Regulation of Body Temperature

Regulation of Body Temperature

Body temperature reflects the persons metabolic rate and overall clinical state. The first thing the doctors do is to take the temperature. If it is too high or too low then something is wrong.

Normal temperatures

Oral (mouth) - 37⁰C
Rectal - 37.5⁰C
Skin surface - Usually around 5⁰C higher than atmospheric temperature (almost always lower than our body temperature)

If the atmospheric temperature is always lower than our body temperature we will always be losing heat to the environment

Body temperature doesn't always stay at 37⁰C. It varies throughout the day and for females, there are also monthly variations.

Body temperature is normally at its highest at the end of our working day (about 6-7 pm). This is when the metabolic rate is at its highest because of the amount of energy that has been required.

Body temperature is normally at its lowest a couple of hours before we get up in the morning (about 4-5 am). This is because the metabolic rate has been gradually falling because require less energy when sleeping.

Monthly variations in women are a result of hormonal changes. Hormones affect metabolic rate and hence heat production. In men testosterone levels remain the same, but for women oestrogen and progesterone levels rise and fall depending on where they are in the menstrual cycle.

Sources of heat in the body

Cellular respiration: When glucose is converted to ATP, 60% of the energy is released as heat
Muscular activity: More energy is required therefore there is an increase in cellular respiration to form more ATP. This results in more heat production. During the muscular activity, frictional heat is also produced by internal structures rubbing against each other (like rubbing hands together)
Assimilation of food e.g. digestion of food and absorption of nutrients

Major ways of getting rid of heat

Radiative heat loss: Results when heat flows from an area that is warmer to an area that is cooler. This can be increased by dilating the blood vessels in the skin; resulting in an increase in warm blood flow. If we want to reduce radiative heat loss, the blood vessels need to constrict, reducing blood flow to the skin
Evaporative heat loss: Heat from the body is transferred to the water (sweat). As it heats up it turns to vapour and then evaporates from the skin
Exhaling: When we exhale the air is warm and wet

Increase in Body Temperature

1. When feeling too hot we can control our temperature behaviourally by removing clothing, having a cold drink, standing under a fan etc.

2. If not sufficient to lower body temperature the first physiological action to occur is the vasodilation of the arterioles in the skin. This increases the flow of warm blood to the surface of the skin, resulting in an increase in radiative heat loss.

3. This is followed by activation of the sweat glands. The sweat absorbs the heat and turns to vapour. This results in an increase in evaporative heat loss.

All of the above happens very quickly, but if exposed to heat over a long period of time the endocrine system will start to secrete less of the hormones adrenaline and thyroxine. These hormones are responsible for increasing metabolic rate, therefore less secretion results in a reduction in cellular respiration, resulting in less heat production.

Decrease in Body Temperature

1. When feel too cold we can control our temperature behaviourally by adding clothing, having a hot drink, standing near a heater etc.

2. If not sufficient to increase body temperature the first physiological action to occur is the vasoconstriction of the arterioles in the skin. This decreases the flow of warm blood to the surface of the skin, resulting in a reduction in radiative heat loss.

3. This is followed by activation of the skeletal muscles. The muscles contract causing shivering. This results in an increase in cellular respiration and hence the production of more heat. Shivering can however be controlled. All the control centre is trying to get you to do is contract muscles to increase cellular respiration and hence heat production. We can contract our muscles by moving around when cold. If do not move around voluntarily the control centre will make your muscles contract involuntarily.

All of the above happens very quickly, but if exposed to cold over a long period of time the endocrine system will start to secrete more of the hormones adrenalin and thyroxine. These hormones are responsible for increasing metabolic rate, therefore more secretion results in increased cellular respiration, resulting in more heat production.

Body Temperature Abnormalities

Fever

A fever is an abnormally high body temperature (adults above 37.8, children above 37.5). It is most commonly caused by a bacterial or viral infection, however, other causes include heart attacks, tumours, surgery, accidents (trauma), and reactions to vaccinations.

The mechanism of fever production is meant to occur because when the body tries to fight the infection, chemicals are released which alter the set point of the hypothalamus to a higher temperature. This causes the effectors that are involved in temperature regulation to react to bring the temperature up to the new set point, i.e. vasoconstriction of the blood vessels in the skin, and contraction of the skeletal muscles. Therefore even though body temperature is rising, the skin remains cold and shivering occurs. This is called a chill.

Once the temperature reaches the new set point the chills disappear, however the body will still regulate the temperature at this higher setting until the stress (infection) is removed. Once the infection has resolved the set point of the hypothalamus will return to normal, but because the body temperature remains high in the beginning, the heat losing mechanisms (vasodilation of blood vessels and sweating) occur to reduce body temperature. The sweating is, therefore, a sign that temperature is starting to fall.

Benefits of fever

Stops growth of some bacteria and viruses
Increases heart rate, hence delivering more white blood cells to the area of infection
Heat speeds up chemical reactions which may help cells to repair themselves faster during an infection

Complications of a fever

Very high fever that is sudden or which has occurred over a prolonged period can be dangerous and cause serious complications e.g.

Dehydration
Permenant brain damage
Death

Heat Cramps

Cramp (painful muscle contractions) can occur due to loss of water and salt from excessive sweating.

Heat Exhaustion and Heatstroke

For information on HEAT EXHAUSTION and HEATSTROKE click on link to NHS choices

Hypothermia

For more information on FEVER click on link to NHS choices

When the body temperature falls below 35 degrees, this is known as hypothermia. It can be caused by:

Exposure to overwhelming cold stress e.g. immersion in cold water
Metabolic diseases e.g. hypoglycaemia, kidney problems
Drugs
Malnutrition

If body temperature falls below 29 degrees the hypothalamus can no longer regulate body temperature, and death is likely to occur if the temperature falls below 25 degrees.

For more information on HYPOTHERMIA click on link to NHS choices