The heart

Click images to enlarge.


The circulatory system is made up of three main parts:

  1. The heart
  2. The blood vessels
  3. The blood

Blood is transported in 2 different circuits:

  1. The pulmonary circuit carries deoxygenated blood from the heart to the lungs (where it picks up oxygen) and back again to the heart. Deoxygenated blood is blood with some of its oxygen taken away
  2. The systemic circuit carries oxygenated blood from the heart to the body (where it delivers oxygen to the working muscles) and back again to the heart. Oxygenated blood is blood packed with oxygen.

systemic and pulmonary circuits
The heart is about the size of your clenched fist and is located just behind the sternum (in between the lungs). It is tilted slightly to the left.
heart position
During exercise, blood supply to some organs is reduced and transported instead to muscles that are working much harder. Blood passes through the heart as follows (also refer to the diagram underneath):

  • Deoxygenated blood (returned from body) is pumped into right atrium
  • Blood is then forced into the right ventricle
  • The deoxygenated blood is then forced into the lungs
  • Blood picks up oxygen as it passes through the lungs
  • The oxygenated blood is then sent to the left atrium from the lungs
  • The oxygenated blood is pushed into the left ventricle
  • It is then sent to different parts of the body

heart structure
Please note that the above picture is a simplified representation of the heart; see the diagram below for the heart’s actual structure.

Passage of blood – structures in heart

(Deoxygenated Blood)
Vena Cava
Right Atrium
Tricuspid Valve
Right Ventricle
Pulmonary Valve
Pulmonary Artery
(Oxygenated Blood)
Pulmonary Vein
Left Atrium
Bicuspid Valve
Left Ventricle
Aortic Valve
heart structure 2

Heart rate

Every time the left ventricle contracts, it forces blood away from the heart to the rest of the body. The blood is forced away, not in a continuous flow, but in a series of small surges which create a wave-like expansion in the arteries. These waves of expansion are called the pulse of the heart, and can be felt at a number of sites around the body including:

  • The radial pulse – below the thumb in the wrist
  • The carotid pulse – in the groove on either side of the windpipe in the neck

Heart rate is measured in beats per minute, and the average adult has a resting pulse rate between 60 and 80 bpm. During exercise, the heart rate increases considerably and can increase to over 200 bpm.

Stroke volume

Stroke volume refers to the amount of blood that is pumped out of the ventricles every beat. During exercise, stroke volume increases – more blood is pumped out every beat. This increase is due to many factors:

  • More blood is sent back to the heart, due to muscles squeezing blood in the veins
  • As more blood is sent back to the heart, it stretches as it fills up
  • As the muscle fibres of the heart stretch, they contract more strongly, pumping more blood out of the heart

Cardiac output

Cardiac output refers to the amount of blood pumped out of the ventricles every minute.

Cardiac Output = Heart Rate x Stroke Volume

As both heart rate and stroke volume increase during exercise, cardiac output must also increase.