The cardiac cycle

Question 1

1. Atria contract ventricles relax

Answer 1

Blood from the lungs flows into the left atrium and blood from the body flows into the right atrium simultaneously.

The atria contract, increasing the pressure in the atria.

The blood in the atria is forced into the ventricles.
The ventricles are relaxed and fill with blood.

Question 2

2. Ventricles contract atria relax

Answer 2

The atria relax and the ventricles start to contract.

Contraction of the ventricles causes the pressure inside the ventricles to increase.

The pressure shuts the atrioventricular valves so that blood does not flow back into the atria.

The blood in the ventricles is forced out of the ventricles and out of the heart through the pulmonary artery or the aorta.

Question 3

3. Atria relax ventricles relax

Answer 3

The blood in the pulmonary artery and the aorta is at high pressure.

The pressure shuts the semi-lunar valves so that blood does not flow back into the ventricles.

Both the ventricles and the atria relax and the atrioventricular valves reopen.

Blood flows into the ventricles and the atria from the pulmonary vein and vena cava.