AS/A-level Biology - The Cardiac Cycle

• The cardiac cycle is when the atria and the ventricles contract and relax together so that the blood that has entered the heart is pumped out creating the heartbeat.
• This happens about 70 times every minute.
• The contractions are called systole and the periods of relaxation are called diastole.

• Blood at low pressure flows into the atria from the veins.
• When they are full, they both go into atrial systole.
• This makes the pressure in the atria higher.
• Right atrium is filled with deoxygenated blood from the body.
• Left atrium is filled with oxygenated blood from the lungs.
• When the atria contract, valves in the veins shut to prevent backflow.
• Blood is then pushed through the right and left atrioventricular valves by the high pressure blood in the atria and the low pressure of blood in the ventricles.

• Ventrical systole (lub)

• The pressure of the blood in the ventricles increases as they fill with blood.

• A+V diastole (dup)

• This pressure pushes open the semi lunar valves (tricuspid valves) at the base of the pulmonary artery and blood leaves the heart.

• The ventricles relax (diastole) and the pressure inside them lowers. The atria are relaxed too.

• The pressure in the pulmonary artery is higher than in the heart, but the semi lunar valve is pushed shut. This stops blood from going back into the heart.

When it comes to AS/A-level Biology, can you differentiate sinoatrial node and atrioventricular node?

• Small patch of heart muscle in the right atrium
• Controls the heart rate
• “The pacemaker” sends an electrical signal to the right and left atrium.
• This wave makes the atria contract.
• It does not pass through the ventricles because of a line of fibrous non-conducting tissue between the atria and the ventricles.
• The wave reaches the other patch of heart muscle called the Atrioventricular Node (AVN).

• This is the node that involves both the atria and ventricles.
• The AVN is deep in the centre of the heart muscle.
• It picks up the wave sent by the SAN and delays it by about 0.1s to let the atria finish contracting before the AVN sends the second wave.
• The AVN uses the Purkyne tissue (a conducting tissue) to the apex.
• The wave is sent up through the ventricles making them contract, pushing the blood through the semilunar valves, aorta and the pulmonary artery.

References:

1. https://biologydictionary.net/cardiac-cycle/

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