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A day at the sauna. How does the body react to change?. Saving energy?. Sayid has decided to save energy by staying in bed all day. How much of his energy do you think this will save?. Surprisingly, the answer is only about 30%.

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a day at the sauna
A day at the sauna

How does the body react to change?

saving energy
Saving energy?

Sayid has decided to save energy by staying in bed all day.

How much of his energy do you think this will save?

Surprisingly, the answer is only about 30%.

The other 70% keeps his body temperature at 37°C, and the solutions around his cells at just the right concentration.

what is homeostasis
What is homeostasis?

The body uses so much energy, even during sleep, because it must maintain a constant internal environment.

This process of keeping things the same is called homeostasis.

A series of automatic control systems ensures that the body maintains a constant temperature, and steady levels of water, ions and blood sugar.

Homeostasis allows the body’s cells to work at their optimum.

why control temperature
Why control temperature?

Environmental temperature is constantly changing. One minute it can be very hot, the next very cold.

Despite this, the body must be kept at a constant temperature of 37°C. Why?

This is the optimum temperature for the body’s enzymes.

Even slight changes in body temperature can have a life-threatening effect on health. If body temperature falls too low, reactions become too slow for cells to survive: too high, and the body’s enzymes are at risk of denaturing.

what is core body temperature
What is core body temperature?

The vital organs located deep within the body, such as the heart, liver and kidneys, are maintained at 37°C. This is the core body temperature.

Skin temperature at the body's extremities, such as the fingers and toes, is usually lower than the core body temperature.

On a warm day, skin temperature may be just 1°C lower than the core body temperature, but on a very cold day it could be up to 9°C lower.

finding the right balance
Finding the right balance

Core temperature is maintained by balancing heat gain and heat loss.

How can heat be gained?

  • movement and exercise
  • shivering
  • vasoconstriction
  • wearing extra clothing.

How can heat be lost?

  • sweating
  • vasodilation
  • removing extra clothing.
how is temperature controlled
How is temperature controlled?


Body temperature is monitored and controlled by temperature receptors in the skin and brain.

These receptors detect changes in the temperature of blood flowing through those areas.

The thermoregulatory centre in the brain is called the hypothalamus.

If body temperature deviates from 37°C, the hypothalamus and skin receptors send out electrical signals that trigger actions or behaviours that increase or decrease heat loss.

why do we shiver
Why do we shiver?

When core body temperature drops, muscles begin to twitch. This rapid and contraction and relaxation of the muscles is called shivering.

Shivering generates heat, which raises body temperature.

Goose bumps involuntarily appear when a person becomes cold. Goosebumps are caused by the tiny muscles at the base of body hairs pulling the hairs erect.

The upright hairs trap an insulating layer of air, which helps reduce heat loss.

vasoconstriction and warming up
Vasoconstriction and warming up

Why do people go pale when they are cold?

When core body temperature falls, blood vessels in the skin get narrower. This is called vasoconstriction.

Vasoconstriction is caused by contraction of the muscular wall of the blood vessels.

This reduces the volume of blood flowing near the skin surface, and reduces the amount of heat lost from the body.

vasodilation and cooling down
Vasodilation and cooling down

Why do people turn red when they are hot?

When core body temperature rises, blood vessels in the skin get wider. This is called vasodilation.

Vasodilation allows a larger volume of blood to flow near the skin surface, transferring heat to the environment. This cools the body down.

Additional cooling occurs with the production of sweat from sweat glands.

As the sweat evaporates it transfers heat away from the body.

temperature control in newborns
Temperature control in newborns

Sayid has a baby sister. Samira was born premature and is too young to control her temperature. An incubator helps to control her temperature, using negative feedback.

The air around Samira is kept at 32°C. Why is it not keep at 37°C?

why is water important
Why is water important?

The human body is about 60-70% water.

Water molecules and ions constantly move in and out of cells, and are essential for all life processes.

Dehydration (loss of too much water from the body) damages cells.

How is water gained and lost?

  • Water is produced by the body during respiration, and absorbed from food and drink.
  • Water is lost from the body in exhaled air, sweat, urine and faeces.
dehydration and its causes
Dehydration and its causes

Just a 1% decrease in body weight due to water loss is enough to cause mild dehydration.

Mild dehydration can cause dizziness, a dry mouth and concentrated urine. Severe dehydration can cause death.

What causes dehydration?

  • heavy sweating
  • low water intake
  • eating salty food
  • breathing dry air
  • caffeine and alcohol
  • diarrhoea.
what is blood glucose
What is blood glucose?

Glucose is a type of sugar used by the body to provide energy.

Sometimes there is too much glucose in the blood, and sometimes there is not enough.

What affects the level of blood glucose?

  • Eating causes blood glucose levels to rise.
  • Vigorous exercise causes blood glucose levels to fall.

How does the body regulate blood glucose levels?

controlling blood glucose
Controlling blood glucose

Between meals, blood glucose levels are topped up from stored deposits in the liver and muscles.

After a meal, blood glucose rises but quickly returns to normal. Where does the excess go? Why not leave it in the blood?

Excess glucose makes the blood plasma and tissue fluid around cells too concentrated. This can severely damage cells, for example, causing crenation in red blood cells.

However, low blood sugar levels can be equally as dangerous, as it can make cells swell up and burst. This is called lysis.

the pancreas and blood glucose
The pancreas and blood glucose


Blood glucose levels are monitored and controlled by the pancreas.

The pancreas produces and releases different hormones depending on the blood glucose level.

  • Insulin is released when blood glucose levels are high – the liver stores excess glucose as glycogen.
  • Glucagon is released when blood glucose levels are low – the liver converts stored glycogen into glucose and releases it into the blood.
glossary 1 3
Glossary (1/3)
  • ADH –The hormone released from the pituitary gland that acts on kidneys and blood vessels to maintain the body’s water balance.
  • dehydration –The loss of too much water from the body.
  • homeostasis – The constant regulation of the body's internal environment.
  • hyperthermia – Dangerously high body temperature.
  • hypothalamus – The part of the brain that helps to regulate the body's internal environment.
  • hypothermia – Dangerously low body temperature.
  • glucose – The main source of energy for the body.
glossary 2 3
Glossary (2/3)
  • glucoregulation –The homeostatic control of the body’s blood sugar level.
  • glycogen – A storage form of glucose, found in the liver and muscles.
  • insulin –A hormone involved in the control of blood sugar, and which is reduced or absent in people with diabetes.
  • kidney – The bean-shaped organ that filters the blood and produces urine.
  • negative feedback – A stabilizing mechanism that slows down or reverses a stimulus.
glossary 3 3
Glossary (3/3)
  • osmoregulation – The homeostatic control of the body’s water and ion balance.
  • pancreas – The organ that secretes the hormone insulin.
  • thermoregulation – The homeostatic control of the body’s temperature.
  • vasoconstriction – The narrowing of the blood vessels in the skin in order to reduce heat loss.
  • vasodilation – The widening of the blood vessels in the skin in order to increase heat loss.