In fetal mammals, oxygenated blood returns to the fetus via the umbilical veins. It then drains into the right side of the heart. There would be no use in sending oxygenated blood into non-aerated lungs, so it is diverted through a channel, the ductus arteriosus, into the aorta of the fetus.

At birth, this structure closes rapidly, allowing blood to pass the lungs of the newborn. A residual band of connective tissue, the ligamentum arteriosum, marks the site of the original structure. The foramen ovale is a developmental aperture between the atria, and it usually closes spontaneously as the heart develops.

The circulatory system of the body is composed of blood vessels. There are three main types of blood vessels: the arteries, the capillaries, and the veins.

Arteries always carry blood away from the heart. Usually the blood is oxygenated; the exceptions are the pulmonary arteries, which carry blood away from the heart to the lungs to become oxygenated. Veins carry deoxygenated blood towards to heart from tissue, with the exception of the pulmonary veins, which carry oxygenated blood from the lungs to the heart. Capillaries allow for exchange between blood and tissue.

Begin by remembering that arteries carry blood away from the heart. Veins carry blood from the body back to the heart. Note that the distinction between the blood in arteries and veins does not lie in the oxygen-content. Rather, it lies in the direction of flow. Arteries always take blood away from the heart, whereas veins always take blood back to the heart.

Arteries carry blood away from the heart. In all but one case, arteries carry oxygen-rich blood. The exception is the pulmonary arteries. They carry oxygen-poor blood away from the heart, to the lungs, to pick up more oxygen. Veins carry blood back to the heart. In all but one case, veins carry oxygen-poor blood. The exception is the pulmonary veins. They carry oxygen-rich blood from the lungs back to the heart.

Veins are the only circulatory vessels that contain valves. These structures contain valves because much of the pumping pressure that originated in the heart has been reduced by the time blood reaches veins, which return deoxygenated blood to the heart. The reduction of force can cause the blood to pool or move very slowly. Since veins rely mostly on gravity, blood flow force (originating from the heart), and muscular contractions to squeeze blood through them, it makes sense for them to have valves to keep blood from flowing backwards in between muscular contractions.

The pulmonary arteries are the only arteries that contain oxygen-poor blood because it carries blood into the lungs to be oxygenated. By definition, arteries carry blood away from the heart and veins carry blood to the heart. However, the pulmonary vein is the only vein in the body that carries oxygen-rich blood (from the lungs back to the heart to be pumped into the rest of the body) and the pulmonary artery is the only artery in the body that carries oxygen-poor blood (to the lungs to be oxygenated before being sent to the heart to be pumped into the rest of the body. The carotid artery is in the neck and delivers oxygen-rich blood to head. The femoral artery is in the leg and delivers oxygen-rich blood to the leg. The aorta is the main artery that delivers blood from the left ventricle of the heart to the rest of the body. 

Blood types are named according to the antigens present on the surface of a person's red blood cells, specifically the ABO and Rh antigens. AB+ blood has all three antigens and therefore none of the antibodies, meaning it can receive any blood type without having an immune reaction to the donor blood. 

O- blood is unique from the other blood types due to its lack of ABO and Rh antigens. Unlike A and B, O is used to denote no antigens rather than an "O type" of antigen. As a result, it can be given to any other blood type since there are no antigens on the red blood cells to have an immune reaction the recipients own natural antibodies. 

The function of the lymphatic system is to keep fluids out of the interstitium and connective tissue by returning it to the blood. Fluid leaks out of capillaries because of their thin walls. The lymphatic vessels collect this fluid and return it to the heart. When the lymphatic system is impaired, this fluid can accumulate in the tissue and cause swelling, known as edema.

Most of the lymph re-enters the venous circulation through the thoracic duct, and the volume is somewhere around a liter to a liter and a half per day.  

Deoxygenated blood enters the heart through the right atrium, where it then flows into the right ventricle. The right ventricle pumps the deoxygenated blood through thepulmonary arteries to the lungs, where it becomes oxygenated. Oxygenated blood returns to the heart through the pulmonary veins, which empty into the left atrium. Blood then flows into the left ventricle, which contracts to pump the blood through the aorta and toward the rest of the body.