Blood vessels are the highway system of the body.

They are tubular structures that transport blood throughout the body.

There are approximately 60,000 miles/100,000kilometers of blood vessels in the human body; this ensures that every cell is within diffusion distance from a capillary.

There are 5 different types of blood vessels:
1. Arteries
2. Arteriole
3. Capillaries
4. Venules
5. Veins

1. ARTERIES
– Arteries are thick-walled blood vessels that carry blood, under high pressure from the heart out toward the extremities of the body.
– The blood in the arteries is usually under high pressure because the blood had just left the heart.
– The arteries carry oxygen-rich blood from the left side of the heart.
– As the arteries bring blood to the outer extremities of the body, they become smaller and smaller in diameter.

2. ARTERIOLES
– The major arteries branch into smaller arteries, eventually becoming arterioles.
– These small blood vessels’ walls contract to control blood flow to the organs.
– The amount of blood that flows into a particular tissue depends on the diameter of the arterioles.
– If an organ needs oxygen the arterioles relax and the arterioles diameter increases in size, increasing the blood flow to that organ.
– The arterioles can also contract, reducing the diameter and therefore the blood flow when needed.

3. CAPILLARIES
– Eventually blood vessels become the thickness of one cell. These very narrow blood vessels are call capillaries.
– While the blood moves through the capillaries, gases, hormones and other nutrients diffuse in and out of the blood and the surrounding tissue.
– Capillaries also collect the waste produced from the cells of surrounding tissue and brings them back toward the heart.
– Capillaries gradually increase in size, becoming larger vessels called venules.

4. VENULES
– When several capillaries join, they form veins called venules.
– Venules are very small blood vessels that allow blood to return from the capillary beds to larger blood vessels called veins.

5. VEINS
– Veins are thin-walled vessels that bring the blood back to the heart.
– The lowest blood pressure is found in the veins.
– Most veins run upward or against gravity, therefore they rely on skeletal activity (like walking and breathing) and muscle contractions (in the legs and other parts of the body) to help move the blood back to the heart.
– Veins also have valves in them that allow the blood to move in one direction and prevent the back flow of blood.