When your heart beats the blood is forced through the veins as vacuum pressure forces de- oxygenated blood into the heart. The pumped blood is shot through capillaries where the oxygen gets to the cells. Then muscular contractions and vacuum pressure of the heart makes it complete the cycle
Deoxygenated blood from the body enters the right atrium. It flows through the tricuspid valve into the right ventricle. Contraction of the ventricle then closes the tricuspid valve and forces open the pulmonary valve. Blood flows into the pulmonary artery. This branches immediately, carrying blood to the right and left lungs. Here the blood gives up carbon dioxide and takes on a fresh supply of oxygen. The capillary beds of the lungs are drained by venules that are the tributaries of the pulmonary veins. Four pulmonary veins, two draining each lung, carry oxygenated blood to the left atrium of the heart. From the left atrium, Blood flows through the mitral valve into the left ventricle. Contraction of the ventricle closes the mitral valve and opens the aortic valve at the entrance to the aorta. The first branches from the aorta occur just beyond the aortic valve still within the heart. Two openings lead to the right and left coronary arteries, which supply blood to the heart itself.
it's a never-ending cycle so i'm indecisive on where to start but oh well - you breathe in air through your lungs, which is then filtered so that oxygen is taken out of the air (because earth's air is made up of hundreds of gases), the oxygen is transported to the heart then to red blood cells that are currently depleted of their load of oxygen, these blood cells carry oxygen to other types of cells in your body until they deplete of oxygen in which case they will have already gone through their vascular route and have returned to the heart to pick up more oxygen.
a few slightly related facts to mention is that red blood cells age and degrade rather quickly compared to other cells, so up to millions of red blood cells are made every few seconds.
also, despite what it may seem, your liver is actually more useful than your heart, having up to 200 different tasks to complete, making it sometimes slightly possible (but nothing to bet on) for you to survive much longer without your heart than your liver