| A | B |
|---|
| arteries | The walls of this type of vessel have a thick layer of smooth muscle and some elastic connective tissue fibers. |
| vein | Blood flows very slowly in this type of vessel because there is not much pressure. |
| capillaries | These vessels are the smallest blood vessels in the body. |
| artery | This type of vessel has strong walls to withstand the pressure of blood surging through it. |
| veins | Most vessels of this type contain one-way valves to prevent blood from flowing backward |
| capillary | This type of vessel has walls that are only a single cell thick. |
| artery | This type of vessel carries blood away from the heart. |
| vein | This type of vessel carries blood back to the heart. |
| capillary | This type of vessel is so small that red blood cells must squeeze through in single file. |
| artery | The aorta is one of the largest of this type of vessel. |
| vein | Because the layers of muscle cells and connective tissue in the walls of this type of vessel are thinner, they may look collapsed under a microscope. |
| capillary | Materials like oxygen, carbon dioxide, nutrients, and wastes can diffuse through the walls of this type of vessel. |
| capillaries | These vessels connect arteries to veins. |
