Final answer:
Blood plasma and interstitial fluid continuously exchange through semi-permeable capillary walls, enabled by osmotic pressures and assisted by the lymphatic system, nourishing cells and removing waste.
Step-by-step explanation:
The continuous exchange between blood plasma and the interstitial fluid is facilitated by the semi-permeable capillary walls that allow water, inorganic ions, and small organic molecules to pass through. This exchange is crucial for delivering nutrients to cells and removing waste. The capillary walls are permeable to substances like glucose, amino acids, urea, but not to large molecules such as proteins. The capillary filtration releases plasma into the interstitial space, which becomes interstitial fluid that bathes the cells.
The composition of interstitial fluid, which includes amino acids, sugars, fatty acids, and other necessary components for cellular function is similar to that of plasma, but without the plasma proteins. However, the continuous exchange is regulated by osmotic pressures; blood colloidal osmotic pressure (BCOP) encourages reabsorption of water back into the capillaries, while the lymphatic system is responsible for draining excess interstitial fluid into the bloodstream.
Moreover, extracellular fluid, which encompasses both plasma and interstitial fluid, maintains the osmotic, anion-cation balance essential for physiological processes. Factors such as capillary hydrostatic pressure and the blood colloidal osmotic pressure create a dynamic equilibrium that facilitates this continuous exchange between plasma and interstitial fluid.