What term describes the transfer of heat through the circulation or movement of a liquid or a gas?

By the end of this section, you will be able to:

• Describe how the body regulates temperature
• Explain the significance of the metabolic rate

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 remains steady at around 36.5–37.5 °C (or 97.7–99.5 °F). In the process of ATP production by cells throughout the body, approximately 60 percent of the energy produced is in the form of heat used to maintain body temperature. Thermoregulation is an example of negative feedback.

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 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. 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.

Figure 1. Click for a larger image. The hypothalamus controls thermoregulation.

Mechanisms of Heat Exchange

When the environment is not thermoneutral, the body uses four 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. For 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 percent 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 hyperthermia. About 15 percent of the body’s heat is lost through convection.
• 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 percent 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. However, the rate at which evaporation occurs depends on relative humidity—more sweat evaporates in lower humidity environments. Sweating is the primary means of cooling the body during exercise, whereas at rest, about 20 percent of the heat lost by the body occurs through evaporation.

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 neutrally temperate environment, while in the postabsorptive state. It measures how much energy the body needs for normal, basic, daily activity. About 70 percent of all daily energy expenditure comes from the basic functions of the organs in the body. Another 20 percent comes from physical activity, and the remaining 10 percent 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. As you age, the BMR generally decreases as the percentage of less lean muscle mass decreases.

Chapter Review

Some of the energy from the food that is ingested is used to maintain the core temperature of the body. Most of the energy derived from the food is released as heat. The core temperature is kept around 36.5–37.5 °C (97.7–99.5 °F). This is tightly regulated by the hypothalamus in the brain, which senses changes in the core temperature and operates like a thermostat to increase sweating or shivering, or inducing other mechanisms to return the temperature to its normal range. The body can also gain or lose heat through mechanisms of heat exchange. Conduction transfers heat from one object to another through physical contact. Convection transfers heat to air or water. Radiation transfers heat via infrared radiation. Evaporation transfers heat as water changes state from a liquid to a gas.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

1. How does vasoconstriction help increase the core temperature of the body?
2. How can the ingestion of food increase the body temperature?

Glossary

basal metabolic rate (BMR): amount of energy expended by the body at rest

conduction: transfer of heat through physical contact

convection: transfer of heat between the skin and air or water

evaporation: transfer of heat that occurs when water changes from a liquid to a gas

metabolic rate: amount of energy consumed minus the amount of energy expended by the body

radiation: transfer of heat via infrared waves

thermoneutral: external temperature at which the body does not expend any energy for thermoregulation, about 84 °F

thermoregulation: process of regulating the temperature of the body

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Updated April 25, 2017

By Peter Solovyev

Heat transfer occurs by three main mechanisms: conduction, where rigorously vibrating molecules transfer their energy to other molecules with lower energy; convection, in which the bulk movement of a fluid causes currents and eddies that promote mixing and the distribution of thermal energy; and radiation, where a hot body emits energy that can act upon another system via electromagnetic waves. Convection and conduction are the two most prominent methods of heat transfer in liquids and gases.

Conduction typically occurs in solids. Electric stove tops use conductive heat transfer to bring a pot of water to a boil: thermal energy is transferred from the hot burner to the cool pot, causing the water's temperature to increase. Conduction happens because of the vibration of molecules. In a solid substance, atoms, arranged very tightly in lattice-like structures, have very little freedom to move around in space. As the burner heats up, the atoms in the metal begin to vibrate faster and faster as their energy increases. When you place the cool pot of water on the burner, you are creating a temperature gradient -- a place for the heat to flow to. Since energy flows from hot things to cooler things, the vibrating atoms of the burner transfer some of their heat to the atoms that make up the metal of your pot of water. This causes the pot's atoms to vibrate, transferring their energy to the water.

Conduction is more common to solids, but in principle it can -- and does -- happen in liquids and gases, just not very well. Because the molecules of fluids have a greater freedom of motion than in solids, there is less of a chance that vibrating molecules will collide with another and transfer energy throughout the fluid. In fact, air is such a poor conductor that it is used to help insulate homes. Some energy-efficient windows have "air spaces" between them that create a pocket of air between the inside of the home and the cold outside air. Because air does not conduct heat very well, more heat stays inside the home since the air makes it difficult for this thermal energy to make its way outside.

Convection is by far the most efficient and common way for heat to be transferred through liquids and gases. It occurs when some regions of a fluid get hotter than others, causing currents in the fluid that move it around to distribute that heat more evenly. Think of a house in the winter time. You may have noticed that the attic is always very warm while the basement is typically cool. This happens because when air heats up, it becomes light, causing it to move up towards the ceiling. Cold air is much heavier and falls to the floor. As the hot air moves to the ceiling and the cold air falls, these two types of air collide and mix, causing the heat from the warm arm to transfer to the cooler air and thus distributing the heat throughout the room.

Radiation occurs when a body gets hot enough to emit electromagnetic energy. The sun is a classic example of radiative heat transfer: it is very far away in space, but it is hot enough for your to feel its heat. You feel this heat because of radiation, and even on a cool day the sun feels warm. Electromagnetic energy can travel through empty space and can cause a target object to heat up from far away. Radiative heat transfer does not commonly occur in liquids and gases.