Question

How does the heart function?

Answer 1

Part 1: Cardiac cycle
The performance/ functioning of the human heart is represented in the form of the cardiac cycle. A series of alternate contraction and relaxation events drive the movement of blood through the chambers of the heart, blood vessels and to all the body parts as well. Cardiac cycle consists of two phases:

• Diastole
• Systole

Part 2: Stages of cardiac cycle
Cardiac cycle comprises three stages:
Joint diastole:

• All the four chambers of the heart are relaxed.
• The blood from pulmonary veins and vena cava gets filled in the left and right atria respectively.
• This creates pressure to force open both the arterio-ventricular valves (tricuspid and bicuspid valves).

Atrial systole:

• An action potential is generated by the SA node which stimulates both the atria to contract simultaneously.
• Blood gets emptied into ventricles.

Ventricular systole:

• The action potential is transmitted by the AV node, bundle of His (AV bundle), bundle branches and purkinje fibers to the entire ventricular musculature.
• This leads to the ventricular systole (contraction).
• The already opened atrio-ventricular valves (tricuspid and bicuspid) get closed to prevent the backflow of blood into the atrias.
• Increased intraventricular pressure causes opening of the semilunar valves.
• This causes the flow of blood, unidirectionally, into the pulmonary artery (for oxygenation) and aorta (for systemic circulation).

Part 3: Blood pumped per cycle
During each cardiac cycle, approximately 70 mL of blood is pumped out by each ventricle into respective blood vessels. Hence, around 5 litres of blood is pumped every minute throughout the body in a resting human.