| 1. |
Blood is a mixture of:
- Plasma . a straw coloured, watery liquid
containing digested foods, mineral salts, hormones and waste
products including carbon dioxide.
- Red blood cells . containing haemoglobin
to carry oxygen round the body.
White blood cells
- phagocytes which engulf bacteria and secrete an enzyme
to kill them;
- lymphocytes which produce antibodies to neutralise harmful
chemicals and kill bacteria and viruses;
- platelets . fragments of cells which help the blood to
clot when the skin is cut.
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| 2. |
Functions of the blood
Blood has three important functions
(jobs):
- It transports nutrients (digested foods),
oxygen and waste products.
- It protects us from harmful bacteria,
viruses and chemicals.
- It controls body temperature by allowing
blood vessels (mainly capillaries) near the surface of the
skin to expand when we get hot and contract when we get
cold.
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| 3. |
The
circulatory system consists of the heart and blood
vessels. Blood circulates around the body in the blood vessels.
There are three kinds of blood
vessel with structures related to their function. |
- Arteries
- carry blood rich in oxygen away
from the heart at high
pressure;
- have thick, muscular walls to withstand the pumping pressure.
- Capillaries
- branch off from the arteries and then rejoin to form veins;
- are very narrow tubes with thin walls through which digested
food,
oxygen and waste products can diffuse.
- Veins
- carry blood lacking in oxygen back
to the heart at low pressure;
- have thinner, less muscular walls than arteries;
- contain valves to prevent the backflow of blood.
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| 4. |
The
heart is a pump with two circuits for the blood.
- One circuit goes from the heart to the
lungs and back again.
- The other circuit goes from the heart
to the rest of
the body and back again.
The diagram on the right shows how the heart is divided
into four chambers - two
atria (singular, atrium) where blood enters and two
ventricles where blood leaves.
The ventricles have thick, muscular walls so that they can
contract and relax continuously, forcing blood around the
body. Every time the muscles in the
ventricles contract, they produce a heart beat. Our hearts
beat about 70 times a minute even when we are resting.
The diagram also illustrates how the blood picks up oxygen
and digested foods and how it gets rid of waste products.
A Deoxygenated
blood enters the right atrium of the heart via the
vena cava.
B Deoxygenated
blood is pumped from the right ventricle, via the pulmonary
artery, to the lungs.
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C
In the lungs, blood picks
up oxygen and gets rid of carbon dioxide.
D Oxygenated blood
travels back to the heart in the pulmonary vein.
E Oxygenated blood
is pumped to vital organs and the whole body from the left ventricle,
via the aorta.
F Digested food is absorbed by blood through
the walls of the small intestine.
G Digested food is processed and stored in
the liver.
H Waste products (urea, water, salts etc.)
are removed from the blood in the kidneys.
I Blood supplies the rest
of the body with food and oxygen. It carries away waste
products. |
| 5. |
Heart
problems
There are three main factors that cause heart problems.
- High blood pressure when the heart is
put under strain because there is a narrowing of the blood
vessels.
- Poor diet leading to someone being overweight
and putting an overload on their heart.
- Smoking which reduces the amount of oxygen
being carried round the body. This causes the heart to beat
faster than normal and can lead to high blood pressure.
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