The body strives to maintain the levels of fluid within the body. Several systems, including the circulatory system, lymphatic system, and renal system, are all responsible for keeping the blood flowing throughout the body and removing unnecessary fluid.

Circulatory defects, such as congestive heart failure and sickle cell anemia, and defects of the kidneys can result in peripheral edema. Diabetes itself does not cause peripheral edema, however, complications of diabetes (such as heart disease) may result in some swelling of the limbs.

The tricuspid and bicuspid valves constitute the atrioventricular valves, which separate the heart's atria from its ventricles. The tricuspid valve separates the right atrium from the right ventricle. The bicuspid valve separates the left atrium from the left ventricle.

The pulmonary and aortic valves constitute the semilunar valves, which separate the heart's ventricles from major arteries. The pulmonary valve separates the right ventricle and the pulmonary artery. The aortic valve separates the left ventricle and the aorta.

There is no such thing as a ventricular valve.

Each capillary consists of a single layer of epithelium, known as simple squamous epithelium. Substances pass between and through these cells, allowing nutrients to pass from the capillary lumen into the interstitium and allowing wastes to pass from the interstitium to the capillary lumen. Red blood cells flow through the capillaries in single file due to the small circumference of the vessel.

Venules are small veins leading away from the capillary network. Arterioles are small terminal arteries leading into the capillary network. Veins are blood vessels carrying blood toward the heart, while arteries carry blood away from the heart.

The pulmonary arteries bring deoxygenated blood from the body into the lungs, where carbon dioxide is exchanged for oxygen. The oxygen-rich blood leaves the lungs to return to the heart through the pulmonary veins. This is the only occurrence in which arteries carry oxygen-poor blood and veins carry oxygen-rich blood.

The thoracic aorta supplies the bronchi, esophagus, pericardium, chest wall, and diaphragm with oxygenated blood. The inferior vena cava collects deoxygenated blood from the lower extremities and transports it to the heart. In the upper limbs, superficial veins merge to form the axillary vein. These empty into the superior vena cava, and then into the heart. 

Carbon dioxide is usually circulated in human blood in the form of , or bicarbonate. This is an important part of the blood buffering system, as the bicarbonate ion is the conjugate base of carbonic acid.