Overview: Circulation of body fluids
The three main body fluids, plasma, tissue fluid and lymph, are in constant circulation, passing through three distinct compartments: Blood vessels (blood), tissue spaces (tissue fluid) and lymphatic structures (lymph). Fluid derived from the blood plasma enters tissue spaces as tissue fluid. From there, tissue fluid flows into lymphatic vessels as lymph which eventually enters the blood stream. At each level, components of the immune system are monitoring these fluids for pathogens, toxins and foreign antigens.
Blood >
Blood consists of a fluid portion, plasma, that carries blood cells and platelets. As blood flows through capillaries in body tissues, hydrostatic pressure forces a filtrate of plasma into the tissue spaces, forming tissue fluid or interstitial fluid. Immune surveillance of blood occurs in the spleen.
Tissue fluid >
Tissue fluid forms as a filtrate of plasma, percolates through connective tissues where it is monitored by MALT. Innate surveillance occurs by macrophage phagocytosis, natural killer cells and inflammatory reactions. MALT also initiates adaptive immune responses, i.e., T and B cell activation.
Lymph >
Tissue fluid enters lymphatic capillaries as lymph. Lymph can contain antigens, bacteria and metastatic cancer cells along with immune cells emigrating from the tissues. Lymph fluid passes through lymph nodes (blue arrows) located along lymphatic vessels where adaptive responses are amplified and additional ones are initiated. Lymph eventually enters the venous side of the blood stream where it mixes with the plasma (red circle). Immune surveillance of blood occurs in the spleen.
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