SA node (sinoatrial node) – known as the heart's natural pacemaker The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.

The correct answer is option 'A' - SA Node.

The heart is a vital organ responsible for pumping blood throughout the body. It has its own electrical conduction system that initiates and coordinates the heartbeat. The electrical signals generated by this system cause the heart muscles to contract and relax, resulting in the rhythmic beating of the heart.

The initiation of the heartbeat begins with the Sinoatrial (SA) node, also known as the pacemaker of the heart. The SA node is a small cluster of specialized cells located in the upper part of the right atrium, near the opening of the superior vena cava. It sets the pace of the heartbeat by producing electrical impulses.

The SA node acts as the natural pacemaker of the heart because it generates electrical signals spontaneously without any external stimulation. It generates electrical signals at regular intervals, causing the atria to contract. This contraction allows the blood to be pushed into the ventricles.

The electrical signals produced by the SA node spread through the atria, causing them to contract and pump blood into the ventricles. This process is known as atrial systole. The electrical signals then reach the atrioventricular (AV) node, which is located in the lower part of the right atrium.

The AV node acts as a gatekeeper, delaying the electrical signals for a brief moment to allow the ventricles to fill with blood completely. This delay ensures that the atria and ventricles are coordinated properly, optimizing the efficiency of the heart's pumping action.

After the delay at the AV node, the electrical signals are conducted to the Bundle of His, which is a bundle of specialized fibers that transmit the signals to the ventricles. The Bundle of His divides into the left and right bundle branches, which then further divide into smaller fibers called Purkinje fibers.

The Purkinje fibers distribute the electrical signals throughout the ventricles, causing them to contract and pump blood out of the heart. This process is known as ventricular systole. The coordinated contraction of the atria and ventricles ensures the efficient flow of blood through the heart.

In summary, the initiation of the heartbeat begins with the SA node, which produces electrical impulses spontaneously. These electrical signals spread through the atria, reach the AV node, and then pass through the Bundle of His and Purkinje fibers to stimulate the contraction of the ventricles.

SA node