Simply put, the heart is a pump made of muscle tissue. Like all muscles, the heart needs a source of energy and oxygen to function. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.
Two different types of cells in our heart enable the electrical signal to control our heartbeat:
ü Conducting cells carry our heart's electrical signal.
ü Muscle cells enable our heart's chambers to contract, an action triggered by our heart's electrical signal.
The electrical signal travels through the network of conducting cell "pathways," which stimulates our upper chambers (atria) and lower chambers (ventricles) to contract. The signal can travel along these pathways by means of a complex reaction that allows each cell to activate one next to it, stimulating it to "pass along" the electrical signal in an orderly manner. As cell after cell rapidly transmits the electrical charge, the entire heart contracts in one coordinated motion, creating a heartbeat.
The electrical signal starts in a group of cells at the top of our heart called the sinoatrial (SA) node. The signal then travels down through your heart, triggering first our two atria and then our two ventricles. In a healthy heart, the signal travels very quickly through the heart, allowing the chambers to contract in a smooth, orderly fashion.
The heartbeat happens as follows:
1. The SA node (called the pacemaker of the heart) sends out an electrical impulse.
2. The upper heart chambers (atria) contract.
3. The AV node sends an impulse into the ventricles.
4. The lower heart chambers (ventricles) contract or pump.
5. The SA node sends another signal to the atria to contract, which starts the cycle over again.
This cycle of an electrical signal followed by a contraction is one heartbeat.
SA node and atria
When the SA node sends an electrical impulse, it triggers the following process:
ü The electrical signal travels from your SA node through muscle cells in our right and left atria.
ü The signal triggers the muscle cells that make your atria contract.
ü The atria contract, pumping blood into your left and right ventricles.
AV node and ventricles
After the electrical signal has caused our atria to contract and pump blood into our ventricles, the electrical signal arrives at a group of cells at the bottom of the right atrium called the atrioventricular node, or AV node. The AV node briefly slows down the electrical signal, giving the ventricles time to receive the blood from the atria. The electrical signal then moves on to trigger our ventricles.
When the electrical signal leaves the AV node, it triggers the following process:
ü The signal travels down a bundle of conduction cells called the bundle of His, which divides the signal into two branches: one branch goes to the left ventricle, another to the right ventricle.
ü These two main branches divide further into a system of conducting fibers that spreads the signal through your left and right ventricles, causing the ventricles to contract.
ü When the ventricles contract, our right ventricle pumps blood to your lungs and the left ventricle pumps blood to the rest of our body.
After our atria and ventricles contract, each part of the system electrically resets itself. The cells of the SA node at the top of the heart are known as the pacemaker of the heart because the rate at which these cells send out electrical signals determines the rate at which the entire heart beats (heart rate).
The normal heart rate at rest ranges between 60 and 100 beats per minute. Our heart rate can adjust higher or lower to meet our body's needs.
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