Capillaries

The intimate relationship between the circulatory system and the tissues is achieved by, at the level of capillaries in the form of capillary network. the capillaries are extremely narrow (7-10u diameter), thin walled microscopic vessels. Their walls consists of a single layer of endothelium, which presents very little resistance to the diffusion of dissolved substance in or out . The cells of the tissues are bathed in tissues which provides a medium through which diffusion of materials can take place. The close approximate between the capillaries and the tissue cells facilitates the exchange of material. It is only here that exchange by diffusion or active transport can occur. The oxygen carried by the blood is diffused out into the oxygen deficient tissue and the carbon dioxide of the tissue cells is diffused in simultaneously. The nitrogenous waste is filtered through the capillaries into the excretory tubules for discharge.