Deoxygenated blood reaches the heart and enters into the right atrium. Pressure in the right atrium increases which causes the right atrioventricular valve to open allowing the passing of the blood into the right ventricle. The right ventricle contracts and the increased pressure leads to the opening of the pulmonary valve. The blood leaves the heart through the pulmonary arteries and enters the pulmonary circulation. The now oxygenated blood returns to the heart through the pulmonary vein and enters into the left atrium. After the pressure increases in the atrium, the left atrioventricular valve opens and the oxygenated blood enters the left ventricle. The left ventricle contracts which leads to the opening of the aortic valve. The blood leaves the heart through the aorta entering the systemic circulation and passes from arteries, to arterioles, capillaries, venules and finally through veins. The blood finally returns to the right atrium through the superior and inferior vena cava.