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OCR Biology Revision - Part 5 - The Heart

Updated on March 7, 2014

Internal Structure of the Heart

The heart consists of 4 chambers:

The Atria

The right atrium:

  • This is one of the two upper chambers of the heart.
  • This chamber receives deoxygenated blood from the body, through the vena cava vein.
  • In the wall of the right atrium is where the sinoatrial (SA) node is located.
  • The sinoatrial node is a mass of specialised tissue which initiates the impulses which stimulate the heart beat.
  • At the bottom of the right atrium is where the atrioventricular (AV) node is located.
  • The atrioventricular node consists of specialised heart tissue which delays the impulse generated by the SA node.

The Left Atrium:

  • The second of the two upper chambers of the heart.
  • Oxygenated blood from the lungs flows into the left atrium through the pulmonary vein.
  • The muscles of both atria are very thin, this is because they do not need to pump the blood very far (only to the ventricles) and therefore they do not create much pressure.

A basic diagram of how the blood enters and leaves the heart (blue lines mean deoxygenated and red lines mean oxygenated).
A basic diagram of how the blood enters and leaves the heart (blue lines mean deoxygenated and red lines mean oxygenated).

The Ventricles

  • When in the atria, the blood flows down into the ventricles through the atrioventricular valves.
  • The atrioventricular valves are small cup shaped flaps of tissue located between the atria and the ventricles.
  • When the ventricles contract the atrioventricular valves fill up with blood and snap shut, this prevents blood flowing back into the atria.
  • The ventricles are separated by a thick wall of muscle called the septum.
  • The septum ensures that the deoxygenated blood in the right ventricle and the oxygenated blood in the left ventricle are kept separate.
  • When the ventricles contract the deoxygenated blood in the right ventricle is pushed through the pulmonary artery which leads to the lungs (in order for the blood to be oxygenated)
  • The oxygenated blood in the left ventricle flows through the aorta which leads to a number of arteries that supply the rest of the body with the oxygenated blood.
  • At the base of the pulmonary artery and aorta are semilunar valves which prevent the blood from flowing back into the ventricles as the begin to relax.
  • The walls of the right ventricle are thicker than that of the atria but still relatively thin. This is because it only needs to pump the blood to the lungs and the lungs are very close to the heart, so it doesn't need to travel far. Also, the blood pressure needs to be kept low because the capillaries in the lungs that the blood will be pumped through are very thin and can easily burst if they are under too much pressure.
  • The walls of the left ventricle are very thick (two or three times bigger than that of the right ventricle), this is because the left ventricle needs to create a lot of blood pressure in order to pump the blood through the aorta and to the rest of the body.

How to read an EGC

  • The waves of a ECG trace are labelled with the letters P, Q, R, S and T.
  • Below is the shape of a healthy persons ECG trace.
  • An elevation of the ST section of the trace can indicate a heart attack.
  • A small and unclear P wave can indicate atrial fibrillation (the heart beat is not coordinated).
  • A deep S wave can indicate an abnormal ventricular hypertrophy (an increase in muscle thickness).

Electrocardiograms (ECG's)

The human body is mainly salty water and so it can conduct electricity.

The electrical activity, initiated by the SA node, that spreads across the heart also spreads outwards away from the heart.

This causes the voltage of the skin to change very slightly and then these potential differences are then detected at various points on the body.

The electrical signals that are produced by the heart are picked up by skin and can be converted into a trace.

The trace of a healthy person has a particular shape and the shape of the trace can sometimes indicate when a part of the heart muscle is not entirely healthy (for instance if a person has an irregular heartbeat).

Key Definitions

Purkyne Tissue:

  • Specially adapted muscle fibres that conduct the wave of excitation from the atrioventricular node down the septum and to the ventricles.

Atrial Systole:

  • The period when the atria contract due to the electrical impulse travelling down cardiac muscle of the heart.

Ventricular Systole:

  • A short period when all for heart valves are closed and the ventricles contract.


  • The period where the blood flows into the heart through the veins and then flows from the atria to the ventricles.

Cardiac Cycle:

  • The sequence of events in one heartbeat.


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      Olivia 2 years ago

      Your website was extremely helpful in enabling me to understand me about an Electrocardiogram and their uses.

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      Jefferson Taylor 3 years ago

      This is really owesome. I get all my information needed for my assignment.