Capillaries are tiny blood vessels that connect arteries and veins in the body. They are so small that red blood cells can only pass through them in single file. This unique structure allows capillaries to deliver oxygen and nutrients from the blood to the surrounding tissues, while also collecting waste products like carbon dioxide. The walls of capillaries are very thin, which makes it easy for substances to move in and out. This process is essential for maintaining healthy cells and tissues. Without capillaries, the body's cells wouldn't receive the oxygen and nutrients they need to function properly.