Thus far, Investigations One through Three have provided students with an opportunity to explore heat by discovering the relationship between heat, kinetic energy, and temperature. Students also discovered how heat is transferred and how materials differ in their ability to conduct heat. Students also explored how heat is transferred during chemical reactions in order to understand endothermic and exothermic processes. In Investigation Four, students will discover how the body uses these principles of heat transfer to regulate body temperature. Students will also explore how changing the volume of a gas can change its temperature through experiments with the Joule-Thompson effect.

Body Heat

Animals maintain body temperature in different ways. Fish, reptiles, and amphibians are poikilothermic, or cold-blooded. Their bodies take on the temperature of their environment. Therefore, these animals must actively adjust their physical position in their environment in order to regulate their body temperature. Mammals and birds, however, are homoeothermic (warm-blooded) animals. Their body temperature is regulated by an area of the brain called the hypothalamus.

In warm-blooded animals, the hypothalamus acts as the body’s thermostat. Normal body temperatures in humans range between 36 ^oC and 37.5^oC (97 ^oF and 100 ^oF), with an average body temperature of approximately 37 ^oC (98.6 ^oF). The body can function normally up to a temperature of approximately 39 ^oC (103 ^oF). It constantly monitors the temperature of the blood as it passes through, and then sends signals to the blood vessels and skin to adjust how the blood is distributed in the body and the amount of perspiration being released.

When Our Body is Too Hot

Water has a tremendous capacity for heat absorption, and the body uses this to its advantage. Blood has a high water content, and so the body adjusts the amount of blood flowing through different areas of the body to regulate its temperature. As core body temperatures rise, the brain signals the blood vessels in the limbs and skin (peripheral vessels) to dilate (increase diameter), allowing more blood to reach these areas where heat can be released through radiation from the skin and convection through evaporation of perspiration. Evaporation of water is an endothermic process because water changes from a liquid to a vapor. This change requires energy. Therefore, the water absorbs heat from the body as it evaporates, and causes the skin temperature to decrease. This helps cool the body.

 Many people associate perspiration with the presence of sweat on their skin. However, perspiration is a continual process. It remains unnoticed until the perspiration being released from the sweat glands is unable to evaporate immediately. This occurs when the body undergoes a high level of physical activity, or when there is a high level of humidity (moisture content) in the air. Physical activity increases the exothermic (heat releasing) reactions in the body responsible for providing energy to cells. This increases the body’s core temperature and causes the brain to direct more blood to the skin.

As humidity increases the amount of additional water that the air can hold decreases, slowing the evaporation of perspiration from the skin’s surface. When humidity is very low, as in the desert or during winter, perspiration evaporates immediately. Water may also evaporate from skin cells, resulting in dry skin.

When Our Body is Too Cold

If body core temperature decreases, the hypothalamus sends signals to the peripheral blood vessels, telling them to contract (decrease diameter) so that less blood circulates to these areas. This decreases heat release through radiation and convection and causes hands and feet to feel cold. This allows more heat to remain in the body core. When body core temperatures drop to a specific critical temperature despite the redirection of blood from the peripheral vessels, the hypothalamus triggers involuntary contractions of skeletal muscles, causing shivering. These contractions require energy, and therefore they stimulate the metabolism of energy stores in the muscle cells. In essence, the body is deliberately generating exothermic reactions to increase body temperature. Finally, in addition to redirecting blood supply and muscular shivering, small muscles in the skin at the base of each hair follicle contract and cause body hairs to “stand on end” or bristle, limiting the airflow near the skin surface to reduce evaporation and retain body heat. In the process, “goosebumps” are formed.

Look at the two pictures above. The top picture is of a Komodo Dragon basking on rocks heated by the sun. The Komodo Dragon is an endangered species of reptile limited to a few small islands in Southeast Asia. Like other reptiles, they are cold-blood and must regulate their internal body temperature by choosing warm locations in their immediate environment. Although the little girl pictured below is a warm-blooded mammal, she is also lying on a hot surface to warm up after leaving the cold water. Notice how the red arrows indicate the transfer of heat by conduction, that is direct contact, with the warm rock or concrete surfaces.

Joule-Thompson Effect

In order to understand the Joule-Thompson effect, it is important to remember that gases expand to fit their container. Furthermore, this expansion requires an increase in the kinetic energy of the gas molecules. The expansion of liquid to a gas is endothermic. Liquid molecules have lower kinetic energies than gas molecules, so they must absorb heat in order to expand into a gas. The Joule-Thompson Effect describes a phenomenon that occurs when a gas is compressed and then expanded.

Compressing a gas by forcing it through a small opening forces molecules closer together. In some cases, this causes the gases to return to a liquid state. As the molecules exit through this small opening into a much larger space, they expand rapidly. However, there is no external heat source to supply the energy for expansion as when a liquid is heated, so heat is absorbed from the surroundings. This causes the temperature of the area around the gas molecules to decrease. This rapid decrease in temperature due to the rapid expansion of a gas is known as the Joule-Thompson Effect. This is why air exiting from a can of compressed air or a car or bicycle tire feels cool. When you purse your lips and blow on hot food, you are using the Joule-Thompson Effect to cool it.

Investigation Four introduces students to the regulation of body temperature by evaporation through experimental models. Students will also investigate the Joule-Thompson Effect by observing how the temperature of their breath can be changed either by pursing their lips or by compressing the end of a straw.