What causes heart beat?

The heartbeat is primarily caused by the electrical impulses generated by the sinoatrial (SA) node, also known as the natural pacemaker of the heart. The SA node, located in the right atrium, periodically produces electrical impulses that spread throughout the heart, causing its coordinated contraction. Here's a detailed explanation of what happens during a heartbeat:

1. Depolarization of SA Node: The SA node spontaneously generates an electrical impulse due to the influx of positively charged ions (sodium and calcium) into the cells. This influx causes the cells in the SA node to depolarize, meaning their electrical potential becomes more positive.

2. Atrioventricular (AV) Node Delay: The electrical impulse travels from the SA node through the right and left atria, causing them to contract and push blood into the ventricles. As the impulse reaches the atrioventricular (AV) node, which lies between the atria and ventricles, there's a slight delay. This delay allows the atria to complete their contraction before the ventricles begin to contract.

3. Activation of Bundle of His: The electrical impulse then travels down a specialized pathway called the bundle of His, which divides into the left and right bundle branches. These branches carry the electrical impulse to the left and right ventricles.

4. Ventricular Depolarization and Contraction: As the electrical impulse reaches the ventricles, it causes their cells to depolarize as well. This depolarization triggers the contraction of the ventricles, forcefully pumping blood out into the arteries (aorta and pulmonary artery).

5. Repolarization and Relaxation: After ventricular contraction, the ventricles undergo repolarization, during which the positively charged ions inside the cells return to their resting states. Repolarization causes the ventricles to relax, allowing them to fill with blood again in preparation for the next heartbeat.

6. Cardiac Cycle: The sequence of events described above constitutes one complete cardiac cycle:

- Systole: The phase during which the ventricles contract and pump blood out of the heart.

- Diastole: The phase during which the ventricles relax and fill with blood.

The heartbeat is regulated by the autonomic nervous system, which can increase or decrease the heart rate depending on the body's needs, such as during exercise or rest.