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Starling forces

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Physiology 3120
Tom Stavraky

Human Physiology Wednesday, September 23, 2009 “EC V” Exchange between Plasma & Interstitial Fluid • If you add an isotonic NaCl load, there will be an increase in volume in the ECF, but no osmosis will occur because there is no concentration gradient • Site of exchange is the capillary endothelium  8nm opening  Most dissolved substances can pass (0.3-1.2nm)  Most large proteins can’t pass  “Bulk flow”  Thus the composition of the interstitial fluid & the plasma is very similar  Filtration = movement OUT of the capillary  Reabsorption = movement INTO capillary • Starling forces: factors that influence fluid exchange at the endothelium; 2 hydrostatic pressures (fluid pressures); 2 colloid osmotic (oncotic) pressures (osmotic pressure created by proteins in plasma and interstitial fluid) 1. Capillary hydrostatic pressure of plasma (P ) c  Pressure from the blood in the capillary  Varies from one end to the other (25mmHg to 10mmHg)  Promotes filtration 2. Interstitial fluid hydrostatic pressurIF(P )  Pressure on the fluid in the interstitial space  Varies from organ to organ (-6 to +6 mmHg)  Can cause filtration (when pressure is negative) or reabsorption (when pressure is positive) 3. Interstitial colloid osmotic (oncotic) pressureIFΠ )  Pressure caused by osmosis due to proteins in the interstitial space  Pressure: +5mmHg  Causes filtration 4. Plasma colloid osmotic (oncotic) pressure (Πp)  Pressure caused by osmosis due to proteins in the plasma  Pressure: +28mmHg (
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