The capillary wall consists of an endothelial cell layer and a basal membrane, visible under the electron microscope. In some organs, such as the brain, there are also additional contractile cells, called pericytes, attached to the outer wall. The endothelial cells of the capillary wall, which are between 25 and 50 micrometer long, as a rule adjoin each other tightly, leaving or not gaps in between, and form and endothelial tube, which measures between 5 and 10 micrometer in diameter. Transport of materials occurs from the capillaries into the surrounding tissue and vice versa, and in both cases through the tube of endothelial cells. The capillary makes it possible the exchange of oxygen, carbondioxide, water, food, and waste materials between surrounding tissue and blood.
There are three types of capillary: 1) continuous capillary, which provide uninterrupted lining, allowing only small molecules, such as water molecules and ions to diffuse through it; 2) fenestrated capillary, which has pores or gaps, through which limited amount of protein diffuse; 3) sinusoidal, which are similar to fenestrated capillaries, but with larger pores in between the endothelial cells to allow red and white blood cells, as well as serum protein, to go through them.