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.

A common misconception is that all veins carry deoxygenated blood. In reality, all veins are responsible for bringing blood back to the heart. Generally, blood traveling toward the heart is deoxygentated. The pulmonary veins, however, bring blood that has just received oxygen from the lungs back to the heart. The pulmonary veins are the only veins in the body to carry oxygenated blood.

Similarly, the pulmonary arteries are the only arteries to carry deoxygentated blood away from the heart. All arteries carry blood away from the heart, but most contain oxygenated blood. The vena cavae are large veins that carry deoxygenated blood from the body back to the right atrium. This blood is then transferred to the right ventricle, and the then pulmonary arteries for transport to the lungs. The path of blood from the heart to the lungs and back is known as the pulmonary circuit.

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.

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.

Veins and arteries are the two primary vessels of the circulatory system. Blood is pumped from the heart into arteries, which branch into smaller arterioles. Arterioles terminate in capillary beds, which are specialized to allow for gas exchange. Blood leave the capillaries through venules, which expand into veins that carry the blood back to the heart. Veins always travel toward the heart and arteries always travel away from the heart.

Efferent arterioles are specialized arterioles in the kidneys that transport blood between two separate capillary beds, known as the renal portal system.

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.  

Angiogenesis is the development and growth of new blood vessels. The process is regulated by a class of peptides called angiogenic factors. Hematopoiesis is the process by which blood cells are made, in the bone marrow. Atherosclerosis refers to the thickening and the loss of the elasticity of the arteries due to deposition of plaques. Hypertension refers to high blood pressure. Vasoconstriction involves the contraction of smooth muscle surrounding vessels, decreasing blood flow and increasing blood pressure in those vessels.

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. 

The statum corneum is the superficialmost layer of the skin and is not a component of blood vessels.

Blood flow in the veins is much slower because the blood is much farther from the left ventricle. Since the force of this powerful contraction is far "behind" venous flow, blood in veins is more likely to flow backward than in an artery. Because of this, veins have valves to prevent backflow much like the valves in the heart prevent back flow between chambers. The reason the blood flow in deep veins is generally faster than that of superficial veins is because muscle contractions help to move blood along in deep veins. A prime example is the lower legs. Contraction of the gastrocnemius (calf) muscle causes blood to move up through the lesser saphenous vein.