HEART

 

Most of us would think of the heart as a pump, sending blood throughout our body. While that is not incorrect, it is not entirely accurate.

Our heart is actually electrical. It is the motor of our body. When electrocuted, the electrical current can disrupt the heart's normal electrical impulses, potentially leading to arrhythmias, cardiac arrest, or even a heart attack.

 

Have you ever wondered  when your heart  has stopped, why would we use the Automated External Defibrillators (AED) to electrically "reboot" the heart? Or why the Electrocardiogram (ECG) machine is used to assess your heart functions?

 

The heart generates its own electrical signal. This electrical signal is produced by a tiny structure known as the sinus node, located in the upper portion of the right atrium. The right atrium is one of four heart chambers and related valves, which include two atria at the top of the heart with two ventricles at the bottom.

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The cardiac electrical signal controls the heartbeat in two ways. First, since each electrical impulse generates one heartbeat, the number of electrical impulses determines the heart rate. In a normal sinus rhythm, that rate will be between 60 and 100 beats per minute.

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The sinus node signal also controls electrical conduction as it "spreads" across the heart. It causes the cells of the heart muscle to contract in the correct sequence, and ensures regular, efficient, and coordinated heartbeats. For this reason, the sinus node is often referred to as the heart's "natural pacemaker."

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The heart is roughly the size of a fist and is located in the mediastinum, slightly to the left of the chest, protected by the rib cage. It is made of cardiac muscle tissue and has four main chambers:

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1. Right Atrium: The upper-right chamber that receives deoxygenated blood from the body via the superior and inferior vena cava. This blood is low in oxygen and high in carbon dioxide.

2. Right Ventricle: The lower-right chamber that receives blood from the right atrium and pumps it to the lungs through the pulmonary artery for oxygenation.

3. Left Atrium: The upper-left chamber that receives oxygenated blood from the lungs via the pulmonary veins.

4. Left Ventricle: The lower-left chamber, which is the strongest and most muscular part of the heart. It pumps oxygen-rich blood to the rest of the body through the aorta.

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The heart has four valves that prevent the backflow of blood and ensure it flows in the correct direction:

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1. Tricuspid Valve: Located between the right atrium and right ventricle, it prevents blood from flowing backward into the atrium.

2. Pulmonary Valve: Between the right ventricle and the pulmonary artery, it prevents blood from flowing back into the ventricle after it has been pumped to the lungs.

3. Mitral Valve: Located between the left atrium and left ventricle, it prevents blood from flowing backward into the atrium after it has been pumped into the ventricle.

4. Aortic Valve: Between the left ventricle and the aorta, it prevents blood from flowing back into the ventricle once it has been pumped into the aorta.

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Blood flows through the heart in a specific sequence:

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1. Deoxygenated Blood Returns to the Heart:

   • Blood that has circulated through the body, delivering oxygen and nutrients, returns to the right atrium via the superior and inferior vena cava.

2. Right Atrium to Right Ventricle:

   • The right atrium contracts, pushing the deoxygenated blood through the tricuspid valve into the right ventricle.

3. Right Ventricle to Lungs:

   • The right ventricle contracts, sending blood through the pulmonary valve into the pulmonary artery, which carries the blood to the lungs for oxygenation.

4. Oxygenated Blood to the Left Atrium:

   • After oxygen exchange in the lungs, the oxygen-rich blood returns to the left atrium through the pulmonary veins.

5. Left Atrium to Left Ventricle:

   • The left atrium contracts, pushing the oxygenated blood through the mitral valve into the left ventricle.

6. Left Ventricle to the Body:

   • The left ventricle, with its thick muscular walls, contracts forcefully, sending blood through the aortic valve into the aorta, which distributes it to the rest of the body.

 

The heart's pumping action is a rhythmic cycle known as the cardiac cycle. This cycle has two main phases:

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1. Diastole: The phase when the heart muscle is relaxed, and the chambers fill with blood. During this time, the atria contract to push blood into the ventricles.

2. Systole: The phase when the heart muscle contracts. The ventricles contract, pumping blood into the arteries (the right ventricle pumps to the lungs, and the left ventricle pumps to the body).

 

Each heartbeat is regulated by electrical signals:

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• Sinoatrial (SA) Node: Known as the "natural pacemaker," the SA node, located in the right atrium, generates electrical impulses that trigger the heart's rhythm. This causes the atria to contract and initiates the cycle.

• Atrioventricular (AV) Node: Located between the atria and ventricles, the AV node delays the electrical signal slightly, allowing the ventricles to fill with blood before they contract.

• Bundle of His and Purkinje Fibers: These pathways carry the electrical signals to the ventricles, causing them to contract and pump blood.

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The heart operates two primary circulatory pathways:

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1. Systemic Circulation:

   • This involves the flow of oxygen-rich blood from the left ventricle to the body through the aorta. It delivers oxygen and nutrients to tissues and returns deoxygenated blood via veins into the right atrium.

2. Pulmonary Circulation:

   • This involves the flow of deoxygenated blood from the right ventricle to the lungs via the pulmonary arteries, where it is oxygenated. Oxygenated blood returns to the left atrium via the pulmonary veins.

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Blood pressure refers to the force that blood exerts on the walls of blood vessels, and it is an important measure of cardiovascular health. It consists of two measurements:

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1. Systolic Pressure: The pressure when the heart contracts and pumps blood into the arteries (the top number).

2. Diastolic Pressure: The pressure when the heart is at rest between beats, as the chambers refill with blood (the bottom number).

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