BLOOD VESSELS

ARTERIES Arteries carry blood away from the heart. Arteries branch into smaller blood vessels called arterioles. Arteries have thick, smooth muscle walls and are very elastic. What is the largest artery in the body? What is the only artery in the body that carries deoxygenated blood?

VEINS Veins carry blood to the heart. Veins have much thinner walls and less elasticity than the arteries. Veins have valves to prevent the backflow of blood. Veins are the confluence of smaller blood vessels called venules. What are the largest veins in the body? What are the only veins in the body that carry oxygenated blood?

Capillaries Capillaries connect arterioles and venules. Capillary walls are only one-cell thick. This enables nutrients and oxygen in the blood to easily pass out of the capillary and into the cells of the body where they are needed. It also enables wastes and carbon dioxide to pass out of the body cells and into the blood in the capillaries.

So, the pathway of blood from the heart is aorta arteries arterioles capillaries venules veins vena cavae right atrium

All exchanges of gases (oxygen and carbon dioxide), nutrients, and wastes take place at the level of the capillaries.

Histology of the Blood Vessels

Artery-Vein Pair Artery is on left, vein on right

Longitudinal Section of a Capillary – Note that it is only as wide as the diameter of a red blood cell.

Do you remember that we said that arteries had a lot of elasticity Do you remember that we said that arteries had a lot of elasticity? Here you can clearly see the internal elastic membrane of an artery.

Here you can see the internal elastic membrane at higher magnification using a different histological stain.

Low magnification of an artery Low magnification of an artery. The artery is the thick, round structure in the center.

Another artery; another stain

Small artery-vein pair. Artery is on left, vein on right Small artery-vein pair. Artery is on left, vein on right. Do you know what a lumen is?

Vein – You can see the valve here Vein – You can see the valve here. Realize you are looking at at a 2-dimensional representation of a 3-dimensional object.

Longitudinal section of a vein Longitudinal section of a vein. Note that this cannot be a capillary since its diameter is more than 1-cell wide.