How to Understand the Cardiac Cycle

There are many concepts in A level Biology where a small adjustment in thought process and understanding can lead to sudden clarity. This is certainly true of the cardiac cycle.

Start with simple concepts and build upon them. Don’t start with looking at the human heart, four chambers, four valves and a great deal of plumbing.

Begin with visualising a single flexible tube, filled with fluid, now imagine you squeeze the middle. The volume decreases, therefore the pressure increases and the fluid will move to where the pressure is lower. Remember this principle. Now consider how we could increase the efficiency by preventing flow in one direction. Introduce two flaps of flesh on the inside of the pipe. These are pushed open and pushed closed by the flow of blood, we call these valves. Most students think of radiator valve or a system where the active opening permits flow, this is the source of confusion. The valves in your circulatory system are completely passive (they are pushed open by the flow of blood).

Now let’s go back to the valve we have added to create two regions of the pipe. On one side of the valve we have a chamber called the atrium, on the other we have a chamber called the ventricle. Now when the muscle (cardiac) surrounding the chamber (ventricle) contracts then volume of the chamber decreases, which increases the pressure (remember the principle) and the blood moves back toward the atrium which pushes the valve between the atrium and the ventricle closed.

Blood leaving the ventricle pushes open the valve between the ventricle and the aorta. The blood flowing through the valve into the aorta increases the pressure and pushes the walls of the aorta outwards. As the ventricle relaxes, the recoil of the aortic walls pushes blood back to the ventricle, which pushes the valve closed.

Think of the four-chambered human heart as two, two-chambered hearts joined back to back, as they pump blood round two completely separate systems (the systemic and the pulmonary).

Remember, as volume decreases, pressure increases. Blood pushes valves open, and pushes them closed. Blood flows to the lowest pressure. Stick to those principles and when you understand them, apply the knowledge to the human heart.