When someone is cold they, will experience vasoconstriction in their hands, allowing less blood to flow to the hands. Their hands will feel colder, since less blood is reaching their hands. When the body is cold, it concentrates blood flow to the core. This maintains a constant, warm blood supply to the vital organs, as well as reduces the blood flow to extremities that have a greater surface area for heat loss to the environment.

During heavy exercise, the body temperature increases. Heat must leave as efficiently as possible, and thus the body's blood vessels will vasodilate. Vasodilation leads to the vessels being closer to the skin, thus allowing quicker heat transfer to the environment. This effect is accomplished, in part, by stimulation from the sympathetic nervous system.

On a cold day, it is vital to keep body at a steady temperature through homeostatic processes. The body will compensate for heat loss by having the blood vessels vasoconstrict. This leads to the vessels being farther from the skin, allowing less heat loss due to blood flow. This also explains why conditions like frostbite generally start at the extremities, where surface area is greatest. These regions have the greatest amount of vasoconstriction, and thus contain the smallest amount of warm blood, leaving them prone to the cold environment due to a lack of internal heating.

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.