As the right atrium contracts, the tricuspid valve opens and blood flows into the right ventricle. When the right ventricle contracts, the pulmonary valve opens and blood is pumped into the pulmonary trunk. The pulmonary trunk then divides into the right and left pulmonary arteries, which carry blood to the lungs. In the lungs, the blood flows through pulmonary capillaries, where oxygen and carbon dioxide are exchanged, and then returns to the heart via the pulmonary veins. The pulmonary veins enter the left atrium, which contracts to open the bicuspid valve and allow blood to enter the left ventricle. When the left ventricle contracts, the aortic valve opens and blood is pumped into the aorta, which carries blood to the systemic arteries. The systemic arteries deliver oxygen and nutrients to the body's tissues through systemic capillaries, and then blood returns to the heart through systemic veins. The venae cavae deliver deoxygenated blood from the systemic veins back into the right atrium, completing the cycle.