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Oncotic pressure, or colloid osmotic-pressure, is a type of osmotic pressure induced by the plasma proteins, notably albumin, [1] in a blood vessel's plasma (or any other body fluid such as blood and lymph) that causes a pull on fluid back into the capillary.
The Starling principle holds that fluid movement across a semi-permeable blood vessel such as a capillary or small venule is determined by the hydrostatic pressures and colloid osmotic pressures (oncotic pressure) on either side of a semipermeable barrier that sieves the filtrate, retarding larger molecules such as proteins from leaving the blood stream.
In medicine, hydrostatic pressure in blood vessels is the pressure of the blood against the wall. It is the opposing force to oncotic pressure. In capillaries, hydrostatic pressure (also known as capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood—a “constant” pressure primarily produced by ...
Increase hydrostatic pressure in vessels: left ventricular heart failure, Decrease oncotic pressure in blood vessels: Cirrhosis (Cirrhosis leads to hypoalbuminemia and decreasing of colloid oncotic pressure in plasma that causes edema) Nephrotic syndrome (also due to hypoalbuminemia caused by proteinuria). Malnutrition (hypoalbuminism)
The components of the Starling forces – hydrostatic pressure, permeability, and oncotic pressure (effective pressure due to the composition of the pleural fluid and blood) – are altered in many diseases, e.g., left ventricular failure, kidney failure, liver failure, and cirrhosis.
Under normal circumstances the SAAG is < 1.1g/dL (11g/L) because serum oncotic pressure (pulling fluid back into circulation) is exactly counterbalanced by the serum hydrostatic pressure (which pushes fluid out of the circulatory system).
Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles. [35] Gravity affects blood pressure via hydrostatic forces (e.g., during standing), and valves in veins, breathing, and pumping from contraction of skeletal muscles also influence blood pressure in veins. [32]
Capillary hydrostatic pressure P c = 0.2 × Arterial Pressure + Venous Pressure 1.2 25mmHg (arteriolar end) 10mmHg (venous end) P i: Tissue interstitial pressure Determined by the compliance of tissue Compliance = volume/Δ pressure Varies by location ≅ −6 mmHg Π c: Capillary oncotic pressure Measured across semipermeabel membrane