Answer and explanation:
Hello, in order to make the explanation easier to follow, I'll describe these processes as a series of steps.
1) Deoxygenated blood is pumped from the heart to the lungs by the pulmonary arteries.
2) When this deoxygenated blood reaches the alveoli, there's a gas exchange through the lung capillaries in which carbon dioxide is changed by oxygen.
3) This now oxygenated blood goes from the lungs back to the heart through the pulmonary veins.
4) The incoming blood of the pulmonary veins is poured into the left atrium.
5) When the Mitral Valve opens, the blood in the left atrium passes to the left ventricle.
6) The blood (remember that is oxygenated) in the left atrium is expelled of the heart thanks to the contraction of the cardiac muscle - this is how it gets out of the heart and reaches the Aorta.
7) The Aorta is the biggest artery in the body. It carries oxygenated blood and transports it to every organ and muscle.
8) Meanwhile, all of the deoxygenated blood from the organs and the muscles will travel back to the organs through many veins that will ultimately drain in the inferior and superior vena cava.
9) Deoxygenated blood from the vena cava will arrive at the right atrium.
10) When the Tricuspid Valve opens, the blood in the right atrium will flow to the right ventricle.
11) The right ventricle (with deoxygenated blood) will pour its content in the pulmonary arteries, thus repeating the cycle again and again.
The process involving the lungs, the heart, and the pulmonary vessels is called Pulmonary Circulation. The part of the circulatory system that involves the heart, the rest of the organs, and the aorta and vena cava is called Systemic Circulation.