Capillary fluid exchange is when nutrients, gases, wastes, and other substances are swapped between the capillary blood and internal body cell tissues. It occurs within the capillaries, and microscopic blood vessels with arterial and venous ends. The heart pumps and sends blood through the arteries to the capillaries, where the fluid leaves the arterial end and re-enters at the venous one due to the change in pressure.
The capillary walls consist of endothelial cells, which are permeable, allowing for the passage of substances in several ways. One of them is diffusion when gases spread with the blood’s concentration gradient. Oxygen and carbon dioxide diffuse in the reverse direction, moving to an area with low concentration. The capillary movement has another transport mechanism, bulk flow, which involves substances moving in the same direction in response to force caused by two types of pressure, hydrostatic and osmotic. Overall, diffusion and bulk flow are two major transport mechanisms.
Pressure is essential for bulk flow to perform its transporting functions. Hydrostatic pressure is primarily responsible for fluid transportation between capillaries and tissues, and it is exerted by the blood volume against the capillary’s walls. They have holes, allowing the fluids to exit the bloodstream and increase hydrostatic pressure in the intestinal fluid. At the venous end, hydrostatic pressure is low because the lymphatic vessels absorb excess fluids from body tissues by filtration.
Osmotic pressure is the movement of the interstitial fluid into the capillaries, which is determined by a difference in solute concentration in the blood and tissue fluid. Oncotic pressure, a form of osmotic one, is induced by proteins, forcing the movement into an area with low water concentration. Ultimately, both diffusion and bulk flow are necessary for capillary fluid exchange, as they equalize concentration levels.
