Capillaries

Capillaries, the network of tubes which communicate on the one hand with the ultimate arterioles, and on the other hand with the smallest branches of the veins. The diameter of a capillary vessel varies in different parts of the human body; some are barely large enough to enable a single red blood corpuscle to traverse them, as in parts of the brain; elsewhere, as in the skin, the capillaries are considerably larger. In some organs the meshes of the network have an elongated form, as in muscle, while in the alveoli of the lung, and in glands, a rounded form prevails. The closeness of the network is in direct correspondence with the vascularity of the part. The walls of a capillary are composed of a single layer of flattened cells, endothelium (q.v.), and permit of the ready interchange of materials between the blood and the tissues.