The ventricles contract and force the blood under pressure, past the semi lunar valves into the arteries. The closing of the bicuspid and tricuspid valves prevents back flow. At the same time the atria and ventricles relax and blood begins to flow back into them from the veins.
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Because blood travels directly from the left atrium to the left ventricle.
The wall of the left ventricle is thicker because it pumps blood to the entire body. The right ventricle pumps blood to the lungs, so it does not have to be as strong as the left ventricle.The oxygenated blood that comes into the left ventricle has to be transported to all the parts of the body. For this reason, the left ventricle has thicker muscle walls that pump blood at a higher pressure than the right ventricle that pumps blood only to the lungs.
The walls of the left atrium contract to force blood through the mitral valve into the left ventricle.
The left ventricle pumps blood to the entire body, while the right ventricle pumps blood only to the lungs, which are immediately adjacent to the heart. Hence, the left ventricle has to pump with greater force, and requires more muscle to do it.
In a mirror.