Cardiac Cycle

It refers to the repeating pattern of contraction and relaxation of the heart. Systole means contraction and diastole means relaxation. The first phase of the cardiac cycle is Joint Diastole where all four chambers of the heart are in a relaxed state. Blood enters the atria through the vena cavae and pulmonary veins. Pressure builds up as the atria get completely filled due to which the AV valves open up and blood rushes into the ventricles. Semilunar valves remain closed at this time. The sino-atrial node generates an action potential that causes the contraction of both atria known as the atrial systole. Blood flow into the ventricle increases by 30%. This action potential travels to the ventricle by the AVN, AV bundle, and the bundle of His. As a result the ventricles contract. This is called ventricular systole. The intraventricular pressure increases causing the closure of the AV valves and the opening of the semilunar valves. The blood rushes out of the ventricles into the pulmonary artery and aorta. The pressure in the ventricles decreases and is called ventricular diastole. The above three steps keep on repeating and constitute the cardiac cycle. There occur 72 cardiac cycles in a minute. One cycle takes 0.8 seconds to occur. The volume of blood pumped out by each ventricle in a cardiac cycle is called as stroke volume which is about 70ml per ventricle. The volume of blood pumped by each ventricle per minute is called cardiac output which is approximately 5L.

Phases of Cardiac Cycle

Following are the different phases that occur in a cardiac cycle:

1. Joint Diastole: In this stage, all the chambers of the heart are in a relaxed state. Both the aortic semilunar valve and pulmonary semilunar valve are closed but the atrioventricular valves remain open, thus causing the chambers of the heart to relax.
2. Atrial Systole: In this phase, blood flows from both atria to both ventricles due to the contraction of the atria.
3. Isovolumetric Contraction: At this stage, ventricles begin to contract. The atrioventricular valves and the semilunar valves remain closed and there is no transformation in volume.
4. Ventricular systole: Here ventricles contract and get emptied. Pulmonary semilunar and aortic semilunar valves get opened.
5. Isovolumetric Relaxation: In this phase, no blood enters the ventricles, and consequently, pressure decreases, ventricles stop contracting and begin to relax. Now due to the pressure in the aorta – pulmonary semilunar and aortic semilunar valves get closed.
6. Ventricular Filling Stage: In this stage, blood flows from the atria into the ventricles. It is altogether known as one stage i.e. Joint Diastole. 

Duration of Cardiac Cycle

Our heart beats 72 times in a minute i.e. 72 cardiac cycles occur in one minute or 60 seconds. Therefore time taken for one cycle to occur is 60/72 i.e. 0.8 seconds. Time taken by the various steps of a cardiac cycle is as follows:

• Atrial systole- 0.1s
• Ventricular systole- 0.3s
• Joint diastole- 0.4s
• Atrial diastole- 0.7 seconds
• Ventricular diastole-0.5 seconds. 

Living organisms are in constant need of nutrients, oxygen, and other essential substances. Also, waste products produced in the body through the various metabolic reactions need to be removed simultaneously. All this transportation is done by the circulating fluid of the body known as blood. Plasma constitutes about 55% of the blood. Cellular elements like RBCs, WBCs, and Platelets form the remaining 45%. The blood is circulated throughout the human body in various types of blood vessels like arteries, veins, and capillaries and is pumped by the heart. Arteries have thick walls and narrow lumen. Blood flows in it under high pressure. Arteries carry blood away from the heart. Veins have thin walls and wider lumen. The blood flows in it under low pressure and has valves to prevent its backflow. Veins carry blood toward the heart. The capillaries are only one cell thick.