The capillaries that contain a complete lining are called continuous capillaries. The lining itself is referred to as the endothelial lining. Variations in this lining determine the "leakiness" of the capillary.
The endothelial linings of capillaries, also called the tunica intima, represent an inner layer with a thickness of only one cell. The endothelial cells of the continuous capillaries create an uninterrupted lining that only permits small molecules such as ions and water to pass through. These substances pass through tight intercellular clefts, or gaps, in the endothelial linings. No special transport mechanism is required and bidirectional diffusion tales place depending on the osmotic gradients existing on either side of the capillary wall. The first individual to observe and describe capillaries was Marcello Marpighi in 1661 after he discovered them in a frog's lung.