Our body temperature must be controlled within a very narrow range so that our body can function properly. A constant core temperature of around 37ºC needs to be maintained. The thermoregulatory centre of the brain triggers changes in effectors, such as sweat glands and muscles, in order to constantly balance our temperature gains and temperature losses.
Maintaining body temperature
Temperature control is the process of keeping the body at a constant core temperature close to 37°C.
Our body can only stay at a constant temperature if the heat we generate is balanced and equal to the heat we lose.
Although our core temperature must be close to 37ºC , our fingers and toes can be colder. This is because energy is transferred from the blood as it travels to our fingers and toes.
The warm thermogram (l) shows the body at normal temperature 37°C (red) - the extremities are cooler (peach and pink areas). The cool thermogram (r) illustrates how the body diverts heat to the core organs to aid survival - the extremities are the coldest areas below 25°C (dark blue).
How our body maintains a constant temperature
Temperature receptors in the skin detect changes in the external temperature. Sensoryand relay neurones [neurones: Nerve cells. They carry an electrical message or impulse when they are stimulated.] transmit this information as impulses [impulse: A nervous impulse is a signal that is transmitted along a neuron or series of neurons.] to the thermoregulatory centre of the brain – the area of the brain responsible for monitoring and controlling temperature.
The thermoregulatory centre also has temperature receptors which detect changes in the temperature of the blood flowing through the brain.
In the event of a change in temperature away from 37oC, the thermoregulatory centre sends electrical impulses to effectors [effectors: organs which have an effect when stimulated (eg muscles or glands)] (predominantly in the skin) which bring about responses that correct the temperature back to 37oC.
When the body is too cold:
- The blood vessels supplying the skin capillaries [capillary: Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins.] constrict [constrict: To get narrow.] , causing less blood to flow nearer the surface of the skin, the skin to become pale in appearance, and a reduction of heat loss.
- The body shivers - the twitching of muscles generates additional heat as their contraction [muscle contraction: A shortening or tensing of the muscle.] causes the muscles to respire [respire: To engage in respiration, the energy-producing process inside living cells.] thus releasing energy to warm the body.
When the body is too hot:
The blood vessels supplying the skin capillaries dilate [dilate: Widened or expanded.] causing more blood to flow nearer the surface of the skin, the skin to become red in appearance, and an increase in heat loss.
The body sweats - which increases heat loss due to the large amount of heat energy required to evaporate [evaporate: The process in which a liquid turns into a gas.] the water.
Note that we sweat more in hot conditions, so more water is lost from the body. This water must be replaced through food or drink to maintain the balance of water in the body. Ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons.] such as sodium ions and chloride ions are also lost when we sweat. They must be replaced through food and drink.
Controlling body temperature
Human enzymes usually work best at 37ºC, which is human body temperature. This can be measured in several places, including the ear, finger, mouth and anus.
There are various ways to measure body temperature, including using a clinical thermometer, heat-sensitive strips, digital probes or thermal imaging cameras.
Extremes of body temperature are dangerous:
- high temperatures can cause dehydration, heat stroke and death if untreated
- low temperatures can cause hypothermia and death if untreated
The body’s temperature is monitored by the brain. If you are too hot or too cold, the brain sends nerve impulses to the skin, which has three ways to either increase or decrease heat loss from the body’s surface:
- Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.
- If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal.
- Blood vessels supplying blood to the skin can swell or dilate - vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. Blood vessels can shrink down again - vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.
Muscles can also receive messages from the brain when you are cold. They respond by shivering, which warms you up.
|Too cold||Too hot|
A - Hair muscles pull hairs on end.
B - Erect hairs trap air.
C - Blood flow in capillaries decreases.
D - Hair muscles relax. Hairs lie flat so heat can escape.
E - Sweat secreted by sweat glands. Cools skin by evaporation.
F - Blood flow in capillaries increases.
A very common mistake in exams is to write that the blood vessels move up and down in the skin. The blood vessels do not move during vasodilation and vasoconstriction.
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