Mini MCAT passage: Studying lymph in a model system

Before entering the blood, lymph passes through lymph nodes. It is carried in and out by lymphatic vessels. Blood vessels are also present in the lymph nodes. Blood and lymph do not mix directly, but the lymph nodes are nevertheless a place where blood and lymph can interact across thin cellular layers with high surface area. These layers are similar to the endothelial walls of capillaries.

A study investigates the role of lymph nodes in changing the composition of lymph fluid. The study uses dog popliteal lymph nodes as an experimental model. Popliteal lymph nodes are located in the knee region. These lymph nodes are removed from dogs, along with the lymphatic vessels and blood vessels that perfuse the lymph nodes. All the vessels are connected to pumps, and lymph and blood are then pumped through the vessels to observe how they interact.

The results of the experiment show that changing the pressure of the blood passing through the lymph nodes has an effect on protein concentration of the lymph. When blood pressure in the blood vessels approaching the lymph node is increased from $40$‍ mmHg to $60$‍ mmHg, the protein concentration of the lymph coming out of the lymph nodes is reduced from $2.3$‍ g/dL to $1.8$‍ g/dL.