BLOOD FLOW THROUGH THE HEART
As mentioned the heart is the pump that creates the circulation of blood throughout the body.
It has 2 functions:
Collect oxygen poor blood from the body and pump this blood to the lungs so it can obtain more oxygen and release waste such as carbon dioxide.
Collect the oxygen rich (oxygenated) blood from the lungs and pump this blood to the rest of the body.
As blood flows through the body, the oxygen is depleted and waste products are collected. The oxygen depleted blood eventually is collected by the Inferior and Superior Vena Cava. The Inferior and Superior Vena Cava travel directly to the right atrium. As the heart pumps, contracting the atriums, it pushes the de-oxygenated blood past the tricuspid valve into the right ventricle. From there, when the heart pumps again, contracting the ventricles, the de-oxygenated blood is pushed past the pulmonary valve, into the Pulmonary Artery. From there, the blood travels to the right and left pulmonary arteries and to the lungs.
The blood, now oxygenated, travels to the heart via the Pulmonary veins. The oxygenated blood is collected into the left atrium. As the heart pumps, contracting the atriums, it pushes the blood past the mitral (bicuspid) valve to the left ventricle. From there, when the heart pumps again, contracting the ventricles, the oxygenated blood is pushed past the aortic valve, into the Aortic Artery and thereby the rest of the body.
It is important to note that blood passively flows from the atriums into the ventricles. However, the ventricles are only filled to 80% capacity passively. It is when the heart contracts the first time, that the remaining 20% of the blood is pushed from the atriums into the ventricles. Once the ventricles are filled, this causes the tricuspid and mitral valves to close, preventing blood traveling back to the atria during ventricular contraction.