Capillaries are the smallest blood vessels in the body, connecting arteries and veins. They are so tiny that red blood cells can only pass through them in single file. Their thin walls, made of a single layer of cells, allow for the exchange of oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues. These tiny vessels play a crucial role in the circulatory system by facilitating the delivery of essential substances to cells and the removal of waste. The extensive network of capillaries ensures that every cell in the body receives the nutrients it needs to function properly.