CIRCULATORY SYSTEM
The circulatory system consists of:

the heart, which is the muscular pump that keeps the blood moving.

the arteries, which carry blood away from the heart

the veins, which return blood to the heart

the capillaries, which are tiny blood vessels that are close to the body’s cells.
The heart pumps the blood around the body. The circulatory system in humans is known as a
double circulatory system because it consists of two systems: one that goes from the heart to
the lungs, and another that goes from the heart to the rest of the body. Tubes called blood
vessels carry the blood around the body. Arteries generally carry oxygenated blood, while veins
generally carry deoxygenated blood.
The system that carries the blood to the lungs is low pressure because the lungs are located
near to the heart. The system that carries the blood to the whole body is high pressure because
the blood must be pumped to the extremities of the body and back.
The heart pumps blood in three stages:
1. Blood flows into the atria.
2. Both atria contract at the same time, forcing blood into the ventricles. When they
contract, the tricuspid and mitral valves open to allow the blood through, and then they
close.
3. Both ventricles contract at the same time, forcing blood through the pulmonary artery
and aorta. When they contract, the pulmonary and aortic valves open to allow the blood
through, and then they close.
All this occurs every time the heart beats (approximately 60-80 times when a person is at rest).
The heartbeat is controlled by the pacemaker.
AN INCREASE IN PHYSICAL ACTIVITY MAKES THE HEART BEAT FASTER:
When the level of physical activity increases, the muscles are also working harder. Therefore,
they need more oxygen. The breathing rate increases to supply the body and muscles with
more oxygen, and the heart beats faster to pump more oxygenated blood to the muscles.
COMPONENTS OF BLOOD
COMPONENT
red blood cells
white blood cells
platelets
plasma
FUNCTION
Red blood cells contain haemoglobin, which oxygen binds to in the
lungs. Red blood cells transport oxygen throughout the body.
To defend the body against infection and to kill bacteria. There are
two kinds of white blood cells: phagocytes, which engulf bacteria,
and lymphocytes, which create antibodies and target bacteria.
To clot blood.
Transports blood cells, ions, soluble nutrients, hormones and
carbon dioxide (which are dissolved in the plasma)
DIFFERENCES BETWEEN ARTERIES AND VEINS
VEINS




Carry blood TO the heart
Have low pressure
Since there is low pressure, they have
valves to prevent backflow of blood
Generally carry deoxygenated blood.
ARTERIES




Carry blood AWAY FROM the heart
Have high pressure
Have thick muscular walls to withstand
pressure
Generally carry oxygenated blood.
The only exceptions to the last point are the pulmonary artery and pulmonary vein. In this case,
the pulmonary vein carries OXYGENATED blood from the lungs to the heart, and the pulmonary
artery carries DEOXYGENATED blood from the heart to the lungs.
The pulmonary ARTERY pumps
deoxygenated blood to the lungs,
where it can receive oxygen and
become oxygenated. The
pulmonary VEIN brings the blood
back to the heart, where the
aorta pumps it to the rest of the
body. The oxygen in the blood is
used up, and the vena cava
brings the deoxygenated blood
back to the heart, and so the
process repeats itself.
The right and left ventricles and
atria are so labelled because the
heart is usually drawn as if being
observed from the front of it.
Coronary heart disease is the narrowing or blockage of the coronary arteries. This can be due
to:
 A build-up of cholesterol and fatty deposits (called plaques) on the inner walls of the
arteries. This can be prevented by eating more healthily.
 Stress
 Smoking (chemicals in tobacco can damage blood vessels) – this can be prevented by
not smoking.