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What is Oncotic Pressure?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

Oncotic pressure is a form of pressure in the circulatory system which encourages water to cross the barrier of the capillaries and enter the circulatory system. In patients with low oncotic pressure, fluid will tend to accumulate in the tissues, resulting in edema. This phenomenon is part of a complex interconnected system which is designed to keep the body in a state of homeostasis, working together with hydrostatic pressure to keep the level of fluids in the blood stable.

The walls of the circulatory system are semi-permeable. Fluids can pass across these membranes, but larger materials such as proteins, also known as colloids, cannot. Hydrostatic pressure is the force which pushes fluids over this membrane and out of the circulatory system, while oncotic pressure is the force which brings fluids back into the circulatory system. When these two forces are in balance, there is no net loss or gain of fluid from the circulatory system. When they are not, a patient can develop medical problems.

An individual's colloid osmotic pressure may be measured to diagnose pulmonary edema.
An individual's colloid osmotic pressure may be measured to diagnose pulmonary edema.

Oncotic pressure occurs as a result of osmosis. When fluids move across the membrane of the capillaries due to hydrostatic pressure, they leave behind a concentration of solutes which were too big to cross the membrane. Fluids tend to flow from areas of low concentration to areas of high concentration, which means that when the solute level rises as fluid leaves the capillaries, fluids are pulled across the membrane and into the capillaries to balance the concentration of the solution on both sides of the barrier.

Oncotic pressure is a form of pressure in the circulatory system which encourages water to cross the barrier of the capillaries and enter the circulatory system.
Oncotic pressure is a form of pressure in the circulatory system which encourages water to cross the barrier of the capillaries and enter the circulatory system.

Sometimes, oncotic pressure may be referred to as colloid osmosis or colloid osmotic pressure, referencing the process and solutes involved. A number of things can influence oncotic pressure, including the amount of colloids in the blood. People with imbalanced concentrations of colloids in their blood may be at risk of low oncotic pressure. Individuals suffering from dehydration or an excess of fluids can also develop imbalances in their oncotic pressure as their bodies cannot compensate quickly enough.

Various forces keep the circulatory system flowing evenly and maintain the balance of fluids in the body.
Various forces keep the circulatory system flowing evenly and maintain the balance of fluids in the body.

Several equations involve oncotic pressure, and there can be medical situations in which it is useful to understand this phenomenon. Doctors and nurses generally learn about oncotic pressure, among a wide variety of other topics, in coursework which covers basic anatomy and physiology, and their studies may include familiarization with equations which involve the various forces which keep the circulatory system flowing evenly and maintain the balance of fluids in the body.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllTheScience researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllTheScience researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...

Discussion Comments

anon1005466

Great explanation. Thank you.

anon230805

This helped me a lot! Thank you!

calpat

The terms hydrostatic and oncotic pressure are new to me. The only pressure I had ever heard of concerining the human body, is blood pressure. Everyone is always talking about having high blood pressure these days.

All three have to do with the circulatory system though, which makes me wonder if blood pressure is related in some way to the other two.

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    • An individual's colloid osmotic pressure may be measured to diagnose pulmonary edema.
      By: Alila Medical Media
      An individual's colloid osmotic pressure may be measured to diagnose pulmonary edema.
    • Oncotic pressure is a form of pressure in the circulatory system which encourages water to cross the barrier of the capillaries and enter the circulatory system.
      By: 3drenderings
      Oncotic pressure is a form of pressure in the circulatory system which encourages water to cross the barrier of the capillaries and enter the circulatory system.
    • Various forces keep the circulatory system flowing evenly and maintain the balance of fluids in the body.
      By: stockshoppe
      Various forces keep the circulatory system flowing evenly and maintain the balance of fluids in the body.