Transport in animals: (f) the cardiac cycle

The idea

The cardiac cycle is the sequence of events in one complete heartbeat, described through the coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles. Teach it as pressure-driven flow: blood moves from higher to lower pressure, and valves open and close passively in response to pressure differences. Atrial systole follows ventricular diastole, pushing the remaining blood through the open atrioventricular valves; ventricular systole then raises ventricular pressure, closing the atrioventricular valves (first heart sound) and opening the semilunar valves once ventricular pressure exceeds arterial pressure. Ventricular diastole lowers ventricular pressure, closing the semilunar valves (second heart sound) and reopening the atrioventricular valves.

What earns the marks

• States that the cardiac cycle is the sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles in one heartbeat.
• Explains that blood flows from regions of higher pressure to lower pressure, so pressure changes drive flow through the heart.
• Describes atrial systole: atria contract, raising atrial pressure and pushing blood through the open atrioventricular valves into the ventricles.
• Describes ventricular diastole: ventricles relax, ventricular pressure falls below arterial pressure, semilunar valves close, and when ventricular pressure falls below atrial pressure the atrioventricular valves reopen.
• Explains that valve opening and closing is passive, driven by pressure differences across the valve, not by active muscle contraction of the valve itself.
• Interprets a pressure–time graph, identifying systole and diastole and the points at which the atrioventricular and semilunar valves open and close.

Common mistakes and how to put them right

• Thinking valves are actively opened and closed by muscles.

  Put it right by explaining that valves open and close passively in response to pressure differences across them.

• Believing blood is actively pumped by the atria into the arteries.

  Put it right by stating that atria deliver blood to the ventricles and the ventricles generate the pressure that ejects blood into the arteries.

• Confusing systole with relaxation.

  Put it right by defining systole as contraction and diastole as relaxation.

• Assuming the semilunar valves open during atrial systole.

  Put it right by linking semilunar valve opening to ventricular pressure exceeding arterial pressure during ventricular systole.

Exam tips

• For each valve event, name the pressure change that causes it, since questions often test the link between pressure and valve action rather than the event alone.
• When reading a pressure–time graph, locate the crossover points between atrial, ventricular and arterial pressure curves, as these mark valve opening and closing.
• Sequence the stages explicitly (atrial systole, ventricular systole, ventricular diastole) so the order of events is unambiguous.