Capillaries connect the arteries to the veins. Arteries carry blood pumped from the heart, while veins return the oxygen-starved blood back to the heart. Capillaries connect the two blood vessels to facilitate the transfer of blood. Capillaries are the smallest blood vessel. Nutrients and wastes are exchanged in the process, transitioning from the arteries to arterioles then to capillaries before transitioning back to the veins through the venules. There are three main types of capillaries: continuous, fenestrated, and sinusoidal. Capillary blood sampling is used to test for blood glucose (for monitoring purposes), hemoglobin, pH, and lactate. The existence of capillaries was suspected in 1653 and first observed and described in 1661 by Marcello Malpighi.