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First teaching 2023

First exams 2025

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The Walls of the Heart (CIE AS Biology)

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The Walls of the Heart

  • The muscular walls of the atria are thinner than those of the ventricles
  • When the atria contract, the thin muscular walls do not generate much pressure, but enough to force blood down into the ventricles, through the atrioventricular valves
  • In contrast, the walls of the ventricles are thicker and more muscular
  • Following contraction of the atria, the ventricles contract and squeeze blood inwards, increasing its pressure and pushing it out of the heart through right and left semilunar valves

Left & right ventricle

  • The muscle of the left ventricle is significantly thicker than the right ventricle
  • This is because the blood leaving the right ventricle travels less distance than blood leaving the left ventricle
  • The blood pumped out from the right ventricle travels to the lungs, whereas blood leaving the left ventricle has to travel to the rest of the body to deliver oxygen for respiration.
  • To reach the rest of the body, the blood leaving the left ventricle must be under high pressure
  • This is generated by the contraction of the muscular walls of the left ventricle
  • The right ventricle generates less pressure from the contraction of its thinner walls, as blood only has to reach the lungs

Worked example

A graph showing pressures changes in the heart is shown below:

Pressure changes in the Left Atrium and Ventricle graphExplain the differences in pressure between left atrium and ventricle in the graph.

The walls of the left atrium are thin, so the pressure generated by their contraction is low. Low pressure is sufficient because blood is forced only a short distance; from the left atrium down to the left ventricle. The muscular walls of the left ventricle are much thicker and generate much higher pressure, as shown on the graph. This is because much more pressure is generated when this thick muscle contracts and squeezes blood with enough force to reach the rest of the body.

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Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!