The heart and the cardiac cycle: pressure graphs made simple

The one rule for valves

Valves open when the pressure behind them is greater than the pressure in front, and close when the pressure in front is greater. Atrioventricular (AV) valves sit between atria and ventricles; semilunar valves sit between ventricles and the arteries. If you can compare two lines on a graph, you can answer every valve question.

Walking through the cycle

• Atrial systole: atria contract, pressure in atria exceeds ventricles, AV valves open and blood tops up the ventricles.
• Ventricular systole: ventricles contract, ventricular pressure rises above atrial pressure so AV valves close. When it exceeds arterial pressure, semilunar valves open and blood is ejected.
• Diastole: ventricles relax, ventricular pressure falls below arterial pressure so semilunar valves close. When it falls below atrial pressure, AV valves open and passive filling begins.

Reading pressure–time graphs

Find the points where lines cross — each crossing is a valve event. The aorta line never falls to zero because elastic tissue in the artery wall recoils, maintaining pressure. The left ventricle reaches a much higher peak than the right because its thicker muscular wall pumps blood around the whole body rather than just to the lungs.

Cardiac output calculations

Cardiac output = stroke volume × heart rate. Heart rate can be found from a graph: count the time for one complete cycle, then heart rate = 60 ÷ cycle length in seconds. A cycle of 0.8 s gives 75 beats per minute. Always check units — stroke volume is often in cm³ and output in dm³ per minute.