Blood Vessels

 Arteries carry blood from the heart out to the capillaries. Their walls are fairly thick and contain elastic fibers and/or smooth muscle fibers.

Capillaries are the tiniest of the blood vessels. They consist of a single layer of simple squamous epithelium rolled into a tube. Their thin walls allow nutrients, oxygen, etc. to freely move out of the blood to reach the cells.

Veins return blood from the capillary beds to the heart. Many veins contain valves which, aided by surrounding muscular movements, help keep the blood moving toward the heart.