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The discovery that capillary flow is pulsatile, and that interference with this normal mode probably results in vasomotion, suggests that the capillary flow is inherently pulsatile. Fluid exchange is therefore likely to be a pulsatile process. However, it has been questioned if there is sufficient pressure in the oscillation of the pulse to drive fluid exchange.

Pulsatile flow profoundly increases the movement of plasma through membranes8 The movement of a simple fluid (such as saline) through a membrane (flux) increases linearly with pressure. Applying a pulsatile pressure with a mean pressure and amplitude the same as the non pulsatile pressure increases flux by about 30% (fig 5)

Fig 5 Increase in flux of saline with pulsatile pressure

The flux of plasma across an identical membrane was non linear with a plateau between 60 and 120mmHg (fig 6).

Fig 6 Pressure flux for human plasma

Pulsatile pressures only produces a small increase in flux over the pressure range 100-200mmHg. However, pusltile pressure produced twice as much flux at 50mmHg (fig 7)

Fig 7 Comparison of pulsatile and non pulsatile pressures on the flux of plasma.

Pulsatile pressure increased flux by 120% at 50mmHg but only 5% between  100 and 200mmHg

Pulsatile pressures in vitro double the flux of plasma at the pressures normally found in the capillary. If this is the same in vivo, pulsatility will have a profound effect on fluid exchange across membranes.

Pulsatility is an essential feature in capillary flow. The inherently pulsatile nature of the capillary bed and the effect of pulsatile pressure on fluid flux need to be included in any theory of blood pressure.

 

PRO as a new theory of fluid exchange

The therapeutic significance of PRO

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Last modified: 07/05/06