Afterload is the amount of work the heart has to do to pump blood to the rest of the body. It's determined by the resistance to flow in the arteries. Blood vessels can become narrower (vasoconstriction) or wider (vasodilation), and this affects afterload.
The heart muscle contracts and relaxes to pump blood. During systole, contraction occurs, which ejects blood from the ventricles into the aorta and other arteries. Then, during diastole, relaxation occurs and blood flows back into the ventricles from the atria.
Afterload directly affects how much force is needed to eject blood from the ventricles during systole. If afterload is high, the ventricles have to work harder to pump blood out, and this can lead to heart failure. There are many factors that can influence the afterload, such as valvular heart diseases, hypertension, and narrowing of arteries by conditions such as atherosclerosis.
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