What does the heart do?
About the size of your fist, the heart pumps blood through the body to deliver oxygen to the body’s tissues. Blood whose oxygen has been expended goes back to the heart and is sent to the lungs. The oxygenated blood returns to the heart to be pumped back into the body. About 2,000 gallons of blood are circulated through our bodies every day.
A healthy heart beats approximately 86,000-100,000 times per day. This assumes an ideal range of 50-70 beats per minute. The old standard was 60-100; however, a resting heart rate higher than 76 beats per minute may be linked to an increased risk of heart attack.
How does blood circulate through the body?
Arteries transport oxygenated blood away from the heart to the body; veins transport deoxygenated blood to the heart from the body. Arteries decrease in size as they move from the heart, and veins increase in size as they approach the heart. The capillaries are thin blood vessels which connect arteries and veins. Capillaries allow oxygen, carbon dioxide, nutrients, and waste products to pass between our organ’s cells. Our bodies contain over 60,000 miles (over 95,000 kilometers) of these blood vessels!
Simply, it travels from the body to the heart, then to the lungs, back into the heart, and into the body. Step-by-step, here is how it works:
- Deoxygenated blood enters the right atrium through the superior and inferior vena cavas.
- Blood passes through the tricuspid valve into the right ventricle.
- Blood passes through the pulmonary (or pulmonic) valve into the lungs to oxygenate.
- In the pulmonary capillaries, blood gains oxygen and releases carbon dioxide.
- Oxygenated blood enters the pulmonary veins and goes into the left atrium.
- Blood passes through the mitral valve and enter the left ventricle.
- Blood enter the aorta and systemic arteries through the aortic valve.
- Blood deposits oxygen into the body’s tissues and absorbs carbon dioxide in the systemic capillaries.
How does the heart beat?
The atria and ventricles (the heart’s chambers) alternately contract and relax. Electrical signals from the brain trigger these muscular reactions. A bundle of cells in the right atrium, called the sinoatrial node, regulates the heartbeat. Its electrical activity spreads through the atrial walls and makes them contract. Another group of cells in the center of the heart, between the atria and ventricles, slows the electrical signal before it enters the ventricles. This delay allows the atria to contract before the ventricles do. The His-Purkinje network sends the electrical impulse to the walls of the ventricles, making them contract.
Let’s visualize this process. Look at the diagram and focus on the right atrium (RA). Imagine that it is full of blood. The RA expands, increasing the volume for the blood to reside in. Since there is the same amount of blood in a larger space, the pressure decreases. This acts like a vacuum and sucks in blood from the vena cavas to fill this empty space. The tricuspid valve (TV) can only open one way, and that is into the right ventricle (RV), so there’s no worry of backflow from the RV to the RA (unless one is diagnosed with TV regurgitation). So, when the RA contracts, it pushes the blood through the TV into the RV. When the RA expands, it closes the TV, and the process continues. The same happens through all of the chambers into the heart.