On average, the body has five liters of blood continually traveling through it. Blood flows through the body via the circulatory system. The circulatory system is made up largely of veins, capillaries, arteries, and the heart.
As blood begins to circulate, the heart contracts the left ventricle and blood is forced into the aorta. This blood is oxygen rich and will be pumped to all parts of the body by way of arteries. On the way back to the heart, deoxygenated blood will travel through veins. The veins bring the deoxygenated blood back to the heart by entering the right atrium, by way of the vena cavea. The right atrium fills with this blood and contracts. This contraction pushes the blood through a one-way valve into the right ventricle. The right ventricle contracts and forces the deoxygenated blood into the pulmonary artery, which leads to the lungs. While in the lungs a gas exchange occurs. During this process waste leaves the blood and oxygen enters it. This oxygenated blood then leaves the lungs through the pulmonary veins and reenters the heart through the left atrium. The heart contracts and pushes the blood through a one-way valve into the left ventricle. It is then forced into the aorta and the process repeats.