Final answer:
Blood plasma exits and re-enters the circulatory system through filtration, driven by a hydrostatic pressure gradient. About 85% of the filtered plasma diffuses back into the capillaries, while the remainder is taken up by the lymphatic system to be filtered and returned to the circulatory system.
Step-by-step explanation:
Blood plasma leaves and re-enters the circulatory system through a process known as filtration. This biological mechanism involves the movement of fluid due to hydrostatic pressure gradients across the endothelial lining of capillaries and into surrounding tissues. Filtration, as opposed to diffusion, relies on this pressure gradient to push both the fluid and the solutes from a region of higher pressure to one of lower pressure. In the kidneys, filtration is a critical step for removing waste from the bloodstream. After plasma has filtered out of the capillaries, approximately 85% of it diffuses back into the capillaries near the venules due to the hyperosmotic condition that arises from the filtration process. The remaining plasma enters the lymphatic system, ultimately returning to the heart after being filtered by lymph nodes.