The three kinds of vessels are arteries, capillaries , and veins. Most veins return blood to the heart after it has given out nourishment to the tissues and take up waste products and pois ons. Blood in veins is called venous blood. It can be seen only under the microscope. Most capillaries are so small that only one blood cell can pass through them at a time.
Capillaries connect the smallest arteries with the smallest veins.
They make up a network of blood vessels throughout the body. The smallest are in the brain and lining of the intestine.