Veins always carry blood towards the heart. The blood in veins is mostly deoxygenated, however the pulmonary vein, which goes from the lungs to the left atrium, carries newly oxygenated blood back to the heart for it to be pumped to the rest of the body.

In contrast, arteries always travel away from the heart and usually carry oxygenated blood, with the exception of the pulmonary arteries. Arteries and arterioles have a thick layer of smooth muscle that helps to regulate blood pressure. Veins may have some smooth muscle, but are not nearly as significant in helping to regulate blood flow.

Arteries are strong elastic vessels that carry blood moving away from the heart. As arteries move away from the heart they become smaller (think of a tree and its branches, the trunk is larger than the branches and as each branch grows out it becomes smaller) and are referred to as arterioles, which connect to capillaries. Capillaries penetrate nearly all tissue; their walls are very thin and allow exchange of materials (oxygen, nutrients) between blood and tissues. Veins are thinner-walled and less muscular. The smallest ones are called venules and connect to capillaries.

The scalp bleeds profusely not only because the densely packed hair follicles demand a greater blood supply, but also because the superficial fascia (that binds the skin to the connective tissue of the occipitofrontalis muscle) prevents vascular dilation. Because of this, clotting is more difficult.

Veins are the most compliant entity within the circulatory system. The compliance of a systemic vein is 24 times that of its corresponding artery. Veins serve as the major blood reservoir within the human body, and are 8 times as distensible as arteries.

Blood flows through the circulatory system as a result of pressure generated by the heart. As blood moves throughout the circulatory system, pressure is lost due to friction generated between blood and blood vessel walls. Therefore, pressure falls continuously as blood moves farther from the heart. Since the body wants to maintain a forward, unidirectional flow of blood, the highest blood pressure is found in the aorta, and the lowest blood pressure is found in the venae cavae - just before emptying into the right atrium.

Blood flow through veins is not very efficient. Slow and weak "pushing" by arteries does not contribute much to blood flow, as the hydrostatic pressure in veins is roughly zero. However, contraction of the diaphragm and skeletal muscles, along with the one way valves in veins (these prevent back flow) are important factors.

Note: Blood clots can occur if blood does not flow properly through veins. This can occur if a person doesn't move enough (for example a long international flight).

During times of stress or physical activity, sympathetic nerves can stimulate blood vessels to constrict and dilate in order to redirect blood to the needed areas in the body. For example, sympathetic innervation can direct blood away from the skin and digestive tracts to facilitate muscle action.

Arterioles are typically surrounded by smooth muscle and can be constricted in order to redirect blood flow. Arteries also contain smooth muscle, but are generally too large to have specific, well-controlled effects on blood flow regulation. Venules have very little smooth muscle, and capillaries have none. Remember that the walls of capillaries consist of only a single layer of endothelium.

Venules are formed from merged capillaries. They progressively merge to form veins. Veins are thin-walled, are under low pressure, and contain the highest proportion of the blood in the cardiovascular system. The largest vein in the body is the vena cava, which returns blood to the heart.

Capillaries have the largest total cross-sectional and surface area. Arterioles are the site of highest resistance in the cardiovascular system. Arteries are thick-walled with extensive elastic tissue and smooth muscle.

The cephalic vein is a superficial vein of the upper limb. It communicates with the basilic vein via the median cubital vein. It travels along the anterolateral surface of the biceps brachii muscle. The cephalic veins empties into the axillary vein (a continuation of the brachial and basilic veins). The cephalic vein terminates at the lateral margin of the first rib where it becomes the subclavian vein.

Arteries and veins differ in a few key ways. Arteries are much thicker than veins in order to compensate for the larger pressure exerted on them. Layers of smooth muscle in arteries is used to modulate this high pressure, allowing the vessel to expand or constrict. Veins have much larger lumens than arteries and have valves in order to prevent the backflow of blood. Veins have much lower pressure, but also must force blood to flow against gravity in order to carry it from the periphery back to the heart. This is accomplished by the venous valves.