The right side of the heart is responsible for collecting deoxygenated blood from the body. The inferior and superior vena cavas carry the deoxygenated blood to the right atrium. From here it is pumped into the right ventricle through the tricuspid valve and then through the pulmonary valve into the pulmonary artery and so to the lungs where it is reoxygenated.
The freshly oxygenated blood is then returned to the left side of the heart via the pulmonary veins. These veins enter the left atrium and the blood is pumped from here to the left ventricle through the mitral valve. Once in the left ventricle the blood is then pumped through the aortic valve into the aorta and around the body.
Blood pressure within the heart and vessels is not all the same. The right side of the heart works under less pressure than the left because it only has to pump blood to the lungs whereas the the left side of the heart has to pump blood all the way around the body and therefore needs to work a higher pressure.
Anatomically the heart is designed to cope with this arrangement, the left ventricle having a thicker/stronger muscle wall than the right.
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