What is oncotic pressure?
Oncotic pressure, or colloid osmotic-pressure, is a form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel's plasma (blood/liquid) that causes a pull on fluid back into the capillary.
Why does oncotic pressure increase along the length of capillaries?
The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration (e.g., in renal glomerular capillaries), because the filtering fluid leaves behind proteins leading to an increase in protein concentration.
How does oncotic pressure affect the interstitium?
The increase in oncotic pressure in the interstitium shifts the equilibrium of oncotic and hydrostatic pressure, resulting in a net fluid accumulation in the interstitium (see Fig. 2-2B ). This fluid movement occurs most prominently in postcapillary venules, in which endothelial cells are sensitive to several inflammatory mediators.
What is the oncotic pressure of colloid?
Colloids contain larger insoluble molecules, such as gelatin. Oncotic pressure values are approximately 290 mOsm per kg of water, which slightly differs from the osmotic pressure of the blood that has values approximating 300 mOsm /L.
What does high oncotic pressure mean?
The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration (e.g., in renal glomerular capillaries), because the filtering fluid leaves behind proteins leading to an increase in protein concentration.
What happens when oncotic pressure decreases?
In tissues, physiological disruption can arise with decreased oncotic pressure, which can be determined using blood tests for protein concentration. Decreased colloidal osmotic pressure, most notably seen in hypoalbuminemia, can cause edema and decrease in blood volume as fluid is not reabsorbed into the bloodstream.
Does increased oncotic pressure cause edema?
A small amount of protein exists in the interstitium and forces some fluid out of capillary walls. This force is the interstitial oncotic pressure. Together, these factors contribute independently or cooperatively to form edema.
What does oncotic pressure do to water?
Oncotic pressure can be understood by recalling the nature of osmosis, which is the passive movement of water from an area high in water concentration, through a semi-permeable membrane, to an area low in water concentration. This movement achieves an equal amount of water in each area.
Does low oncotic pressure cause edema?
Edema occurs when there is a decrease in plasma oncotic pressure, an increase in hydrostatic pressure, an increase in capillary permeability, or a combination of these factors.
What is the purpose of oncotic pressure?
Oncotic Pull Colloid osmotic pressure (COP), the osmotic pressure exerted by large molecules, serves to hold water within the vascular space. It is normally created by plasma proteins, namely albumin, that do not diffuse readily across the capillary membrane.
How does albumin increase oncotic pressure?
However, albumin is theoretically advantageous over crystalloids for its potential to increase a patient's oncotic pressure. Its short half-life limits the effects of albumin. As mentioned above, the strength albumin has over crystalloids is that it leads to an increase in intravascular oncotic pressure.
What is the difference between osmotic and oncotic pressure?
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.
What increases edema?
Your risk of edema may be increased if you take certain medications, including: High blood pressure medications. Nonsteroidal anti-inflammatory drugs. Steroid drugs.
How does oncotic pressure affect blood pressure?
Osmotic agents increase the oncotic pressure of the blood; this pulls water from tissues and increases the volume of the blood acutely. The increased blood volume will inhibit renin release, thus increasing renal blood flow.
What is oncotic pressure in simple terms?
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Why does decreased plasma protein cause edema?
Decreased serum protein reduces the oncotic pressure of the blood, leading to loss of fluid from the intravascular compartment, or the blood vessels, to the interstitial tissues, resulting in edema.
Why does albumin increase oncotic pressure?
However, albumin is theoretically advantageous over crystalloids for its potential to increase a patient's oncotic pressure. Its short half-life limits the effects of albumin. As mentioned above, the strength albumin has over crystalloids is that it leads to an increase in intravascular oncotic pressure.
Why does oncotic pressure vary?
The oncotic pressure of plasma, 25 mmHg, normally does not vary significantly with location in the body because the plasma is regulated and it mixes rapidly. The oncotic pressure of the interstitial fluid varies depending on the leakiness of the capillaries to plasma proteins.
How does osmotic pressure affect the blood?
Osmotic agents increase the oncotic pressure of the blood; this pulls water from tissues and increases the volume of the blood acutely. The increased blood volume will inhibit renin release, thus increasing renal blood flow.
What is the pressure of interstitial fluid?
Because the fluid encounters resistance along the capillary, its pressure falls from the arteriolar end to an average of 12–25 mmHg at the venule end. The interstitial fluid hydrostatic pressure is generally thought to be small, but it depends on the location.
What is the average capillary hydrostatic pressure?
Average values at the arteriolar end are 32–36 mmHg. Because the fluid encounters resistance along the capillary, its pressure falls from the arteriolar end to an average of 12–25 mmHg at the venule end.
Why is it difficult to measure interstitial fluid oncotic pressure?
Direct measurement of interstitial fluid oncotic pressure is difficult because samples cannot be obtained easily. Because lymph drains the interstitial fluid, the oncotic pressure of lymph approximates the interstitial fluid oncotic pressure, and this differs with location.
What is the NFP of a filtration system?
Net filtration pressure (NFP) is the total pressure that promotes the filtration and is dependent on the GBHP, capsular hydrostatic pressure (CHP) and blood colloid osmotic pressure (BCOP) ( Fig. 18.3 ). NFP ranges to 10 mmHg that is presented in Eq. (18.2).
Which pressure promotes filtration?
The glomerular blood hydrostatic pressure (GBHP) promotes the filtration, whereas the colloid oncotic pressure opposes it. Thus the GFR greatly relies upon these two factors, which has been depicted in Eq. (18.1).
What is the oncotic pressure of albumin?
Albumin generates about 70% of the oncotic pressure. This pressure is typically 25-30 mmHg. The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration (e.g., in renal glomerular capillaries), because the filtering fluid leaves behind proteins leading to an increase in protein ...
What pressures affect transcapillary fluid exchange?
There are two hydrostatic and two oncotic pressures that affect transcapillary fluid exchange. Click on the following links to learn more about these pressures: tissue (interstitial) oncotic pressure.
What is the osmotic pressure of a capillary?
Because the capillary barrier is readily permeable to ions, the osmotic pressure within the capillary is principally determined by plasma proteins that are relatively impermeable. Therefore, instead of speaking of "osmotic" pressure, this pressure is referred to as the "oncotic" pressure or "colloid osmotic" pressure because it is generated by colloids. Albumin generates about 70% of the oncotic pressure. This pressure is typically 25-30 mmHg. The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration (e.g., in renal glomerular capillaries), because the filtering fluid leaves behind proteins leading to an increase in protein concentration.
How does interstitial pressure affect interstitial protein concentration?
The oncotic pressure of the interstitial fluid depends on the interstitial protein concentration and the reflection coefficient of the capillary wall. The more permeable the capillary barrier is to proteins, the higher the interstitial oncotic pressure. This pressure is also determined by the amount of fluid filtration into the interstitium. For example, increased capillary filtration decreases interstitial protein concentration and reduces the oncotic pressure. A reduction in the interstitial oncotic pressure increases the net oncotic pressure across the capillary endothelium (π C - π i ), which opposes filtration and promotes reabsorption thereby serving as a mechanism to limit capillary filtration. In a "typical" tissue, tissue oncotic pressure is about 5 mmHg (i.e., much lower than capillary plasma oncotic pressure).
What is the post to precapillary resistance ratio?
In many tissues, the post-to-precapillary resistance ratio is about 0.2, which means that precapillary resistance (mostly arteriolar) is about 5-times greater than postcapillary (venular) resistance. When this ratio is 0.2, a given change in arterial pressure is only about one-fifth as effective in changing capillary pressure as a comparable change in venous pressure. If this ratio increases, as occurs with arteriolar vasodilation, then arterial pressure has a greater influence on capillary pressure, which rises. Conversely, arteriolar constriction decreases this ratio and decreases capillary pressure.
How is hydrostatic pressure determined?
This hydrostatic pressure is determined by the interstitial fluid volume and the compliance of the tissue interstitium, which is defined as the change in volume divided by the change in pressure. The more fluid that filters into the interstitium, the greater the volume of the interstitial space (V i) and the hydrostatic pressure within that space (P i ). In some organs, the interstitial compliance is low, which means that small increases in interstitial volume lead to large increases in pressure. Examples of this include the brain and kidney, which are encased by rigid bone (brain) or by a capsule (kidney). In contrast, soft tissues such as skin, muscle and lung have a high compliance and therefore the interstitial space can undergo a large expansion with a relatively small increase in pressure. As interstitial volume increases, interstitial pressure increases, which can limit the amount of filtration into the interstitium because this pressure opposes the capillary hydrostatic pressure. In other words, as the hydrostatic pressure gradient (P C - P i) decreases owing to the rise in interstitial pressure, fluid filtration will be attenuated. However, large increases in tissue interstitial pressure can lead to tissue damage and cellular death. Normally, P i is near zero. In some tissues it is slightly subatmospheric, whereas in others it is slightly positive.
Does arterial pressure increase or decrease with arteriolar constriction?
Conversely, arteriolar constriction decreases this ratio and decreases capillary pressure.
Overview
Oncotic pressure, or colloid osmotic-pressure, is a form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel's plasma (blood/liquid) that causes a pull on fluid back into the capillary. Participating colloids displace water molecules, thus creating a relative water molecule deficit with water molecules moving back into the circulatory system within the lowe…
Etymology
'Oncotic' by definition is termed as 'pertaining to swelling', indicating the effect of oncotic imbalance on the swelling of tissues.
The word itself is derived from onco- and -ic; 'onco-' meaning 'pertaining to mass or tumors' and '-ic', which forms an adjective.
Description
Throughout the body, dissolved compounds have an osmotic pressure. Because large plasma proteins cannot easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will, to some extent, balance out the tendency for fluid to leak out of the capillaries. In other words, the oncotic pressure tends to pull fluid into the capillaries. In conditions where plasma proteins are reduced, e.g. from being lost in the urine (proteinuria), ther…
Physiological impact
In tissues, physiological disruption can arise with decreased oncotic pressure, which can be determined using blood tests for protein concentration.
Decreased colloidal osmotic pressure, most notably seen in hypoalbuminemia, can cause edema and decrease in blood volume as fluid is not reabsorbed into the bloodstream. Colloid pressure in these cases can be lost due to a number of different factors, but primarily decreased colloid pro…
External links
• Overview at cvphysiology.com