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Category: Medical

Topic: Fluid Dynamics

Level: Paramedic

Next Unit: Intracellular and Extracellular Fluid Balance

18 minute read

The STARLING HYPOTHESIS of fluid exchange is an attempt to explain how hydrostatic and oncotic (colloid osmotic) forces contribute to the movement of fluid across capillary membranes.

HYDROSTATIC PRESSURE: This is the pressure within capillaries created by the blood pressure from the pumping action of the heart, capillary wall tensile strength and elasticity, and osmotic pressure of particles in solution there. With the osmotic gradient being the same at both the arteriole and venule ends of this fluid exchange, it is the extra force from hydrostatic effects that gets things to pass out of the capillaries.

OSMOTIC GRADIENT: The osmotic gradient is a pressure caused by water molecules that forces water to move from areas of higher water pressure to areas of lower water pressure. Across a semipermeable membrane, as exists at the capillary wall, this results in movement of fluid.

ONCOTIC PRESSURE: Oncotic Pressure (Colloid Osmotic Pressure) is a type of osmotic pressure exerted by proteins (colloids), principally albumin, in a blood vessel's plasma that usually tends to pull water into the intravascular space. say you didn't major in chemistry...

Everything likes equilibrium. Equilibrium is harmony. When one thing presents to another, which is bound to upset the equilibrium due to differences in their relative constitutions, physics allows for them to equilibrate. In biological systems, there are (at least) two different types of equilibrium (quiet on the surface, but working hard at the molecular level to maintain equilibrium):

  1. Equilibrium BETWEEN two different compartments on either side of a semipermeable membrane, such as the capillary wall, the equilibrating of which causes things to pass across.
  2. Equilibrium WITHIN each compartment between different types of molecules--solute and solution. This is where the ability to dissolve solutes into solution comes from (you can only put so much sugar in tea before it can dissolve no more added sugar); alternatively, where solute coming out of solution comes in (ask someone with a kidney stone!). 

Equilibriumbetween compartments: Things exactly the same on both sides of a semipermeable membrane is a steady state where nothing changes. This is never the situation for any extended period of time in biological systems, and it is the struggle to maintain equilibrium which allows passage of things across membranes.

Equilibrium within a compartment: Assume each side of that membrane has particulate matter and liquid matter, both contributing their influence toward the equal pressure on each side. If you were to add a combination of [particulate + liquid] to each side equally, nothing changes--pressure-wise--between the two sides. There will be no movement across the membrane. Lucky for us, this is rare; otherwise, we'd die.

The plot thickens... Besides the equilibrium between the two chambers of [particulate + liquid] on either side of the membrane, there is also the equilibrium within each chamber--that of the equilibrium between the particulate and the liquid molecules. Together they create a pressure result which doesn't stop unless an equilibrium established between them.

Within one chamber: If you add more particulate matter, to maintain the same ratio of pressure between the two types of molecules, more liquid will be needed. Take out some particulate, and there is too much liquid for the previously established equilibrium necessary for a steady state. 

Between the two chambers, however... add particulate matter to one side, to maintain balance liquid will be drawn across the membrane to re-establish the equilibrium within that side. (However, this also means less liquid on the other side means unloading some particulate or getting liquid back again. And so it goes...)

In the capillaries, hydrostatic pressure increases movement across the capillary wall by pushing fluid and solute OUT of the capillaries, while capillary oncotic pressure (pressure from the particulates, known as colloid osmotic pressure) pulls fluid into the capillaries and/or prevents fluid from leaving. 

Albumin and its associated cations provide approximately 60% to 70% of the plasma oncotic pressure and globulins provide the remaining 30% to 40%.


Starling's Hypothesis

Net filtration = the permeability of the capillary wall x [hydrostatic - oncotic pressure].

For reference, the elements involved in the calculation are:

  • hydrostatic pressure
  • capillary (Pc) hydrostatic pressure
  • interstitium (Pi) oncotic pressure
  • capillary (pc ) oncotic pressure
  • interstitium (pi )

Don't memorize this--you won't be using it in the field. However, this equation describes how what is filtered (and how much) depends on both

  1. the permeability (porosity) of the capillary wall and the
  2. difference between the hydrostatic and osmotic pressures on either side of the arteriole/venule interface where interstitial fluid dynamics occur.

It suggests that the primary purpose of the circulatory system is to deliver fuel to cells and remove cellular waste.

At the arteriole (arterial) end: the idea is that when the capillary pressure was greater than the osmotic pressure at the arterial end of the capillary, that it would result in fluids moving OUT of the capillary.

At the venule (venous) end: On the other end of the capillary, the venous end, the venous pressure was lower than the osmotic pressure, and so fluids moved back into the capillary. Fluid balance is the state where the volume of liquid absorbed at the venous end is equal to the volume of liquid filtered out at the arterial end of the capillary.


Move Over, Starling! What We Know Today

With recent, more sophisticated observations, this exchange has been discovered to have much more complexity, and therefore Starling's explanations have been found to be oversimplified at best, incorrect at worst.

The Reality: Because of a constant hydrostatic pressure driving fluids out, it is now known that fluid passed into the interstitial space is NOT re-absorbed back into the capillaries, but collected by the lymphatic channels which then return it back into the circulation. The relationship is therefore at a different location (lymphatics), but the math is the same, so Starling need not turn over in his grave.



EDEMA is a palpable swelling produced by expansion of the interstitial fluid volume. It is not found in normal subjects because, with edema, fluid collecting in the tissues of the body is caused by an alteration in capillary dynamics that favors not only an increase in net filtration, but also inadequate removal of the additional filtered fluid by lymphatic drainage. It may accumulate due to:

  • Elevated capillary hydraulic (hydrostatic) pressure.
  • Increased capillary permeability.
  • Disruption of endothelial glycocalyx--a network of membrane-bound proteoglycans and glycoproteins that covers the endothelium, the damage of which affects the integrity of the capillary wall.
  • Decreased interstitial compliance (ability to resist extra volume).
  • Lower plasma oncotic pressure.
  • Combination in varying amounts of all of the above.
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