Thermoregulation and Vasomotor Responses

Thermoregulation is the control of body temperature, and in humans it keeps the core temperature near 37 °C through negative feedback, using sweating, shivering and changes in the diameter of skin blood vessels (vasodilation and vasoconstriction). In brief: when the body is too hot, skin arterioles widen (vasodilation) and sweat glands release sweat, which cools the body as it evaporates. When the body is too cold, the arterioles narrow (vasoconstriction), sweating stops and muscles shiver to produce heat. The control center is the hypothalamus.

Enzymes work properly only within a narrow temperature range, so keeping a stable temperature is vital. This guide explains how the body gains and loses heat, how the hypothalamus acts as a thermostat, the responses to heat and cold, how endotherms and ectotherms differ, works through a heat-loss calculation and corrects common mistakes.

Why is body temperature controlled?

Chemical reactions in cells are catalyzed by enzymes, and their rate depends on temperature. If the temperature is too low, reactions are too slow. If it is too high, enzymes lose their shape (denature) and stop working, and proteins can be damaged. In humans, the core temperature normally lies in the range of 36.5 to 37.5 °C. A fever above about 38 °C is a sign of illness, and a core temperature above about 40 °C is dangerous. Hypothermia, a fall below 35 °C, also becomes dangerous, and below 28 °C the heart may stop.

How does the body gain and lose heat?

Heat is generated mainly by metabolism, particularly in the liver, in the muscles and in the heart. At rest, the body produces about 80 to 100 watts of heat, and during heavy exercise, it can produce more than ten times this amount. Heat can also be gained from the environment.

Heat is lost by four routes:

• Radiation: infrared radiation emitted from the skin. This is the main route (about 60 percent) at room temperature.
• Conduction: direct transfer to cooler objects in contact with the body, such as a cold chair.
• Convection: transfer to moving air or water. Wind and water remove heat faster, which is why wind-chill makes cold air feel colder.
• Evaporation: water evaporating from the skin and lungs takes up heat. Evaporating 1 gram of water absorbs about 2.4 kilojoules.

How does the hypothalamus control temperature?

The thermoregulatory center in the hypothalamus acts as the thermostat. It receives information from two kinds of receptors: thermoreceptors in the hypothalamus itself, which detect the temperature of the blood that flows through the brain (the core temperature), and thermoreceptors in the skin, which detect the temperature of the surroundings. The hypothalamus compares this information with a set point of about 37 °C and sends nerve impulses to the effectors: sweat glands, muscles in the walls of the skin arterioles, skeletal muscles and hair erector muscles. The result is negative feedback, in which any change from the set point triggers a response that reverses it.

What happens when the body is too hot?

• Vasodilation: the muscle in the wall of the skin arterioles relaxes, so they widen. More blood flows close to the surface of the skin in the capillaries, and more heat is lost by radiation and convection. The skin looks red and flushed.
• Sweating: sweat glands secrete sweat onto the skin. Its evaporation takes heat from the body. A person can produce 1 to 2 liters of sweat per hour in the heat, although sweat that drips off without evaporating is not effective.
• Hair flattens: the erector muscles relax, and the hairs lie flat, so there is less trapped air. This effect is small in humans.
• Behavior: seeking shade, removing clothing, drinking water, reducing activity.

What happens when the body is too cold?

• Vasoconstriction: the muscle in the arteriole walls contracts, so they narrow. Less blood flows to the skin surface, and less heat is lost. The skin looks pale, and the hands and feet feel cold.
• Shivering: rapid involuntary contractions of skeletal muscles release heat from respiration. Shivering can increase heat production by several times.
• Sweating stops.
• Hair erection: the erector muscles pull the hairs upright, which traps a layer of insulating air (this is the cause of “goosebumps” in humans, which has no great effect, but is very effective in furry animals).
• Hormones: adrenaline and thyroxine increase the metabolic rate. Babies have brown fat, which produces heat directly.
• Behavior: wearing more clothes, moving into warmth, curling up, being more active.

Note that the blood vessels themselves do not move closer to or further from the skin surface. What changes is the diameter of the arterioles, which controls how much blood flows into the capillaries of the skin.

Responses to heat and cold compared

Worked example: heat loss by sweating

A runner produces heat at a rate of 1 000 watts (joules per second) during a race. Suppose that all of the extra heat must be removed by evaporation of sweat, and that evaporation of 1 gram of water absorbs 2 400 joules. Resting heat production is about 100 W, which is removed by other routes.

1. Extra heat to remove: 1 000 − 100 = 900 W, i.e. 900 J every second.
2. Sweat needed per second: 900 ÷ 2 400 = 0.375 g/s.
3. Per hour: 0.375 × 3 600 = 1 350 g, which is about 1.35 liters of sweat per hour that must evaporate.
4. For a 2-hour run: about 2.7 liters of water, equal to about 3.9 percent of the body mass of a 70 kg runner.

This shows why a person needs to drink during long exercise in the heat. The calculation also illustrates why humid conditions are dangerous: when the air is already saturated with water vapor, sweat cannot evaporate, and the cooling mechanism fails, with a risk of heat exhaustion or heat stroke. Sweat also contains salts, which have to be replaced during long events.

How do other animals regulate temperature?

Endotherms (birds and mammals) produce their own heat and maintain a high, constant temperature, which allows activity in cold conditions but needs a lot of food. Ectotherms (fish, amphibians, reptiles and invertebrates) rely on the environment, so their activity depends on the temperature. They regulate by behavior, such as basking or seeking shade. Some mammals hibernate to save energy during winter. Animals in cold climates have insulation (fur, blubber), and animals in hot climates may have large ears, like elephants, to lose heat.

How was thermoregulation studied?

Santorio Santorio in the early seventeenth century built one of the first thermometers for use on patients. Carl Wunderlich in the 1860s measured the temperatures of thousands of patients and established 37 °C as the average normal value, with fever as a sign of disease. The role of the hypothalamus in temperature regulation was shown by experiments in the early twentieth century, including work by Henry Barbour and Walter Hess.

Which mistakes are common?

Mistake 1: Saying that blood vessels move toward the skin surface

The vessels stay in place. Their diameter changes, and so does the amount of blood that flows through them.

Mistake 2: Thinking that sweating cools the body because sweat is cold

Cooling comes from the evaporation of sweat, which takes heat from the skin.

Mistake 3: Confusing the thermostat with the receptors

The hypothalamus is the control center. Receptors are in the skin and the hypothalamus. Effectors are the glands and muscles.

Mistake 4: Describing shivering as a way to lose heat

Shivering produces heat by muscle contraction. It helps to raise body temperature.