To understand how blood travels through the human body, we can begin in the heart. The heart is divided into four chambers: the upper chambers, called the left and right atria, and the lower chambers, which are called the left and right ventricles. The left ventricle pumps oxygen-rich blood out of the heart and into an artery called the aorta. From the aorta, blood branches out into a series of ever smaller tube-like vessels: arteries, arterioles and finally capillaries. The capillaries deliver the blood's oxygen and nutrients to cells. Then the oxygen-poor blood makes the journey back: from capillaries, to vessels called venules, to veins, and finally back to the heart.
Before blood can begin its journey around the body again, it must travel to the lungs to release carbon dioxide and take in oxygen. The right atrium accepts the oxygen-poor blood and delivers it to the right ventricle, which pushes the blood through an artery to the lungs. The blood then returns in a vein from the lungs to the left atrium, where it waits before passing to the left ventricle and back through the aorta again.
While your blood is traveling through your body, it undergoes another important process: waste filtration. When blood leaves the body's tissues to travel back to the heart and lungs, it carries excess nutrients and other items that your body cannot use.
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