Homeostasis

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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.
Surprisingly, the answer is only about 30%.
How much of his energy do you think this will save?
The other 70% keeps his body temperature at 37°C, and
the solutions around his cells at just the right concentration.

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.

Keeping comfortable

The organs of homeostasis

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

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
Core temperature is maintained by balancing heat gain
and heat loss.
How can heat be gained?
How can heat be lost?
 movement and exercise
 shivering
 vasoconstriction
 wearing extra clothing.
 sweating
 vasodilation
 removing extra clothing.

Too hot…or too cold?

Finding the right balance

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

What’s in skin?

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
Why do people go pale when they are cold?
Vasoconstriction is
caused by contraction
of the muscular wall of
the blood vessels.
When core body temperature falls, blood vessels in the
skin get narrower. This is called vasoconstriction.
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
Why do people turn red when they are hot?
Additional cooling occurs
with the production of
sweat from sweat glands.
Vasodilation allows a larger volume of blood to flow near
the skin surface, transferring heat to the environment.
This cools the body down.
When core body temperature rises, blood vessels in the
skin get wider. This is called vasodilation.
As the sweat evaporates
it transfers heat away
from the body.

Responding to change

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?

How do incubators work?

Negative feedback in the body

Thermoregulation: true or false?

Why is water important?
The human body is about 60-70% water.
 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.
How is water gained and lost?
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.

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.

Controlling water levels

How do the kidneys work?

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.
How does the body regulate blood glucose levels?
 Eating causes blood glucose levels to rise.
 Vigorous exercise causes blood glucose levels to fall.
What affects the level of 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
Blood glucose levels are
monitored and controlled
by the pancreas.
The pancreas produces
and releases different
hormones depending on
the blood glucose level.
pancreas
 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.

Maintaining safe glucose levels

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)
 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)
 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.

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Homeostasis

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