Difference Between Osmotic Pressure and Oncotic Pressure

Main Difference – Osmotic Pressure vs Oncotic Pressure

Osmosis is the process of the net movement of water through a semi-permeable membrane by diffusion due to the concentration gradient. Osmosis is one of the key processes occurring in a biological system as it directly affects the cellular content and water levels by transporting water and other molecules across biological membranes. Osmotic pressure and oncotic pressure are two phenomena that occur due to osmosis. The main difference between Osmotic Pressure and Oncotic Pressure is that osmotic pressure is the pressure needed to stop the net movement of water across a permeable membrane which separates the solvent and solution whereas oncotic pressure is the contribution made to total osmolality by colloids. In this article, the difference between osmotic and oncotic pressure will be discussed.

What is Osmotic Pressure

Osmotic pressure is defined as the pressure needed to stop the net movement of water across a permeable membrane which separates the solvent and solution. The osmotic pressure of a solution mainly depends on the number of solutes or particles and the degree of ionization. Thus, it is referred to as a colligative property.  Osmotic pressure can be calculated by Van’t Hoff equation as below.

Osmotic pressure = n [(number of particles) x (concentration/molecular weight)] x R (universal gas constant) x T (absolute temperature)

Osmotic pressure is measured by an osmometer, which uses one or more colligative properties of the solution. In biological systems, controlling osmotic pressure is called osmoregulation. Osmoregulation is an essential process, and the body has evolved various homeostasis mechanisms to maintain the osmoregulation within the body. Solutions with the same osmotic pressure are called isosmotic. If we consider two solutions with different osmotic pressures, the solution with the higher osmotic pressure is called hyperosmotic, whereas the solution with lower osmotic pressure is known as hypo-osmotic.

What is Oncotic Pressure

The contribution made to total osmolality by colloids in a solution is defined as the oncotic pressure. Hence, it is also known as colloid osmotic pressure. The oncotic pressure can be measured using an oncometer. In animal body, mainly proteins exert the oncotic pressure. In blood plasma and capillaries, albumin is responsible for about 75% the total oncotic pressure. The oncotic pressure of blood plasma is about 25-28 mmHg, which represents about 0.5% of total plasma osmotic pressure.

Filtration and reabsorption present in capillaries

Difference Between Osmotic Pressure and Oncotic Pressure

Definition

Osmotic pressure is the pressure needed to stop the net movement of water across a permeable membrane which separates the solvent and solution.

Oncotic pressure is the contribution made to total osmolality by colloids.

Measured by:

Osmotic pressure is measured by osmometer.

Oncotic pressure is measured by oncometer.

Contribution Factor

The number of solutes or particles and the degree of ionization determine the osmotic pressure.

Oncotic pressure is determined by the number of colloids in a solution.

Image Courtesy:

“Osmotic pressure” by Nkonopli – Own work. (Public Domain) via Wikimedia Commons

“2108 Capillary Exchange” by OpenStax College – Anatomy & Physiology, Connexions Web site.  Jun 19, 2013. (CC BY 3.0) via Commons