Describe how Starling forces determine fluid flux within the pulmonary capillary bed.
The equations for nett fluid flux and for nett filtration pressure were incorrect in many answers.
Better answers presented the equations and discussed each of the elements as relevant to the
pulmonary capillary bed, including difference from systemic capillary beds.
Mention of the role of lymphatics and of the effect of surfactant, left atrial pressure, gravity and
posture gained marks, also.
The equation for "nett fluid flux" is usually this:
Qf = k × Am × ΔP / (η × Δx)
Where
- Qf is the net movement of fluid,
- k is the filtration constant of the capillary membrane,
- Am is the area of the capillary walls (all of them),
- ΔP is the net pressure balance between the hydrostatic and oncotic pressures,
- η is the viscosity of the fluid, and
- Δx is the thickness of the capillary wall.
However, there is no mention of capillary wall thickness or fluid viscosity in the college answer, and - judging by the rest of their comments, a better choice would probably have been the Starling equation, which looks like this:
Jv = Lp S [ (Pc - Pi) - σ(Πc - Πi) ];
It is easier to break down its components into something relevant and specific to the pulmonary circulation:
Starling, Ernest Henry. "On the absorption of fluids from the connective tissue spaces." Classic Papers in Critical Care 19 (1896): 303.
Woodcock, T. E., and Thomas M. Woodcock. "Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy." British journal of anaesthesia 108.3 (2012): 384-394.
Erstad, Brian L. "The Revised Starling Equation: The Debate of Albumin Versus Crystalloids Continues." Annals of Pharmacotherapy (2020): 1060028020907084.
Krogh, August, E. M. Landis, and A. H. Turner. "The movement of fluid through the human capillary wall in relation to venous pressure and to the colloid osmotic pressure of the blood." The Journal of clinical investigation 11.1 (1932): 63-95.
Levine, O. Robert, et al. "The application of Starling's law of capillary exchange to the lungs." The Journal of clinical investigation 46.6 (1967): 934-944.
Mellins, Robert B., et al. "Interstitial pressure of the lung." Circulation research 24.2 (1969): 197-212.
Staub, Norman C. "Pulmonary edema." Physiological Reviews 54.3 (1974): 678-811.