Starling’s Law for the Capillary-Interstitial Fluid Transfer is Wrong?

For nearly a decade I have been encouraging physicians to question the century-old medical school physiology shibboleth concerning fluid volume filtration from followed by absorption to the lumen of an archetypal Capillary; the filtration – absorption model. At steady state, the extended Starling principle holds that the hydrostatic capillary pressure difference declines between arteriole and venule while the colloid osmotic pressure difference across the endothelial glycocalyx layer which opposes trans endothelial filtration also declines, resulting in filtration along the whole length of a capillary; the filtration – no absorption model. This is the first time I have considered Ghanem’s hypothesis of a fluid absorption – filtration model.

Dr Ghanem’s description of a magnetic field-like fluid circulation occurring in a constructed hydrodynamic model of a biological capillary is a fascinating contribution to the conversation. It is widely accepted that blood flow through distensible branching vessels has many characteristics that make comparisons with the way a fully Newtonian fluid flows along a straight rigid tube extremely difficult. Dr Ghanem offers what he believes to be a more appropriate hydrodynamic model for the microvasculature. The Figure below is drawn from an experiment he designed and shows some interesting results.

From right to left, we see an ‘arterial’ pulse pressure of approximately 120/55 mmHg driving fluid through a rigid orifice (representing a muscular capillary sphincter) into a rubber tube of 18 cm length (representing a distensible capillary). At 1, 3, 5, 7 and 9 cm the rubber tube is penetrated by a bevelled needle connected to a water manometer acting as a simple pressure transducer. With the bevel facing upstream Ghanem recorded pressures of 38, 31, 20, 15 and 12.5 mmHg up to 9 cm along the rubber tube which he called the Flow pressures (FP, along the top of the above diagram). Ghanem suggests, however, that to measure the pressure driving fluid filtration through slits in the side wall we need to know what he calls the Side pressure, obtained by rotating the bevelled needles to face downstream (SP, along the bottom of the above diagram). In this orientation the pressure profile is fully reversed. At 1, 3, 5, 7 and 9 cm along the rubber tube Ghanem recorded pressures of -34, -17, -5.5, -3.5 and -0.7 mmHg. Ghanem therefore deduces that interstitial fluid is absorbed into a porous capillary immediately after blood has left the capillary sphincter, and that hydrostatic pressure rises (absorption reduces) as blood passes further along the capillary until it becomes positive and filtration of plasma water to the interstitium begins to occur.

Ghanem’s second deduction is that this pressure profile and absorption -> filtration model causes a magnetic field-like fluid circulation around the capillary. Fluid filtered at the venular segment flows through the interstitium countercurrent to the blood flow in the adjacent capillary and enters the capillary at its origin from a capillary sphincter. In a more recent communication, Ghanem offers a photograph of fluid filtering from a porous rubber tube in a bench top experiment conducted in Eastbourne, England, where Dr Ghanem was employed as a National Health Service Urologist. One easily observes that filtration increases along along the length of this hydrodynamic model of a capillary, quite the reverse of the standard teaching of declining filtration pressure as blood passes along a capillary.

There is much more to be said about Ghanem’s Model. Starling himself pointed out that if the interstitial pressure became higher than the capillary pressure, the capillary would collapse, but I presume Dr Ghanem would counter that flow pressure keeps the capillary open. I will notify Dr Ghanem of this blog, and invite him to join a conversation which I will continue in the coming days. Why not join in and add your own thoughts and reactions?

By admin

after more than a quarter of a century of intensive care medicine consultancy in one of the UK's largest teaching hospitals Dr Woodcock is on a mission to ensure the steady state Starling principle is known and understood by every student and every practitioner.

Leave a comment

Your email address will not be published. Required fields are marked *