AQA B3 Topic 3
Homeostasis
Removal of waste & water control (osmoregulation)
Key words:
Excretion – the removal of waste products from the body eg. carbon dioxide & urea
Homeostasis – the maintaining of a constant internal environment
Internal conditions have to
be kept within a narrow
range of values so the cells
can function properly. This is
homeostasis.
Examples of homeostasis:
• removal of waste products
• water & ion concentration
• body temperature
(thermoregulation)
• blood glucose levels
Waste products have to be removed from the body so levels do not
become toxic.
Waste product
Why is it produced?
How is it removed?
Carbon dioxide
Produced through
aerobic respiration
Through the lungs
when we breathe out
(exhale)
Urea
Produced in the liver
when excess amino
acids are broken
down
The kidneys remove
it from the blood and
make urine – which is
temporarily stored in
the bladder
What happens if the water or ion content is wrong?
• Too much water may & leave the cells damaging them.
Ions are lost from the body via - sweating & urine
Ions are gained via – eating & drinking
Water control – the kidney
Key words:
Urea – produced from the breakdown of amino acids – toxic
Urine – mixture of water, excess ions and urea created in the kidney & stored in the bladder
Selective reabsorption – when the kidney takes back different amounts of water and ions into the blood
depending on the bodys’ demand for them
A healthy kidney produces urine by:
Water is lost from the
body via
• Exhaling
• Sweating
• Urine
Water is
gained
via
• Drinking
• Eating
1. Filtering the blood – glucose, amino acids, mineral ions, urea
& water move from the blood to kidney tubles by diffusion.
Proteins & red blood cells are too large to be filtered out the
blood.
2. Reabsorbtion- all the sugar via active transport
3. Selective reabsorbtion - the dissolved ions needed by the
body via active transport
4. Selective reabsorbtion - as much water as the body needs is
selectively reabsorbed by osmosis
5. Releasing urea, excess ions and water as urine
Little drunk & hot day = little dark yellow, concentrated urine
Lots drunk & cool day = lots of dilute, almost colourless urine.
Kidney failure - dialysis
If a persons kidney stops working properly there are 2 options: 1) Dialysis
Dialysis
• Treatment by dialysis restores the
concentrations of dissolved substances in the
blood to normal levels and has to be carried
out at regular intervals.
2) Kidney transplant
Advantages
• Available to all kidney patients (no shortage)
• No need for immune-suppressant drugs
In a dialysis machine a person’s blood flows
between partially permeable membranes.
The dialysis fluid contains the same
concentration of useful substances as the
blood.
This ensures that glucose and useful mineral
ions are not lost.
Urea passes out from the blood into the
dialysis fluid.
Disadvantages
• Patient must limit their salt and protein intake between dialysis sessions
• Expensive for the NHS
• Regular dialysis sessions (8hrs) – impacts on the patient’s lifestyle
Kidney failure – kidney transplant
Key words:
Antigen – proteins on the surface of cells which identify a cell as ‘self’ or foreign to the body
Antibody - protein used in the immune response to fight off foreign cells
Kidney transplant
A diseases kidney is replaced by a healthy donor kidney.
Organ rejection is a problem as the antigens on the
surface of the donor kidney are recognised by the
immune system as foreign and can be attacked by the
patients antibodies
To reduce the chances of this happening 2 precautions
are taken:
1) Immune-suppressant drugs are give
2) A donor kidney with a similar ‘tissue type’ is used
Advantages
•Patients can lead a more normal life without
having to watch what they eat and drink
•Cheaper for the NHS overall
Disadvantages
•Must take immune-suppressant drugs which
increase the risk of infection
•Shortage of organ donors
•Kidney only lasts 8-9 years on average
•Any operation carries risks
Temperature control - thermoregulation
Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This centre
has receptors which are sensitive to the temperature of the blood.
Receptors in the skin also send impulses to the thermoregulatory centre about the skin temperature.
Consequences of your core
body temperature raising too
high:
• Enzymes are denatured so
can no longer catalyse
reactions in your cells.
Consequences of your core
body temperature dropping
too low:
• Enzyme reaction rate is
slowed.
• Not enough energy is
released & cells begin to die
Temperature control - thermoregulation
Responses to decreased body
temperature:
Increased body
temperature
1. Vasoconstriction narrowing of blood vessels
in the skin. Blood flow
through the skin is
reduced, therefore heat
loss is decreased. Body
temperature increases.
2. Shivering - tiny muscles
under the skin contracting
and relaxing very quickly.
The muscle cells release
heat.
3. Body hair rises away from
the skin, trapping a layer
of air next to it, insulating
the body. Erector muscles
contracting.
Decreased body
temperature
Responses to increased body
temperature:
1. Vasodilatation - widening of blood
vessels. Blood flow is increased and
more heat is lost through the skin.
Nerve impulses pass along the
nerves from the thermoregulatory
centre to the muscles in the walls of
blood cells, stimulating contraction
and causing them to narrow. When
the muscles relax the blood vessels
widen.
2. Sweat is produced. Heat from the
body evaporates the water in sweat.
 when it is hot more water is lost
so more has to be taken in.
3. Body hair lies flat against the skin,
preventing air becoming trapped
next to it.
Blood glucose control
Blood glucose concentration is controlled by the release of hormones from the pancreas
Insulin
Hormone released from the
pancreas when blood glucose
levels have increased.
Allows glucose to move from the
blood to the cells where it is used
or stored as glycogen in the liver.
Glycogen
Hormone released from the
pancreas when blood glucose
levels fall.
Causes glycogen stored in the
liver to be converted into glucose
and released into the blood
Problems with blood glucose control - diabetes
Diabetes is a disease where a persons blood glucose concentrations are not controlled properly because of
either a lack of insulin (type 1) or cells not responding to insulin (type 2)
Type 1 Diabetes
1. Pancreas does NOT produce any or enough insulin
2. Inject insulin daily (into the fat layer beneath the
skin)  traditionally this was from pigs, but now
we use genetically engineered human insulin as it
is more effective.
3. Exercise and diet are used to help lower blood
glucose levels too
Type 2 Diabetes
1. Cells respond less well to insulin (they become resistant)
2. Risk factors of developing it are:
1. High-fat diets
2. Lack of exercise
3. Getting older
4. Obesity
3. Control by changes in diet and exercise eg.
- eating a balanced diet with fewer carbohydrates
- Losing weight
- Regular exercise