The cardiac cycle and blood cells

Last time

Before we start: last time, in The heart, double circulation and the heartbeat, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

One heartbeat, three phases

In less than a second, your heart fills, squeezes and relaxes. Valves snap open and shut in a precise order, driven by pressure changes. If you can read a pressure graph, you can say exactly which valves are open at any moment. After that, we look at the cells the heart is pumping.

Atrial systole

During atrial systole the atria contract. Their pressure rises above the ventricles, so the atrioventricular valves are open. The last bit of blood is pushed into the ventricles. The semilunar valves stay shut, because the pressure in the aorta is higher than in the ventricles. This phase is short, about zero point one of a second, because the atria only need to top up the ventricles.

Ventricular systole

The ventricles contract. Ventricular pressure rises above the atrial pressure, which snaps the atrioventricular valves shut. This makes the first heart sound. When ventricular pressure rises above the pressure in the aorta, the semilunar valves open, and blood is forced out into the arteries. This phase lasts about zero point three seconds. The ventricles are emptying, while the atria begin to fill with blood again.

Diastole

The ventricles relax and their pressure falls below the aortic pressure. The semilunar valves shut, with the second heart sound. Pressure in the ventricles falls below that in the atria, and the atrioventricular valves open. Blood flows from the atria into the ventricles, even before the atria contract. Diastole is the longest phase, about zero point four seconds. Adding the three phases gives a cycle of zero point eight seconds, which is seventy-five beats a minute.