Diffusion
The most important method of capillary exchange is simple diffusion . Substances such as O2, CO2, glucose, AAs and most hormones enter and leave the capillaries by simple diffusion. O2 and most nutrients are of higher concentration in the blood and therefore diffuse down their concentration gradient to the interstitial fluid and into the body’s tissues. CO2 and waste products are of higher concentration in the interstitial fluid and therefore diffuse down their gradient into the blood. Most substances in the blood or interstitial fluid can cross the walls of a capillary by diffusing through:
Intercellular clefts or Fenestrations
Endothelial cells
Water-soluble substances such as glucose, and amino acids pass across the capillary wall through intercellular clefts or fenestrations. Lipid-solublematerials such as O2, CO2 and steroid hormones may pass through capillary walls directly through the lipid bilayer of endothelial cell plasma membranes. Most plasma proteins and RBCs cannot pass because they are too large. In sinusoids (a type of capillary), the intercellular clefts are so large that they allow even proteins and blood cells to pass. For example: hepatocytes synthesize and release many plasma proteins (albumin, and fibrinogen) which diffuse into the blood through sinusoids. A small amount of substances may cross capillary walls by transcytosis . Substances in blood plasma become enclosed within tiny pinocytic vesicles that enter endothelial cells by endocytosis, cross the cell and exit by exocytosis. This method is mainly for large, lipid-insoluble molecules. Example: the hormone insulin.
256
Made with FlippingBook Publishing Software