Also known as Colloidal Osmotic Pressure. Oncotic pressure is a form of pressure in the circulatory system that allows water to cross the capillaries barrier and penetrate the circulatory system. In patients with low oncotic pressure, the fluid will tend to accumulate in the tissues, which can result in edema.
This phenomenon is part of a complex interconnected system that is designed to keep the body in a state of homeostasis, along with the hydrostatic pressure maintains the level of fluids in the blood.
The walls of the circulatory system are semi-permeable. Liquids can pass through these membranes, but larger materials, such as proteins, also known as colloids, can not.
The hydrostatic pressure is the force that pushes the fluids on this membrane and outside the circulatory system, while the oncotic pressure is the force that brings the fluids back into the circulatory system.
When these two forces are in equilibrium, there is no loss or gain of fluids from the circulatory system. When this is not the case, a patient can develop medical problems.
The fluids tend to flow from areas of low concentration to the high concentration areas, which means that when the solute level increases as the fluid exits the capillaries, is removed through the membrane and into the capillary to balance the concentration of the solution on both sides of the barrier.
Sometimes, the oncotic pressure can be referred to the osmosis as colloid or colloidal osmotic pressure, making reference to the process and the solutes in question.
A number of things can influence oncotic pressure, including the amount of colloids in the blood. People with unbalanced concentrations of colloids in their blood may be at risk for low oncotic pressure.
Individuals suffering from dehydration or excess fluid can also develop imbalances in oncotic pressure because their bodies can not compensate quickly enough.
The measurement of oncotic pressure
The colloidal osmotic pressure (oncotic pressure or) is measured using a colloid osmometer employing a semipermeable membrane and measures the change in pressure in the reference chamber when a solution is placed in the test chamber.
The pore size of the membrane in these instruments is 20 to 30 kilodaltons and therefore they are freely permeable to small electrolytes.
The normal osmotic pressure of colloids is 20 to 25 mm Hg. High values in adolescents are common and in critically ill patients, but are not considered to justify treatment.
Single-digit values (commonly observed in patients with portocaval shunts) also need to be treated, but there is a need to do so slowly. The rapid administration of colloids to these patients has caused edema.