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The left atrium is the upper right chamber of the heart. The lungs deliver oxygenated blood into this part of the heart, which then passes the blood through into the left ventricle, the lower right chamber of the heart. Blood comes into the left ventricle from the four pulmonary veins and is passed through to the left ventricle through the bicuspid valve. The left ventricle is then responsible for pumping the oxygenated blood around the body.
The human heart is comprised of four chambers: the left atrium, left ventricle, right atrium and right ventricle. The upper chambers are referred to collectively as "atria," and are responsible for receiving blood from the veins. A person's left atrium is actually located on the right-hand side of the heart, as it is viewed by someone facing him, but each person’s own left atrium is on his left-hand side. The heart’s atria have thin walls.
Newly oxygenated blood enters the left atrium through the pulmonary veins, two of which come from each lung. The blood then passes through the atrioventricular opening, guarded by the bicuspid valve. This valve has two flaps and allows blood to pass through into the left ventricle. It closes as the left ventricle contracts to prevent blood from being sent back up into the left atrium.
The sinoatrial node, located in the right atrium, is responsible for controlling the contraction of the atria, and its signal is what sends the blood through the mitral valve from the left atrium. The walls of the left atrium are made from myocardium, a strong muscle that makes up most of the structure of the heart. The myocardium muscle contracts as a result of the cardiac fibers that are inside it.
After the blood has been passed through to the left ventricle from the left atrium, it stays in there until the ventricle fills up. After the ventricle is full, it contracts. At the moment of contraction, the bicuspid valve opens and the aortic valve opens. The blood cannot pass back through into the left atrium, so it is sent through into the aorta, and from there to the rest of the body. The aorta is the largest blood vessel in the body, measuring about 1 inch (2.5 cm) in diameter.