The heart is a pumping organ that keeps the blood flowing
around the body, the walls of the heart are made mostly out of
The Human heart beats 100 000 times a day on average.
You can feel a pulse in your wrist or neck (where the vessels are close to the surface). This is
the blood being pushed by another beat.
When doctors use a stethoscope, they actually hear the valve closing.
What are valves?
The heart has valves to make sure that blood goes
in the right direction - they prevent the blood from
The heart has four chambers, and this is how it uses them:
Deoxygenated blood comes from the vena cava into the right atrium,
which contracts to push the blood into the right ventricle.
Oxygenated blood comes from the pulmonary vein and into the left atrium, the left
atrium contacts pushing the blood into the left ventricle.
The right ventricle contracts, pushing the blood
to the pulmonary artery and out of the heart.
The left ventricle contracts, pushing the blood to the aorta and ot of the heart.
Once the blood has left the heart through the pulmonary artery
or the aorta, the blood pumps to the organs or cells through
arteries and returns through veins.
The cycle restarts with the atria filling up again.
The main function of the circulatory system is to get food and
oxygen to every cell in the body - transporting vital resources.
Another function is the removal of waste products such as carbon
dioxide and urea - removing waste products.
The Double Circulatory System
The body has a double circulatory system - two circuits joined together.
The first circuit pumps deoxygenated blood to the lungs, where the blood becomes oxygenated. The blood then
travels back the heart.
The second system pumps oxygenated blood to all the organs and cells, and then travels back to the heart deoxygenated just
to be pumped back to the lungs.
There are 3 types of blood vessel:
These carry blood AWAY from your heart.
The heart pumps the blood
out at high pressure so the
artery walls are strong and
The walls are thick
compared to the hole
in the middle of the
artery, called the
They contain thick layers of muscle to make them
strong, and elastic fibres to allow them to stretch
and spring back,
These carry blood TO your heart.
Capillaries eventually join up to form veins.
The blood in veins is at a lower pressure so the walls don't need to be as
thick as the artery walls. They also have a bigger lumen than arteries to help
the blood flow despite the low pressure.
Veins have valves to keep the blood flowing in the right direction.
These are involved in the exchange of material at tissues.
Capillaries are REALLY small, too small to see infact. They carry blood really close to every cell in
the body to exchange substances with them.
They have permeable walls, so substances can diffuse in and out of them. They supply
food and oxygen, and take away waste such as carbon dioxide.
Their walls are usually only one cell thick. This increases the rate of diffusion by decreasing the
distance over which it occurs.
They have a very small lumen.
Red blood cells
The job of the red blood cells is to carry oxygen from the lungs to all the cells in the body.
They have a concave shape to give a large surface area for absorbing oxygen.
They don't have a nucleus, this is an adaptation that allows them to carry more oxygen.
Red blood cells contain a pigment called haemoglobin, it's what makes the cell look red.
In the lungs, haemoglobin combines with oxygen to make oxy-haemoglobin. In body tissues, the reverse happens, oxy-haemoglobin splits up into haemoglobin and oxygen, releasing oxygen into the cells.
White blood cells
White blood cells can change shape o ingest unwelcome microorganisms.
They produce antibodies to fight microorganisms, as well as antitoxins to neutralise any toxins produced by bacteria.
Unlike red blood cells, white blood cells have a nucleus.
Platelets are small fragments of cells. They have no nucleus.
They help the blood clot at a wound, and stop blood pouring out and microorganisms from getting in.
A lack of platelets can cause excessive bleeding and bruising.
Plasma is a pale straw-coloured liquid which carries just about everything:
It carries red and white blood cells, platelets, nutrients like
glucose and amino acids, carbon dioxide from the organs to the
lungs, urea from the liver to the kidneys, hormones, antibodies
Blood clotting and transfusions
If you get a cut, you don't want to lose all of your blood - that is why
clotting is so handy. Sometimes an injury is so bad and you lose lots of
blood so you need to replace it with a transfusion.
When you're injured, your blood clots to prevent too much
bleeding. A clot is a mesh of protein fibres that 'plugs' the
Clots are formed by a series of chemical reactions that take place when platelets in
your blood are exposed to a damaged blood vessel.
Too little clotting means you can bleed to
death (or more likely get many bruises). Too
much clotting can cause strokes and deep vein
People who are at risk of strokes and DVT can
take drugs to reduce their risk. Warfarin, heparin and
aspirin all help prevent the blood from clotting.
Haemophilia is a genetic condition where the blood
doesn't clot easily because a clotting factor is missing in
the body - this missing clotting factor can be injected.
If you had an accident or a surgery, you may lose a lot of blood and you
may need a transfusion. But you need a certain blood group to be
You can get blood from a blood donor - but it must be a compatible blood type to yours. This is because:
On your red blood cells, there are special antigens. These determine your blood type. An
antigen is a substance that can trigger a response from the immune system.
There are two main antigens - A and B. If you have A antigens, you are group A.
If a & B antigens, you are AB. If only B antigens, you are B, and if you have no
antigens you have group O.
However, the blood plasma contains Anti-A or anti-B antibodies.
If an Anti-A antibody meets an A antigen, the blood
clumps together. This is agglutination (and it's not
If an Anti-B body meets a B antigen, the blood clumps together too.
So, you have to put the patient's antibodies and the donors antigens to mind before performing a
transfusion, or there may be some serious damage caused.