Capillaries feed the heart: They are the tiniest vessels that bridge the smallest arteries to small veins called venules. From there, blood passes into veins that serve as tributaries to larger veins before entering the heart.

Capillaries are the smallest, thinnest blood vessels in the whole body. They receive blood from the arterioles and form networks called capillary beds, which are the locations where gases are exchanged and nutrients and other substances are exchanged for waste products with the tissues. White blood cells may also leave the circulatory system through the capillary walls during immune-system responses.

Venules are the smallest, thinnest veins. They receive blood from the capillaries and deliver that blood into larger veins. The walls of the veins have the same three layers as the arteries: the tunica intima, the tunica media, and the tunica adventitia.


Veins differ from arteries in that veins may have valves that project into their lumens to prevent the backflow of blood, they have larger lumens, and they have less smooth muscle. Also, the blood pressure in veins is lower than that in the arteries.

Veins can be classified as deep or superficial.

  • Deep veins: The deep veins usually run alongside arteries and frequently share the same names as those arteries. Examples of some deep veins include

    • The posterior tibial vein of the ankle and leg

    • The external and internal iliac veins of the pelvis

    • The brachial vein of the arm

    Deep venous thrombosis is a blood clot (thickened clump of blood) that forms in a vein. It can break loose and form an embolus (a blood clot that isn’t attached to the walls and can travel through the vein), which can be life threatening if it reaches the lungs, causing a pulmonary embolism (a blood clot lodged in an artery of the lung).

  • Superficial veins: The superficial veins are closer to the surface of the body and are often visible through the skin. They include

    • The great saphenous vein of the lower extremity

    • The cephalic vein of the upper extremity

    • The median cubital vein just anterior to the elbow

    Varicose veins are swollen veins that may have a tortuous appearance and may become painful. They’re often visible in the lower extremities and are more likely to affect women who stand for long periods of time. Compression stockings that squeeze the leg and assist in moving the blood through the veins may be prescribed to treat varicose veins. Other treatments include laser surgery that closes the vein or vein stripping (a surgical procedure that removes the vein).