What is the role of the lymphatic system in tissue fluid balance and immunity?

• A. Pumps blood through capillaries
• B. Produces red blood cells
• C. Filters arterial blood
• D. Drains excess interstitial fluid, returns proteins to circulation, and supports immune cell transport; returns fluid to venous system via subclavian veins

Answer: (D)

The main idea here is how the lymphatic system helps keep tissue fluid balanced and supports immunity. Interstitial fluid that bathes tissues comes from the plasma leaking out of capillaries. Most of this fluid is reabsorbed back into the blood, but a portion, along with proteins and fats, drains into tiny lymphatic vessels. This lymphatic drainage prevents tissue swelling and returns proteins to circulation, eventually sending the fluid back into the bloodstream via large ducts that empty into the subclavian veins (the thoracic duct on most of the body and the right lymphatic duct for the upper right side).

Beyond drainage, the lymphatic system is a highway for immune cells. Lymph transports lymphocytes and antigen-presenting cells to lymph nodes, where the immune system checks for pathogens and mounts responses. The combination of reabsorbing excess fluid and proteins and delivering immune cells to sites of immune surveillance explains why this role is essential for both tissue fluid balance and immunity.

For the other options: pumping blood through capillaries is the job of the cardiovascular system, producing red blood cells is a function of bone marrow, and filtering arterial blood is not a primary role of the lymphatic system.