Question

Mention the stages of cardiac cycle.

Answer 1

Hint: The heart is a muscular organ located between the lungs in the chest cavity. The contraction and relaxation of the heart muscles results in pumping blood throughout the body. Then movement of muscles of the heart takes place in the form of a cycle called cardiac cycle. In common terms, the cardiac cycle is known as the heartbeat.

 Complete step by step answer: The heart is a muscular organ present in the chest cavity. The heart is responsible for pumping blood throughout the body through blood capillaries. There are two types of capillaries – the arteries, that carry oxygenated blood from the heart to the different parts of the body, and the veins that carry deoxygenated blood from different parts of the body to the heart.
Cardiac cycle is a sequence of events that begin at the end of a heartbeat and continue till the beginning of the next heartbeat. It can be described as the alternate contraction and relaxation of the cardiac muscles that results in pumping blood.
In order to understand the stages of cardiac cycle, we must first understand the anatomy of the heart.
The heart is located in the middle mediastinum. It is surrounded by a two layered sac called pericardium. The heart is divided into four chambers – the atrium on top and the ventricles below. There are two atrial chambers – left and right. Similarly, there are two ventricular chambers – left and right. The atrium and ventricle are separated by valves called atrioventricular valves. The valve between the right atrium and ventricle is known as the tricuspid valve, and the valve between the left atrium and ventricle is known as the bicuspid valve. These valves allow the flow of blood into the ventricles and prevent backflow of blood into the atria.
The right atrium receives deoxygenated blood from the body through two veins – the superior and inferior vena cava. On the wall of the right atrium, a group of cells called the Sinoatrial node or SA node is present. The SA node continually sends electrical signals that lead to the contraction of the heart.
The pulmonary artery connects the right ventricle to the lungs while the pulmonary vein connects the lungs to the left atrium.
The cardiac cycle events are organized into a systole or contraction phase and diastole or the relaxation phase. There are four main stages of the cardiac cycle that can be described as follows:
Atrial diastole – Blood enters the right artery from the vena cava(s) while in the left artery, blood enters through the pulmonary vein. At this stage the AV valves are closed. As the atria fill with blood, the pressure in the atria exceeds the pressure in the ventricles (on the same side). This results in opening of the AV valves.
Atrial systole – As the AV valves open, the ventricles start filling up with blood. The Sinoatrial node generates an electrical impulse that results in atrial contraction and expulsion of any amount of blood present in the atria into the ventricles.
Ventricular diastole – During the very early stages of ventricular diastole, the bicuspid and tricuspid valves are closed. The intraventricular pressure is low. This is known as isovolumetric relaxation. During this phase the heart is in a relaxed and expanded position. During the late diastole period, the intraventricular pressure becomes very low and the AV valves open. The ventricles receive an inflow of blood from the atria. Before the end of this period, ventricles receive all the blood from the atria and are in the pre-load condition.
Ventricular systole – During this phase, the electrical signal from the SA node arrives at the atria and eventually transmitted to the ventricles allowing them to contract. As the ventricles contract, the pressure in them exceeds that of the corresponding atria leading to the closure of the AV valves. This stage is known as Isovolumetric contraction. As the pressure mounts in the ventricles, the ejection phase of the cardiac cycle sets in. During this period, the left ventricle pumps blood into arteries to be carried to the body while the right ventricle pumps blood into the pulmonary artery to be carried to the lungs. After the ventricular systole, the amount of blood left in them is known as the afterload.
Post the ejection phase of ventricular systole, the ventricles re-enter the Isovolumetric relaxation phase. The cardiac cycle begins again.

Note: The cardiac cycle is the rhythmic movement of the atria and ventricles that results in heartbeat. The cycle contains four major phases of contraction or systole and contraction or diastole that causes the blood to move in and out of the chambers. Blood pressure fluctuates throughout the cycle. The coordinating events of the cardiac cycle results in efficient circulation of blood throughout the body.