What is the smallest type of blood vessel in the human body?

Capillaries are the smallest type of blood vessel in the human body, playing a critical role in the circulatory system. They are the sites where the exchange of oxygen, carbon dioxide, nutrients, and waste occurs between the blood and the body's tissues. Their walls are incredibly thin, composed of a single layer of endothelial cells, which facilitates this exchange.

Arteries, while vital for transporting oxygen-rich blood away from the heart, have thicker walls to withstand high pressure; therefore, they are larger than capillaries. Veins carry deoxygenated blood back to the heart and also have larger diameters, though they possess valves to prevent backflow. Venules are small vessels that collect blood from capillaries and merge to form veins, but they are still larger than capillaries. Thus, capillaries represent the smallest and most specialized type of blood vessel necessary for nutrient and gas exchange in the body.