Oncotic pressure is a force that encourages water to enter the circulatory system. It works with hydrostatic pressure to maintain fluid balance. When imbalanced, it can cause edema. Oncotic pressure occurs due to osmosis and is affected by blood colloid concentrations, dehydration, and excess fluids. Physicians and nurses learn about it in basic anatomy and physiology courses.
Oncotic pressure is a form of pressure in the circulatory system that encourages water to cross the capillary barrier 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 that aims to keep the body in a state of homeostasis, working together with hydrostatic pressure to keep the fluid level in the blood stable.
The walls of the circulatory system are semipermeable. Fluids can pass through these membranes, but larger materials like proteins, also known as colloids, cannot. Hydrostatic pressure is the force that pushes fluids over this membrane and out of the circulatory system, while oncotic pressure is the force that pushes 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’re not, a patient can develop medical problems.
Oncotic pressure occurs due to osmosis. As fluids move across the capillary membrane due to hydrostatic pressure, they leave behind a concentration of solutes that were too large to pass through the membrane. Fluids tend to flow from areas of low concentration to areas of high concentration, meaning that as the solute level rises as the fluid leaves the capillaries, the fluids are pulled across the membrane and into the capillaries to balance the solution concentration on both sides of the barrier.
Sometimes, oncotic pressure may be referred to as colloidal osmosis or colloidal osmotic pressure, referring to the process and solutes involved. Several things can affect oncotic pressure, including the amount of colloids in the blood. People with imbalanced blood colloid concentrations may be at risk for low oncotic pressure. Individuals experiencing dehydration or excess fluids may also develop imbalances in their oncotic pressure as their bodies cannot compensate quickly enough.
Several equations involve oncotic pressure, and there may be medical situations where understanding this phenomenon is helpful. Physicians and nurses generally learn about oncotic pressure, among a wide variety of other topics, in courses that cover basic anatomy and physiology, and their studies may include familiarization with equations involving the various forces that maintain the system circulatory system that flows smoothly and maintains fluid balance in the body.
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