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There are two major problems with this hypothesis;

1 The osmotic pressure is not constant down the length of the capillary

2 Capillary flow is pulsatile

These problems are discussed below.

1 Effect of pore size on membrane osmotic pressure (MOP)

Pore size has a profound effect on the membrane osmotic pressure (MOP). In general the smaller the pore size the larger the MOP . Plasma is a complex mixture of molecules with a range of particle sizes. Small molecules such as glucose and urea can pass through small pores whereas large molecules such as albumin and the globulins are retained by much smaller pores. The osmotic pressure across the capillary membrane is determined by the pore size at that point in the wall (see fig 2).

Measurement of the osmotic pressure of human plasma across membranes with a range of pore sizes gives a curvilinear relationship between pore size and MOP (fig 3)

Fig 3 Change in MOP of human plasma with membrane pore size8.

At small pore sizes (1-3,000 Daltons) molecules such as glucose, di and monovalent ions are large enough to contribute to the MOP. The smaller the pore size the greater their contribution. Between 10,000 and 30,000 Daltons the major contributing is albumin. At 100,000 Daltons Immunoglobulins contribute most of the MOP.

The fact that COP varies with pore size was not known by Starling.

For more of the PRO story please click on one of the following links

Pulsatility in the capillary

PRO as a new theory of fluid exchange

The therapeutic significance of PRO

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Last modified: 07/05/06