The main function of the cardiovascular system occurs at the capillaries. Exchange of metabolic substrates and waste products occurs across the thin-walled capillary endothelium. This function is well suited to the capillaries not only because of the low diffusion distances across the capillary (most cells are only 20-30 micrometers (um) from a capillary), but also because blood flow velocity is lowest in the capillaries and the capillaries present a very large surface area for exchange. Different capillary beds have different types of endothelium, that is, continuous endothelium with tight junctions between cells; fenestrated endothelium with openings or “windows” within the cells; and discontinuous endothelium with large openings between cells (Figure 18-1). Therefore the structure of the capillary wall will affect transport across the walls, especially transport of large molecules. Tissue function is directly related to tissue capillary structure. The brain capillary endothelial cells have tight junctions to help prevent transfer of many blood-borne molecules into the brain. Liver and the gastrointestinal tract on the other hand are functioning in transport of molecules into and out of the circulation and therefore have capillary endothelial cells that are fenestrated or discontinuous.
The capillary anatomy contributes greatly to the permeability properties of the capillary. Three types of capillaries are the continuous with very low permeability, fenestrated (windows) with greater permeability characteristics and the most permeable (or leaky) are the sinusoid or discontinuous types of capillaries.
The main mechanisms by which materials cross the capillary wall are diffusion, bulk flow, and pinocytosis. Diffusion is the process by which most materials cross the endothelium. Diffusion of a material depends on a concentration gradient and the properties of the material that is diffusing. Lipid-soluble materials can pass through endothelial cells as well as between cells. Therefore substances such as carbon dioxide, oxygen, and lipid-soluble drugs, such as anesthetics, can diffuse readily across the capillary wall. Substances that are water soluble such as glucose, ions, and amino acids can diffuse through pores or fenestrations of the capillary wall. In addition, the size of water-soluble materials affects diffusion; molecules such as glucose with a molecular weight of 180 g/mole can usually diffuse readily whereas molecules such as albumin which has a molecular weight of approximately 69,000 diffuse very little. Globulin, with a molecular weight of approximately 120,000 g/mole essentially does not diffuse.
Diffusion, therefore, is directly proportional to the diffusion properties of the substance that is moving, the area available for diffusion, and the concentration gradient of the substance; and inversely proportional to the thickness of the membrane across which diffusion is occurring. In studying diffusion across the capillary wall, diffusion is determined by the concentration of the substance, the surface area available for diffusion, and the permeability properties of the solute and the capillary wall. The surface area available for diffusion can be ...