Human circulation is a closed system that may be divided into systemic and pulmonary circulation. The first one is a loop from the heart to the body. The second one is a loop from the heart to the lungs. In the heart, firstly, deoxygenated blood from the body travels via Vena Cava to the right atrium. Then, it is pumped to the right ventricle via the tricuspid valve. Next, it is pumped to the lungs via the pulmonary artery. In the lungs, CO2 is exchanged for O2. The blood returns to the heart by pulmonary veins. It is pumped to the left atrium and then to the left ventricle. Next, the blood is pumped to the aorta to the rest of the body.