39

key concepts

39.1

Animals Are Composed of Organs Built from Four Types of Tissues

39.2

Physiological Systems Maintain Homeostasis of the Internal Environment

39.3

Biological Processes Are Temperature-Sensitive

39.4

Body Temperature Depends on the Balance between Heat In and Heat Out of the Body

39.5

Body Temperature Is Regulated through Adaptations for Heat Production and Heat Loss

Heat Limits Physical Performance

The 2008 New York City Marathon took place on a cold, clear, windy day in November. For the third time, the first-place woman in this 41-kilometer race was world record holder Paula Radcliffe. Radcliffe had also been expected to win the women’s marathon in the 2004 Olympics. But that race took place on an extremely hot (high of 34°C), humid day in Athens. Overcome by heat stress, Radcliffe collapsed 6 kilometers from the finish line.

Elite runners generally have their best times when temperatures are below 10°C. The 2012 Boston Marathon coincided with an unseasonable April heat wave, with temperatures exceeding 27°C. During the race, 120 runners were rushed to hospitals with severe heat stress.

When a person’s internal body temperature rises above 40°C, major organs begin to fail, a condition known as heat stroke. Every year some athletes suffer heat stroke, which leads to death in a high percentage of cases. Soldiers in desert environments are at extreme risk of heat stroke, as are workers in many occupations, including firefighting, agriculture, and construction.

Heat stroke is a particular danger for those who are active in the heat because working muscles generate heat. The blood carries that heat out of the muscles and distributes it throughout the body, raising the temperature of the body’s internal tissues. Some of the heated blood flows to the skin, where heat can be lost to the environment, but humans are subject to the problems faced by all mammals in losing excess heat. First, their normal internal temperatures are not far from the environmental temperatures that cause heat stress, so they don’t have much of a safety zone. Second, most mammalian skin surfaces are covered with an insulating layer of fur—great for conserving body heat in cold environments, but an impediment to heat loss in warm ones. Even human skin is an insulator.

A major evolutionary adaptation in mammals for heat loss is efficient heat-loss portals of non-furred areas such as the nose, tongue, and footpads. In these areas, specialized blood vessels can open up and act like radiators to disperse heat (conversely, these portals can close down to conserve heat). Humans are not furred, but our evolutionary ancestors were, and we retain these mammalian blood vessel adaptations in our hands, feet, and face (which is why we blush).

How can we increase heat loss from the body to protect against heat stress?