Colloid osmotic pressure, also referred to as oncotic pressure, is a measurement of pressure exerted within the cardiovascular system by proteins found in blood plasma. The special nature of these protein cells helps ensure that fluids pass in and out of the capillaries at the proper rate. Maintaining the proper pressure ensures that the body tissues maintain the proper levels of liquid and that too much liquid does not escape from the capillaries. Under normal conditions, oncotic pressure tends to cause fluid to be drawn into the capillaries. Variations can lead to buildup of fluid in the tissues, a condition known as edema.
Osmotic pressure in general refers to the pressure that must be maintained on either side of a cell membrane to prevent a solution, or solids dissolved in a liquid, from passing through that cell membrane. Colloidal osmotic pressure refers specifically to the relevant pressure for a solution of plasma proteins. These proteins are relatively large molecules, meaning that they do not readily pass through the cell membrane. Their size also affects the tendency of liquids to move out of the capillaries, maintaining the proper amount of liquid and pressure inside these tiny blood vessels.
Another way in which colloid osmotic pressure affects the cardiovascular system is in intravenous (IV) therapy. Fluids used for IV treatment typically contain either colloid or crystalline solubles, depending on the substance required for the relevant treatment. A common example of an IV therapy using a crystalline solution is intravenous saline used to counter dehydration. This solution contains sodium chloride, or salt, which occurs naturally in a crystalline form. In order for the solutions to remain within the circulatory system, appropriate pressure must be maintained within the blood vessels.
Measuring an individual's colloid osmotic pressure is one way that medical professionals diagnose pulmonary edema, and it can be used to calculate a patient's likelihood of surviving a critical case. This condition, in which fluids escape from the circulatory system and fill the lungs, can be fatal if not treated promptly. It can occur as a result of cardiovascular disease or because of a rapid change in air pressure such as that experienced when climbing at high altitudes. Pulmonary edema also is closely associated with congestive heart failure. Treatment includes maintaining oxygenation of the body tissues as well as treating the underlying cause.