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 sides, 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 the 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)

Cardiac Cycle

The cardiac cycle is the process that occurs when the heart beats. There are 2 parts:

Diastole - The heart ventricles relax to allow blood to fill the heart

Systole - Blood is pumped out of the heart as a result of the ventricles contracting

When both these processes are completed, it is known as 1 cardiac cycle. The process is continuous.

Cardiac output

Cardiac output (CO) is the volume of blood the heart can pump out, usually in litres per minute. CO depends on stroke volume and heart rate. The quicker the heart beats, the higher cardiac output will be as the heart will pump more blood around the body.

This is shown as:

Cardiac Output (Q) = Heart Rate (HR) x Stroke Volume (SV)


There are two circuits within the body through which blood flows. Inbetween each circuit the blood returns to the heart.


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.


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:


  • 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


  • 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


  • 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 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


  • These are fragments of cells which help blood to clot at wounds


  • This is a straw coloured liquid which carries all the blood cells as well as hormones, waste products and digested foods

Blood Pressure

Blood exits the heart under pressure. As the heart beats it forces blood into the arteries and around the body. As the blood passes through the systemic circulation, as it gets further away from the heart, it decreases in pressure. This is why 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:


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 blood pressure

Having inreased blood pressure increases your risk of suffering from angina, heart attack, and stroke.