What controls your heartbeat?

The heartbeat is controlled by a group of specialized cardiac muscle cells called the sinoatrial (SA) node, also known as the natural pacemaker of the heart. Located in the right atrium, the SA node generates electrical impulses that spread throughout the heart, causing it to contract and pump blood.

Here's a step-by-step explanation of how the SA node controls the heartbeat:

1. Generation of Electrical Impulses: The SA node spontaneously generates electrical impulses due to the unique properties of its cells. These cells exhibit rhythmic depolarization, which means their membrane potential decreases over time without any external stimuli.

2. Spread of Impulses: Once generated, the electrical impulses from the SA node travel through the heart tissue. From the SA node, the impulses spread to the nearby atrial muscle fibers, causing the atria (upper chambers of the heart) to contract.

3. Atrioventricular (AV) Node Delay: The electrical impulses then reach the atrioventricular (AV) node, which is located between the atria and ventricles (lower chambers of the heart). The AV node introduces a slight delay in the conduction of impulses, allowing the atria to fill completely with blood before ventricular contraction.

4. Activation of the Bundle of His: From the AV node, the impulses travel down a specialized pathway called the bundle of His, which divides into the left and right bundle branches. The bundle branches carry the impulses to the left and right ventricles.

5. Ventricular Contraction: The electrical impulses reach the Purkinje fibers, a network of specialized conduction fibers within the ventricles. The Purkinje fibers rapidly transmit the impulses throughout the ventricular muscle, causing the ventricles to contract and pump blood out of the heart.

6. Heart Rate Regulation: The SA node's inherent rhythmicity determines the heart rate. However, the heart rate can be influenced by various factors, such as the autonomic nervous system (sympathetic and parasympathetic divisions) and hormonal signals. These influences can increase or decrease the rate of impulse generation in the SA node, thereby altering the heart rate.

Overall, the sinoatrial (SA) node acts as the natural pacemaker of the heart, initiating and coordinating the electrical impulses that lead to the contraction of the atria and ventricles, resulting in a regular and synchronized heartbeat.