Final answer:
The lymphatic system maintains blood volume and pressure by collecting and returning interstitial fluid back to the bloodstream and plays a critical role in immune defense and digestion.
Step-by-step explanation:
The role of the lymphatic system in relation to the cardiovascular system is primarily to maintain blood volume and, consequently, pressure. This is achieved through the return of lymph to the bloodstream. When blood travels through capillaries under pressure, some components like water, oxygen, and nutrients filter out into the tissue spaces between cells, forming interstitial fluid. The interstitial fluid that is not reabsorbed directly by the blood vessels is collected by the lymphatic system, which transports it through lymph capillaries and vessels, and eventually returns it to the bloodstream near the heart. The lymphatic system also plays a crucial role in host defense by the transport of immune cells and absorption of fatty acids from digestion.