Blood returning through the pulmonary veins empties into the left atrium. Some of the oxygen is contained in fluid, or plasma, but most is contained in the red blood cells, which are designed to carry oxygen. Once in the left atrium, blood flows through another one-way valve called the mitral valve into the left ventricle.
The left ventricle is the heart's main pumping chamber. As the left ventricle contracts, the mitral valve closes and the aortic valve opens. Blood is forced through the one-way aortic valve into the aorta, which is the main artery of the body and somewhat larger than your thumb.
The aorta first heads upward toward the neck, then makes a U-turn at the top of the chest just before the neck and heads down through the chest and into the abdomen toward the pelvis. It divides into two arteries, known as iliac arteries, which supply the pelvis and legs with oxygenated blood. In the chest and abdomen, the aorta gives off numerous branches to supply blood to the brain, the heart muscle itself, and other organs, muscles, and tissues.
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