RSE and GM sees the body fluid compartments existing in a state of dynamic equilibrium, not as static buckets of fluid. The work of Robert Hahn shows that we can reasonably approximate extracellular fluid kinetics in clinical practice to a two-compartment model, the central and tissue volumes. In traditional Starling view, the filtration Jv is balanced by capillary absorption of tissue fluids, plus some lymphatic return. But modern experiments prove instead the steady state Starling principle; there is no absorption at steady state, so filtration Jv is entirely balanced by lymph return. Some fluid of the afferent lymph is reabsorbed to the plasma by lymph node capillaries while the protein-rich remainder procedes to the lymphatic duct. In the diagram below, the more important factors determing Jv, Qlymph and fluid output are listed in red boxes.
Reflect on the fact that we now have to recognise a vital secondary circulation, the circulation of tissue fluids bathing the cells of the tissues.
When we infuse an isotonic salt solution it initially expands both the free-flowing plasma and the intravascular gel phase. The expanded intravascular volume can increase the cardiac preload with only a small dilution of the red cells.
Now contrast the situation of infusion of a colloid solution. The free-flowing plasma volume increases, but the gel phase is slower to increase and may even be dehydrated by increased plasma oncotic pressure. See the diagram below. Notice how red cell dilution (anaemia) is caused with a similar increase in the intravascular volume and preload. The Colloid Delusion.
Jv increases, and Vt increases if Qlymph does not immediately rise to compensate. What can we do to stimulate Qlymph and reduce expansion of Vt? A norepinephrine infusion will do the trick.
Three superimposed J curves illustrating the effects of low, normal and high plasma oncotic pressure on the relationship between Jv and Pcap. At lower capillary pressure Jv is close to zero regardless of the plasma oncotic pressure. Always use isotonic crystalloid solutions for resuscitation. Above the J point plasma oncotic pressure starts to matter, but why would you want to transfuse a patient who is already hypervolaemic and at risk of oedema?