What is colloidal osmotic pressure

This is in contrast to most crystalloid particles such as electrolytes, glucose, and other metabolites, which pass readily through the membrane. COP should be thought of as the osmotic pressure exerted by plasma proteins and their associated electrolytes, because the electrolytes contribute significantly to the COP. Oncotic pressure is defined as the osmotic pressure exerted by colloids in solution, so the terms COP and oncotic pressure can be used interchangeably; colloid oncotic pressure, a commonly used misnomer, is redundant.

Osmolality is the concentration of osmotically active particles solute per kilogram of solution. The size and charge of the particles does not matter when determining the osmolality; only the number of particles in solution is relevant. The Starling hypothesis states that fluid flux at the capillary level is controlled by a balance between hydrostatic pressure and osmotic pressure gradients between the capillaries and interstitial space. This equation shows the importance of plasma COP in maintaining a normal fluid balance between the intravascular space and the interstitial space Figure If the COP in the capillaries drops lower than the COP in the interstitium, fluid will move out of the vessels and edema formation will be favored.

Of all the variables included in this equation, we can clinically manipulate only the COP and the capillary hydrostatic pressure. Increasing capillary hydrostatic pressure by administering intravenous fluids will tend to increase edema formation.

Several explanations have been given for this decrease including; supine positioning during labor and delivery, blood loss during delivery, and administration of crystalloid fluids during labor.

Reference range is:. Normotensive Pregnancy. Hypertensive Pregnancy. COP values fall during tocolytic therapy, with the lowest values occurring nine hours after initiation of therapy. In plasma, the oncotic pressure is only about 0. This may be a small percent but because colloids cannot cross the capillary membrane easily, oncotic pressure is extremely important in transcapillary fluid dynamics.

The principle is to have 2 chambers which are enclosed and separated from each other by a semi-permeable membrane which is:. Relative to this membrane, the colloids are the only effective solutes present. The reference chamber contains isotonic saline and the test solution is added to the sample chamber. If the test solution contains colloids, water moves from the reference chamber to the sample chamber.

The decrease in pressure in the test chamber is detected by a pressure transducer strain gauge mounted between the two chambers. Modern oncometers can provide accurate results with samples as small as 50 microlitres in a couple of minutes.

Osmotic pressure and oncotic pressure can be measured by suitable instruments. They can also be determined for a solution by appropriate substitutions in the van't Hoff equation.