Oncotic pressure

Above, we see a representation of fluid flow in the presence of colloids, with the left side representing surrounding tissues and the right representing whole blood. The presence of colloids can increase the flow towards the high concentration of colloids by creating colloid osmotic pressure in an otherwise state of equilibrium.
In the illustration above, we see how the osmotic pressure changes over the length of the capillary, with oncotic pressure remaining the same. Overall direction of fluid flow in relation to equal bidirectional flow is shown by the orange and black lines, respectively.

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. Participating colloids displace water molecules, thus creating a relative water molecule deficit with water molecules moving back into the circulatory system within the lower venous pressure end of capillaries.

It has the opposing effect of both hydrostatic blood pressure pushing water and small molecules out of the blood into the interstitial spaces within the arterial end of capillaries and interstitial colloidal osmotic pressure. These interacting factors determine the partition balancing of extracellular water between the blood plasma and outside the blood stream.

Oncotic pressure strongly affects the physiological function of the circulatory system. It is suspected to have a major effect on the pressure across the glomerular filter. However, this concept has been strongly criticised and attention has been shifted to the impact of the intravascular glycocalyx layer as the major player.[2][3][4][5]

  1. ^ Moman, Rajat N.; Gupta, Nishant; Varacallo, Matthew (2021), "Physiology, Albumin", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 29083605, retrieved 2021-12-09
  2. ^ Levick JR, Michel CC (July 2010). "Microvascular fluid exchange and the revised Starling principle". Cardiovascular Research. 87 (2): 198–210. doi:10.1093/cvr/cvq062. PMID 20200043.
  3. ^ Raghunathan K, Murray PT, Beattie WS, Lobo DN, Myburgh J, Sladen R, et al. (November 2014). "Choice of fluid in acute illness: what should be given? An international consensus". British Journal of Anaesthesia. 113 (5): 772–83. doi:10.1093/bja/aeu301. PMID 25326478.
  4. ^ Woodcock TE, Woodcock TM (March 2012). "Revised Starling equation and the glycocalyx model of transvascular fluid exchange: an improved paradigm for prescribing intravenous fluid therapy". British Journal of Anaesthesia. 108 (3): 384–94. doi:10.1093/bja/aer515. PMID 22290457.
  5. ^ Maitra, Sayantan; Dutta, Dibyendu (2020-01-01). "Chapter 18 - Salt-induced inappropriate augmentation of renin–angiotensin–aldosterone system in chronic kidney disease". In Preuss, Harry G.; Bagchi, Debasis (eds.). Dietary Sugar, Salt and Fat in Human Health. Academic Press. pp. 377–393. ISBN 978-0-12-816918-6. Retrieved 2021-12-10.

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