Control of body temperature is the first example of homeostasis we will look at in this series. For humans, normal body temperature is 37°C. It is important to maintain this temperature as it is the optimum temperature for enzymes involved in many metabolic processes such as respiration. Humans are endotherms, meaning that they generate heat internally through respiration (although behavior is also involved e.g. going to sit near a radiator). Some organisms are ectotherms and do not have the ability to maintain their body temperature internally, so must rely entirely on their behaviour. This article is about how humans control their body temperature using physiological responses. Scroll to the end for a summary flow diagram.
Control of body temperature
Remember that homeostasis involves negative feedback loops. A negative feedback loop needs to begin with a stimulus being detected by a receptor. Thermoreceptors in the skin detect the temperature of the skin, and thermoreceptors in the hypothalamus (an area of the brain) detect the temperature of the blood. These receptors send impulses via sensory neurones to the hypothalamus. The hypothalamus acts as the coordination center and sends nerve impulses via motor neurones to various effectors. These include erector pili muscles, skeletal muscles, sweat glands, arterioles, the thyroid gland, and the adrenal glands. The responses will be different depending on whether body temperature is too high or too low. Read on to see how the effectors bring about responses to bring body temperature back to 37°C.
Increasing body temperature
- Shivering: skeletal muscles contract rapidly to produce more metabolic heat from increased respiration.
- Vasoconstriction: arterioles constrict to restrict the flow of blood to capillaries near the surface of the skin to reduce heat loss through radiation.
- Hairs stand up: the hairs on the skin are controlled by muscles called the erector pili muscles. They contract to make the hairs stand up and trap an insulating layer of air near the skin to reduce heat loss.
- Less sweat produced: sweat glands reduce sweat production to reduce heat loss.
- Adrenaline (from the adrenal gland) and thyroxine (from the thyroid gland): these hormones are released to increase metabolic rate, so more heat is produced from respiration.
Decreasing body temperature
- Sweating: sweat glands produce more sweat, which cools the skin as it evaporates (remember the properties of water).
- Hairs lie flat: the erector pili muscles relax so that the hairs on the skin lie down. There is no insulating layer of air trapped next to the skin so heat can be lost more easily.
- Vasodilation: arterioles dilate to allow more blood to flow through capillaries close to the skin, so more heat is lost through radiation.
Summary
