The constancy of the internal environment: proteins in plasma

The simple fact that there are normal concentrations of various proteins and peptides in blood has broad and surprising implications for how these molecules are released into and removed from the bloodstream by the cells of the body. If, as widely accepted, vesicle transport mechanisms such as exocytosis and endocytosis account for these events, then complex, presently unknown and seemingly unlikely mechanisms must exist to coordinate the rates of separate transport processes. The basis for this conclusion as well as the sole alternative method of transport, movement across permeable membranes, are discussed.