Pressure and volume changes and associated valve movements during the…

The idea

One cardiac cycle has three stages. In diastole all chambers relax, ventricular pressure falls below atrial pressure so the atrioventricular valves open and the ventricles fill passively, and ventricular volume rises. In atrial systole the atria contract, atrial pressure rises above ventricular pressure, the atrioventricular valves are already open and atrial contraction completes ventricular filling. In ventricular systole the ventricles contract, ventricular pressure rises sharply above atrial pressure and closes the atrioventricular valves; when it rises above the pressure in the aorta and pulmonary artery the semilunar valves open and blood is ejected, so ventricular volume falls. The essential principle is that valves are passive: each opens when the pressure behind it is greater than the pressure in front, and closes when that reverses.

What earns the marks

• Stating that a valve opens when the pressure behind it is greater than the pressure in front of it.
• Identifying the point where two pressure curves cross as a named valve opening or closing.
• Linking contraction of a chamber to a decrease in its volume and an increase in its pressure.
• Explaining that the closed atrioventricular valves during ventricular systole prevent backflow into the atria.
• Calculating heart rate in beats per minute from the duration of one cycle read from the graph.

Common mistakes and how to put them right

• Saying valves open and close by themselves, or that muscles open them.

  Put it right: Valves are passive and respond to pressure differences.

• Naming the wrong valve because the atrial, ventricular and aortic curves were never identified first.

  Put it right: Label the curves before answering.

• Describing ventricular systole as the stage when blood flows into the ventricle.

  Put it right: Blood enters the ventricle during diastole and atrial systole, and leaves during ventricular systole.

• Giving heart rate in beats per second, or dividing by the wrong time value when converting.

  Put it right: Read one full cycle in seconds and use 60 divided by that value.

• Claiming ventricular pressure is always higher than atrial pressure.

  Put it right: During diastole and atrial systole atrial pressure exceeds ventricular pressure, which is why the atrioventricular valves open.

Exam tips

• Label the three curves on a cardiac cycle graph before answering anything: atrium, ventricle, aorta.
• Every valve answer needs the words pressure greater than; the valve shuts to stop backflow is only half an answer.
• To find heart rate, read the length of one full cycle in seconds and work out sixty divided by that value.