next to the skin.
Goosebumps help reduce convection to maintain a slightly warmer layer of air and prevent additional heat loss.
Radiation is the transfer of heat via infrared waves. This occurs between any two objects when their temperatures differ. A radiator can warm a room via radiant heat. On a sunny day, the radiation from the sun warms the skin. The same principle works from the body to the environment. About 60% of the heat lost by the body is lost through radiation.
Evaporation is the transfer of heat by the evaporation of water. Because it takes a great deal of energy for a water molecule to change from a liquid to a gas, evaporating water (in the form of sweat) takes with it a great deal of energy from the skin. At rest, about 20% of the heat lost by the body occurs through evaporation. However, the rate at which evaporation occurs depends on relative humidity—more sweat evaporates in lower-humidity environments.
did you know
Sweating is the primary means of cooling the body during exercise.
terms to know
Conduction
The transfer of heat through physical contact.
Convection
The transfer of heat between the skin and air or water.
Radiation
The transfer of heat via infrared waves.
Evaporation
The transfer of heat that occurs when water changes from a liquid to a gas.
2a. Behavioral Mechanisms of Thermoregulation
Besides the biological mechanisms to reduce or increase heat, humans have several behavioral mechanisms that allow them to regulate their body temperature.
IN CONTEXT
When the environment is cold, humans will often do the following to increase heat production or conserve heat loss:
Wear warm clothing
Drink hot liquids
Change posture - hunching over or wrapping arms around the chest to conserve heat
Exercise, such as by jumping up and down, to increase body heat
Seek shelter
Conserving Body Heat - A man is hunched over with his arms wrapped around his chest to prevent additional heat loss.
Conversely, when the environment is hot, many humans will choose to wear light-colored, loose clothing that reflects radiant energy. They may also seek a cool shelter or shade to reduce radiant energy.
Staying Cool - A woman in light-colored clothes sits in shade to cool off on a hot day.
3. Metabolic Rate
The metabolic rate is the amount of energy consumed minus the amount of energy expended by the body. The basal metabolic rate (BMR) describes the amount of daily energy expended by humans at rest, in a thermoneutral environment, while in the postabsorptive state. It measures how much energy the body needs for normal, basic, daily activity. About 70% of all daily energy expenditure comes from the basic functions of the organs in the body. Another 20% comes from physical activity, and the remaining 10% is necessary for body thermoregulation or temperature control. This rate will be higher if a person is more active or has more lean body mass.
Your own metabolic rate fluctuates throughout life. As you age, the BMR generally decreases as the percentage of muscle mass decreases. By modifying your diet and exercise regimen, you can increase both lean body mass and metabolic rate. Aging is known to decrease the metabolic rate by as much as 5% per year. Additionally, because males tend to have more lean muscle mass than females, their BMR is often higher; therefore, males tend to burn more calories than females do. Lastly, an individual’s inherent metabolic rate is a function of the proteins and enzymes derived from their genetic background. Thus, your genes play a big role in your metabolism. Nonetheless, each person’s body engages in the same overall metabolic processes.
There are many formulas that have been proposed to calculate BMR, and various factors affect the accuracy and limitations of different models. The Mifflin–St Jeor equation is considered among the most accurate formulas that have been proposed, and is as follows for males and females:
Male BMR = 10 × W + 6.25 × H − 5 × A + 5
Female BMR = 10 × W + 6.25 × H − 5 × A − 161
Where W represents weight (in kilograms), H represents height (in centimeters), and A represents age (in years).
try it
Directions: Using the equations above, what would your BMR be?
Remember to convert your weight to kilograms and your height to centimeters!
terms to know
Metabolic Rate
The amount of energy consumed minus the amount of energy expended by the body.
Basal Metabolic Rate (BMR)
The amount of energy expended by the body at rest.
summary
In this lesson, you learned about how the body maintains energy and heat balance by thermoregulation. You first reviewed thermoregulation, in which the body regulates its core temperature. You then explored the different biological mechanisms of heat exchange that the body uses to maintain homeostasis, as well as behavioral mechanisms of thermoregulation used by humans to maintain body temperature. Finally, you learned about the average amount of energy expended as a result of the body’s metabolic rate and basal metabolic rate.
Terms to Know
Basal Metabolic Rate (BMR)
The amount of energy expended by the body at rest.
Conduction
The transfer of heat through physical contact.
Convection
The transfer of heat between the skin and air or water.
Evaporation
The transfer of heat that occurs when water changes from a liquid to a gas.
Metabolic Rate
The amount of energy consumed minus the amount of energy expended by the body.
Radiation
The transfer of heat via infrared waves.
Thermoneutral
The external temperature at which the body does not expend any energy for thermoregulation, about 84 °F.
Thermoregulation
The process of regulating the temperature of the body.
