- Physiology
- Normal values
- Difference between oncotic pressure and osmotic pressure
- Oncotic pressure and mortality
- References
The oncotic pressure or colloid is a force exerted by albumin and various proteins in blood plasma that contributes to fluid movement level of the capillary membranes. It is the main force that keeps fluid within the vasculature.
In order to understand what oncotic pressure is, it is pertinent to first understand that the body is divided into several compartments where total body water is distributed: two thirds of this is confined within cells. This compartment is called the intracellular space (ICS).
The remaining third is distributed in the extracellular space in the following way: a quarter is inside the blood vessels (plasma), and the remaining three quarters are located in a space that surrounds all the cells of the organism known as the interstitial space.
Finally, each of these compartments is separated by semipermeable membranes; that is, membranes that allow the passage of some elements and restrict that of others. As a general rule, semi-permeable membranes allow free passage of water and restrict the passage of proteins through it.
This concept is fundamental to understand and distinguish osmotic pressure (water) from oncotic pressure (proteins). Osmotic pressure is the physicochemical force that drives the passage of water from one compartment to another, based on the presence of elements that generate the chemical attraction of water in each of these compartments.
These elements must not be able to freely pass through the membrane, since that would limit their function of dragging water to one side or the other in a net way; it is here when the oncotic pressure comes into force.
Physiology
The oncotic pressure is nothing more than the gradient that proteins establish in a certain compartment to drag water since, due to their chemical nature, these cannot cross membranes but have a negative polar charge, which is why they attract water molecules.
This pressure plays a fundamental role in maintaining the water balance (the net difference between the supply and the loss of water) of the body tissues.
Thanks to a perfect balance between this pressure and the hydraulic pressure inherent to the blood vessels exerted by the pumping of the heart (hydrostatic pressure), the exchange of oxygen, nutrients and toxic waste can occur at the level of the various body tissues and their blood vessels corresponding, known as capillaries.
A change in colloidosmotic pressure is usually an important determinant in the development of a systemic or pulmonary edema. Suffering from a protein deficit in the blood, which can be caused by different reasons, makes it difficult to retain fluids in the body compartments where you want to keep it.
This results in the passage of water into a compartment where it should not normally be present: the interstitial space. The presence of fluid in the interstitial space is known as edema. As a clinical tool, oncotic pressure measurement represents a contribution to the diagnosis of diseases whose cardinal symptom is edema.
Edema does not develop until the plasma oncotic pressure is below 11 mmHg. Lymph flow keeps proteins out of the interstitial space, keeping oncotic pressure in this compartment to a minimum and thus preventing edema.
Normal values
The average value of the oncotic pressure in the plasma of a subject in the resting position is 20 mmHg. However, the values in moving subjects usually present an 18% increase in oncotic pressure, an effect attributed to the decrease in plasma (water) volume caused by exercise.
At different intervals, the oncotic pressure usually presents fluctuations of 10% in the subject (increase and decrease in values).
Albumin provides approximately 60% to 70% of the plasma oncotic pressure and globulins provide the remaining 30% to 40%. Four molecules of albumin are found for every molecule of globulin and it has more anionic charge.
Several studies show a gradual decrease in oncotic pressure in older people, and they also show lower oncotic pressure in women compared to men.
Difference between oncotic pressure and osmotic pressure
Osmotic and oncotic pressure share a relationship. The difference between the two can be understood by remembering the nature of osmosis, which is the basis in both pressures.
Osmosis is the passive movement of water from an area of high water concentration, through a semi-permeable membrane, to an area of low water concentration. This movement achieves an equal amount of water in each area.
Osmotic pressure is the minimum pressure necessary to stop the internal flow of solvent through a semi-permeable membrane. On the other hand, oncotic pressure is the type of osmotic pressure in which pressure is applied by albumin and proteins in the plasma of a blood vessel, to bring water into the circulatory system.
The Pleffers method and the Berkeley and Hartley method are the most famous for determining osmotic pressure, although now in modern times an apparatus known as an osmometer is used to measure osmotic pressure, while the level of oncotic pressure is measured through the oncometer.
The osmotic pressure is directly proportional to the temperature and the concentration of solute in the solution, while the oncotic pressure is directly proportional to the number of colloids in a solution.
Oncotic pressure and mortality
In critically ill patients, a correlation has been found between low oncotic pressure and mortality.
For example, a study with 99 subjects with cardiorespiratory deficiencies showed that all those with an oncotic pressure below 10.5 mmHg perished, while those with a pressure greater than 19 mmHg survived.
Oncotic pressure measurement in critically ill patients is often a reliable source when predicting life expectancy.
References
- Osmotic pressure and oncotic pressure, (2015), deranged physiology: derangedphysiology.com
- Osmotic pressure vs. Oncotic pressure: what's the difference ?, sf, difference.wiki: difference.wiki
- Oncotic pressure, nd, health and wellness: lasaludi.info
- Alberto basilio olivares, jesús carlos briones, jesús antonio jiménez origel, manuel antonio díaz de león ponce, nd, colloidosmotic pressure (pco) as a prognostic indicator in trauma. Preliminary report, journal of the medical association of critical medicine and intensive therapy: medigraphic.com
- Oncotic pressure, 2014, sciencedirect: sciencedirect.com
- Colloid osmotic pressure: its measurement and clinical value, (1977), cma journal: ncbi.nlm.nih.gov
- Ann lawrie, sf, oncotic pressure, school of health sciences: nottingham.ac.uk
- Dr. Bevan, (1980), colloid osmotic pressure: onlinelibrary.wiley.com