Deoxygenated blood begins the cycle through the entire human body in the superior vena cava. From there it enters the right atrium of the heart then through the right atrioventricular valve into the right ventricle. The contraction of the right ventricle forces the blood through the pulmonary semilunar valve leading into the pulmonary artery. The pulmonary artery is the only one in the body that carries blood without oxygen. The valves of the heart prevent blood from flowing backwards.
The pulmonary artery brings the deoxygenated blood to the lungs where it is split into tiny capillaries in the alveoli. It is these thin walled capillaries that allow for oxygen and carbon dioxide transfer to provide oxygen to the rest of the body. This fully oxygenated blood then returns to the left atrium by way of the pulmonary veins. The left atrioventricular valve allows the blood to flow into the left ventricle. The blood leaves the heart into the systemic circulation through the aorta after passing the aortic semilunar valve.
After leaving the main aorta the blood starts flowing down the thoracic aorta, to the inferior mesenteric artery, the femoral artery, the popliteal artery and split to the anterior tibial artery, peroneal artery and posterior tibial artery which brings the blood to the toes. From these final arteries, the blood vessels branch off to smaller capillaries such as the same as in the lungs and alveolus until the vessel walls are thin enough to transfer items such as oxygen, carbon dioxide and nutrients. The capillaries start to join again until the form vessels large enough to be considered veins. To return, the blood flows from the dorsal digital veins, to the fibular vein, femoral vein, iliac vein, to the inferior vena cava. As the inferior vena cava joins the superior vena cava the cycle starts all over again.