How Does the Heart Work?

A heartbeat

The heart has four chambers - two atria and two ventricles. The walls of these chambers are made mainly of special heart muscle. The chambers have to contract (squeeze) in the correct order for the heart to pump blood correctly with each heartbeat.
 

The sequence of each heartbeat is as follows.

• The sinoatrial node (SA node) in the right atrium is like a tiny in-built 'timer'. It fires off an electrical impulse at regular intervals. (About 60-80 per minute when you are resting and faster when you exercise. This controls your heart rate.) Each impulse spreads across both atria which causes them to contract. This pumps blood through one way valves into the ventricles.
  
  
• The electrical impulse gets to the atrioventricular node (AV node) at the lower right atrium. This acts like a 'junction box' and the impulse is delayed slightly. Most of the tissue between the atria and ventricles does not conduct the impulse. However, a thin band of conducting fibres called the atrioventricular bundle (AV bundle) acts like 'wires' and carries the impulse from the AV node to the ventricles.
  
  
• The AV bundle splits into two - a right and left branch. These then split into many tiny fibres (the Purkinje system) which carry the electrical impulse throughout the ventricles. The ventricles contract and pump blood through one way valves into large arteries.
  • The arteries going from the right ventricle take blood to the lungs.
  • The arteries going from the left ventricle take blood to the rest of the body.
  
  
• The heart then rests for a short time (diastole). Blood coming back to the heart from the large veins fill the atria during diastole.
  • The veins coming into the left atria are from the lungs (full of oxygen).
  • The veins coming into the right atria are from the rest of the body (depleted of oxygen).

The blood supply to the heart

Like any other muscle, the heart muscle needs a good blood supply. The coronary arteries take blood to the heart muscle. These are the first arteries to branch off the aorta - the large artery that takes blood to the body from the left ventricle.

The right coronary artery mainly supplies the muscle of the right ventricle.

The left coronary artery quickly splits into two and supplies the rest of the heart muscle.

The main coronary arteries divide into many smaller branches to supply all the heart muscle.

© EMIS and PIP 2006   Updated: June 2006   PRODIGY Validated