Humans have a double circulatory system. This means that blood passes through out heart twice in each cycle.
Let us start from the right atrium.
The right atrium is filled with deoxygenated blood that come from the superior and inferior vena cava.
Deoxygenated blood from the right atrium then enters the right ventricle via the tricuspid valve. When the right ventricle is filled, the tricuspid valve closes to prevent back flow of blood.
The high pressure inside the right ventricle causes the pulmonary valve to open. The muscles of the right ventricle push the deoxygenated blood into the pulmonary artery. The pulmonary valve closes to prevent back flow of blood.
The pulmonary artery carries the deoxygenated blood to the lungs. There, gas exchange occurs at the alveoli. The blood becomes oxygenated.
The pulmonary vein carries the oxygenated blood from the lungs to the left atrium of the heart.
The oxygenated blood then goes to the left ventricle via the mitral valve. When the left ventricle is filled, the mitral valve closes to prevent back flow of blood.
The high pressure inside the left ventricle causes the aortic valve to open. The muscles of the left ventricle push the oxygenated blood into the aorta. The aortic valve closes to prevent back flow of blood.
The aorta then carries the oxygenated blood to the rest of the body.
Gas exchange occurs in the body tissues and the blood again becomes deoxygenated.
The inferior and superior vena cava carry deoxygenated blood from the body tissues to the right atrium and the whole process happens again.
Why is the left ventricle more muscular than the right ventricle?
The left ventricle needs to be more muscular than the right ventricle.
This is because the right ventricle only needs to push the blood to the lungs which is a short distance.
However, the left ventricle needs to push blood to the rest of the body, which is a much greater distance. It needs more force to do so, and hence, is more muscular than the right ventricle.