• 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

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.