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Biology of the Blood Vessels


The Manual's Editorial Staff

Last full review/revision Apr 2020| Content last modified Apr 2020
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What are blood vessels?

Blood vessels are hollow tubes like pipes that carry blood through your body. The blood delivers oxygen and nutrients to all parts of your body and removes waste products, such as carbon dioxide.

  • There are 2 main kinds of blood vessels—arteries and veins

  • Arteries carry fresh blood from your heart to your organs

  • Veins carry waste-filled blood back to your heart

  • Arteries and veins are connected by microscopic vessels called capillaries

Arteries have thick walls lined with muscle. Arteries need to be strong because blood pressure is highest in the arteries. Muscles of your arteries constantly squeeze and relax to help adjust your blood pressure.

Veins have thin walls with only a little muscle. Blood pressure is lower in the veins. Veins can get wider to handle increased blood. Some veins have valves to keep the blood from flowing backward.

Your blood vessels and your heart are parts of the cardiovascular system.

Blood Vessels: Circulating the Blood

Blood travels from the heart in arteries, which branch into smaller and smaller vessels, eventually becoming arterioles. Arterioles connect with even smaller blood vessels called capillaries. Through the thin walls of the capillaries, oxygen and nutrients pass from blood into tissues, and waste products pass from tissues into blood. From the capillaries, blood passes into venules, then into veins to return to the heart.

Arteries and arterioles have relatively thick muscular walls because blood pressure in them is high and because they must adjust their diameter to maintain blood pressure and to control blood flow. Veins and venules have much thinner, less muscular walls than arteries and arterioles, largely because the pressure in veins and venules is much lower. Veins may dilate to accommodate increased blood volume.

Blood Vessels: Circulating the Blood

What can go wrong with my blood vessels?

If a blood vessel is cut or tears, you bleed, either on the outside or internally within your organs. Other common blood vessel problems include:

  • Atherosclerosis (hardening of the arteries), in which fat deposits block your arteries

  • Aneurysms, which are bulges in weak areas of the walls of your arteries

  • Blood clots can form in your veins (called deep vein thrombosis) or in your arteries, causing sudden blockage

  • Vasculitis, in which your blood vessels become inflamed

  • Varicose veins, in which veins near the surface of your skin get enlarged and twisted

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