The Osmosis Unit

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Colloid Osmotic pressure (COP)

The term colloid osmotic pressure has been used to describe the osmotic pressure of the plasma particles which are too large to pass through the capillary pores6-8. It has been assumed that this is the same as the osmotic pressure (osmotic gradient) generated across the capillary wall. However, the osmotic gradient is dependent on the pore size of the membrane as well as the number of large particles in the plasma 10,11,21.

In practice COPís are measured using a colloid pressure osmometer. A sample of test solution in injected into a chamber on one side of a membrane with a known pore size. A reference solution (usually saline) is added to the other side of the membrane. The pressure generated across the membrane is measured with a transducer (see figure 2).

Injection of the test solution into the chamber above the membrane increases the osmotic pressure in this compartment. This draws fluid out of the transducer compartment reducing the pressure on this side of the membrane. This reduction in pressure is measured by the transducer, which is digitally displayed on the control panel.

As pore size changes COP the pore size of membrane used (e.g. COP10,000) should be quoted. While taking measurements the small varyations in the membrane pore size should be eliminated by standardising the readings against a reference solution with a known COP.

Factors that contribute to the COP

The major factors that contribute towards the COP are the concentration of non diffusible molecules, the membrane pore size and the physical attraction between solute molecules and the membrane. These are discussed in turn below.

a Concentration of non diffusible solute molecules

The osmotic gradient increases with increasing concentration of non diffusible solute molecules. However, this is a non linear relationship. The curve obtained follows the shape of a parabola, a second order polynomial relationship11 (see figure 3).

The standard formula relating osmotic gradient to concentration appears to be

OG = ac2 + bc + I

where c is the concentration of the non diffusible molecules. The term a is a constant for a particular solute (i.e. a constant for a particular starch solution) and appears to relate to the membrane polarisation factor. Factor b changes with the pore size of the membrane and appears to relate to the particle size of the compound and the pore size of the membrane. Factor I is the integer and relates to the difference in solvent species on either side of the membrane. The relationship between OG and concentration is illustrated in figure 10.

Figure 4 Relationship between concentration and osmotic gradient

The curve of the line is dictated by the membrane polarisation factor a, the slope by the particle size/pore size factor b and the intercept by c. Measurements of COP must not be taken on diluted samples unless the factors a, b and c have been deduced for the relevant solutes.

b Effect of pore size on osmotic gradient

Change in pore size has a significant effect on osmotic gradient10. Measurement of osmotic gradient of serum over a range of pore sizes from 1000 to 100,000 Daltons demonstrated the following relationship11 (see figure 5).

Analysis of the osmotic graidents of plasma components has demonstrated that the small molecules such as glucose, di and monovalent ions start to contribute to the numerical value of OG at pore sizes of 10,000 Daltons and below. The smaller the pore size the greater their contribution. Between 10,000 and 30,000 the major contributant is albumin. At 100,000 Daltons Immunoglobulins contribute all of the

6 mmHg osmotic gradient. As the pore size increases from the venous to the arterial end of the capillary it is likely that different plasma components are likely to contribute

different numerical amounts to the effective osmotic gradient at that point in the capillary.

c Effect of the reference solution

The choice of reference solution has an effect on the integer of the COP curve. Measuring the COP curves of Hespan using saline or ringers solution as the reference solutions produced the following curves (see figure 6).

Osmotic gradients should be measured against a standard reference solution in order for results to be comparable.


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Last modified: 07/05/06