Homeostasis in humans
Conditions inside our body need to be maintained in a steady state - blood sugar level, concentration of the blood and temperature are monitored and regulated carefully. This is called homeostasis.
Body temperature and the thermoregulatory centre
Body temperature
Body temperature is one of the factors controlled by homeostasis. The human body maintains the temperature that enzymes work best at, which is around 37°C. This process is controlled by the thermoregulatory centre, which is contained in the hypothalamus in the brain. It contains receptors sensitive to the temperature of the blood. The skin also has temperature receptors and sends nervous impulses back to the thermoregulatory centre.
Too hot
When we get too hot, sweat glands in the skin release more sweat. The sweat evaporates, transferring heat energy from the skin to the environment.
Blood vessels leading to the skin capillaries become wider - they dilate - allowing more blood to flow through the skin and more heat to be lost to the environment. This is called vasodilation.
Too cold
When we get too cold, skeletal muscles contract rapidly and we shiver. These contractions need energy from respiration and some of this is released as heat.
Blood vessels, which lead to the skin capillaries, become narrower - they constrict – which allows less blood to flow through the skin and conserve the core body temperature. This is called vasoconstriction.
The hairs on the skin also help to control body temperature. The hairs lie flat when we are warm and rise when we are cold. If we are too cold, nerve impulses are sent to the hair erector muscles which contract. This raises the skin hairs and traps a layer of insulating air next to the skin.
The control of body temperature is an example of a negative feedback mechanism. It regulates the amount of:
- shivering (rapid muscle contractions release heat)
- sweating (evaporation of water in sweat causes cooling)
- blood flowing in the skin capillaries
Vasoconstriction and vasodilation
The amount of blood flowing through the skin capillaries is altered by vasoconstriction and vasodilation.
| Too cold | Too hot | |
|---|---|---|
| Process | Vasoconstriction | Vasodilation |
| Arterioles | Get narrower | Get wider |
| Blood flow in skin capillaries | Decreases | Increases |
| Heat loss from skin | Decreases | Increases |
These diagrams show the processes that take place when vasoconstriction and vasodilation occur.
Generally, when the body temperature is too low, a variety of processes happen - vasoconstriction, sweating stops and shivering starts.
When the temperature is too high, different processes happen - vasodilation and sweat production, which both transfer energy from skin to the environment, resulting in a cooling effect.
The two diagrams show the role of the blood vessels in maintaining body temperature. Hairs on the skin help to regulate temperature. When cold, the erector muscle of each hair forces them to stand upright. This traps a layer of warm air near the skin which provides insulation. When hot, hairs on the skin lie flat to reduce body temperature.
Sweat glands also release sweat which travels to sweat ducts to sweat pores on the surface of the skin. When this sweat evaporates, it removes heat and cools the body down.