Pathways of material exchange between blood and interstitial fluid
Substance exchange between blood and interstitial fluid occurs through the capillary wall. The main exchange methods are as follows:
Diffusion: refers to the thermal movement of solute molecules in a liquid. It is the main method of material exchange between blood and interstitial fluid. Substances inside and outside the capillary can diffuse through the tube wall as long as its molecular diameter is smaller than the pores in the capillary wall. Its diffusion rate is proportional to the concentration difference of the substance on both sides of the capillary wall. The concentration of oxygen and glucose in the blood is higher than that of the interstitial fluid, so oxygen and glucose diffuse from the plasma into the interstitial fluid; while the concentration of carbon dioxide in the plasma is lower than the interstitial fluid, it diffuses from the interstitial fluid into the plasma. The speed of diffusion is related to factors such as the concentration difference of the solute molecules on both sides, the diffusion distance, the area of the diffusion interface, the temperature, and the size of the solute molecules.
Swallowing: The swallowing activity of capillary endothelial cells is to enclose the material on one side through the cell membrane, so that it enters the cytoplasm to form a swallowing vesicle, which is transported to the other side of the cell and expels the cell. This transfer mode is very slow and is an important way to exchange some non-fat-soluble macromolecular substances (such as plasma proteins).
Filtration and reabsorption When the hydrostatic pressure on both sides of the capillary wall is not equal, water and solute molecules will flow through the pores of the capillary wall and flow from the high pressure side to the low pressure side. Its direction and amount depend on the effective filtration pressure of the capillaries. When the effective filtration pressure is positive, the plasma liquid is filtered through the capillary wall to form tissue fluid; when the effective filtration pressure is negative, the tissue fluid is reabsorbed into the blood vessels to form plasma. This method is not important for the material exchange between blood and tissue fluid, it only accounts for a small part of the total material exchange, but it is of great significance for maintaining the dynamic balance between blood and tissue fluid.