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The Circulatory System Explained
The circulatory system consists of the heart, blood vessels and blood itself. Its
function is three-fold. Transport, temperature control and protection.
Transport - of substances such as Oxygen and Carbon Dioxide
Control of body temperature - blood moves towards the skin to cool us down, as excess heat can escape
easier
Protection - in the form of our immune system. Blood carries white blood cells which help fight disease. Platelets
also clot the blood to stop us from bleeding
The unique thing about the human circulatory system is that we have a double pump (the heart) and a double
circulation!
The Heart
The heart is made of cardiac muscle and has two side, right and left, which is why it is called a double pump.
Overall the heart has four chambers, two on the left and two on the right.
This is how it works:

Blood enters the atrium on right side of the heart (deoxygenated)
 It moves down into the right ventricle
 Blood is pumped out of the heart to the lungs to pick up Oxygen and get rid of Carbon Dioxide
 Blood returns to the heart and into the left atrium (oxygenated)
 It moves through to the left ventricle
 From here the blood is pumped out to the body, via the aorta (largest artery in the body)
Circulation
There are two circuits within the body through which blood flows. In between each circuit the blood returns to the
heart.
Systemic
This circuit takes oxygenated blood from the left side of the heart, around the body. When the blood returns to the
right side of the heart, it is deoxygenated, as the oxygen has been mostly used by the muscles and organs in
order to make energy.
Pulmonary
This circuit takes deoxygenated blood from the right side of the heart to the lungs where it can pick up more
oxygen. It then returns this newly oxygenated blood to the left side of the heart where the cycle begins again!
Blood Vessels
There are three types of blood vessels within the circulatory system:
Arteries:

 Carry blood away from the heart
Carry oxygenated blood (with the exception of the pulmonary artery which carries deoxygenated blood
to the lungs)
 Thick, strong, elastic walls
 Smaller arteries are called arterioles
Veins

 Carry blood back to the heart
Carry deoxygenated blood (with the exception of the pulmonary vein which carries oxygenated blood
from the lungs to the heart)
 Contain valves to make sure the blood travels in the right direction when under lower pressures
 Thinner walls
 Smaller veins are called venules
Capillaries

 The smallest blood vessels which connect veins and arteries
Travel deep inside muscles and organs to supply the nutrients and oxygen
 Have walls only one cell thick to allow exchange of these substances
Blood
Blood has four main components:
Red blood cells

These are disc shaped cells which carry haemoglobin to combine with Oxygen
White blood cells

These fight against disease by using antibodies and antitoxins
Platelets

These are fragments of cells which help blood to clot at wounds
Plasma

This is a straw coloured liquid which carries all the blood cells as well as hormones, waste products and
digested foods
Blood Pressure
Blood in under pressure because the when the heart pumps it forces blood into the arteries. As the blood passes
through the systemic circulation it decreases in pressure, the further it gets from the heart. This is why the veins
contain valves, to prevent the blood from flowing back the wrong way.
The reason we have a pulse is because of the changes in pressure when the heart beats and then relaxes. The
pressure when the heart beats is known as the systolic pressure and is the higher number. When the heart
relaxes the pressure drops and is called the diastolic pressure. This is why blood pressure, when measured using
a sphygomomanometer, is shown as two numbers one over the other, like this:
120/80
Blood pressure can be affected by lots of things. It is not good to have either high blood pressure, or low blood
pressure. Both can cause health risks. The following can affect your blood pressure:


Age - blood pressure usually increases with age as the artery walls get furred up, decreasing the space
within them
 Gender - Men often experience higher blood pressure
Exercise - whilst exercising the heart pumps harder and faster, increasing blood pressure. However, in
the long-term, exercise decreases blood pressure
 Stress - Stress raises the blood pressure
Having an increased blood pressure increases your risk of suffering from angina, heart attack and stroke.