lymphatic system

Meaning: The lymphatic system is an open-ended network of vessels, lymph nodes, and organs such as the spleen, thymus, and tonsils. Blood capillaries filter fluid into the interstitial space. Most is reabsorbed, but the excess enters blind-ended lymph capillaries and becomes lymph. Lymph flows one way, pushed by skeletal muscle contraction, breathing, and valves rather than by a heart, and it drains into veins near the heart through the thoracic duct and right lymphatic duct. It has three jobs: fluid balance, fat absorption (chylomicrons enter lacteals in intestinal villi), and immunity (nodes filter lymph, and B and T cells meet antigen there).

On the MCAT: Expect fluid-balance questions that use capillary hydrostatic and oncotic pressures. If lymph drainage is blocked, or plasma protein falls and lowers oncotic pressure, interstitial fluid builds up as edema. Digestion passages test routes: absorbed fats travel through lymph and reach the blood at the subclavian veins, whereas amino acids and monosaccharides go through the hepatic portal vein to the liver. Swollen nodes signal an active immune response.

Common confusion: Students treat the lymphatic system as a second closed circuit. It is not. Lymph capillaries begin blindly, and lymph ends up back in the blood. Also, lymph nodes filter lymph, while the spleen filters blood.