Evaporative Cooling - Jess Herr

Author

Jess Herr

Principle(s) Illustrated

Thermoregulation - The human body functions best at a temperature of about 98.6°F or 37°C. Minor fluctuations from this temperature are not a concern, and in some situations our bodies may even increase our core temperature to fight off infectious disease. As we know, there is a much greater fluctuation in the temperature of our environment. In the United States, people are able to live and survive in temperatures as high as 134.1°F (56.7°C) in Death Valley and as low as -80°F (-62°C) in Alaska.

Our bodies are capable of maintaining a fairly constant internal temperature despite external temperature fluctuations, which is called thermoregulation. The hypothalamus in the brain is responsible for controlling thermoregulation. There are mechanisms in place to increase the body temperature when the environment is cold and mechanisms to decrease the body temperature when the environment is hot. Of course, in extreme temperatures our bodies require some help (warm or cool clothing, fire, air conditioning, etc.) so of course you would not be able to walk through Alaska in the middle of winter in your swimsuit!

Sudoriferous Glands – Sudoriferous or sweat glands secrete water and ions through pores in the epidermis. Some of these glands also secrete uric acid and urea, which can create the odor associated with sweat. Eccrine sweat glands produce odorless sweat and are found covering the body, while apocrine sweat glands secrete odorous sweat and are located only in specific parts of the body, such as the armpits.

Evaporative Cooling - When the outside temperature is high, or the muscles of the body are creating excess heat during exercise, the body must find a way to maintain a lower internal body temperature. Perspiration, or sweating, effectively cools down the body through a process known as evaporative cooling. The skin will start to sweat at 37°C, and normal perspiration will produce an average of 1.5 L/hour of sweat. In extremely warm climates, the body can produce up to 3.5 L/hour of sweat.

It is not the actual sweat produced that cools the body, but the evaporation of the sweat from the skin. Water has a high heat of vaporization, which means it takes a lot of heat energy to cause water to turn from a liquid into a gas. Therefore, the heat produced by the body is absorbed by sweat and removed from the skin as sweat evaporates. This activity will allow you to feel the cooling sensation created by evaporative cooling.

Standards

• HS-LS1-3. Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.

• NGSS Science & engineering standards

  • • • • • 1. Asking questions (for science) and defining problems (for engineering)

            2. Developing and using models

• NGSS Cross-cutting concept standards

• • Energy and Matter

Questioning Script

Prior knowledge & experience:

Students have all at some point or another experience the sensation of sweating, and most students will tell you that sweat can cool them off. If you probe further, very few can explain why this is the case.

Root question:

• 1. Which felt cooler – alcohol or water?

  2. What caused the cool feeling on your skin?

  3. Define the concept “high heat of vaporization.”

  4. Explain how sweating causes evaporative cooling.

  5. From what you have learned, why is it important to drink water during exercise?

Target response:

• 1. Alcohol

  2. Evaporative cooling- heat produced by the body is absorbed by sweat and removed from the skin as sweat evaporates.

  3. It takes a lot of heat energy to cause water to turn from a liquid into a gas

  4. Heat produced by the body is absorbed by sweat and removed from the skin as sweat evaporates.

  5. If the body sweats to maintain a regular internal temperature then water is leaving the body and should be replaced.

Common Misconceptions:

Sweat cools us down because the body/sweat it hotter than the outside environment so heat just moves from the body to the environment.

Photographs and Movies