Physiology 3120 Lecture Notes - Starling Equation, Extracellular Fluid, Colloid

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26 Nov 2011
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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 (Pc)
Pressure from the blood in the capillary
Varies from one end to the other (25mmHg to 10mmHg)
Promotes filtration
2. Interstitial fluid hydrostatic pressure (PIF)
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
3. Interstitial colloid osmotic (oncotic) pressure (ΠIF)
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 (almost 6x greater)
Causes reabsorption
Starling-Landis Equation
Calculates net fluid movement (NFM)
NFM = Kf [ (Pc – PIF) – (Πp – ΠIF) ]
Kf = filtration coefficient
Represents permeability of the capillary and surface area of endothelium
Varies in different tissues (high in kidney, compared to muscle)
Constant in any single capillary bed
Assume = 1 unless otherwise stated
+ value = filtration
- value = reabsorption
End up with net filtration over the entire capillary bed (+8mmHg at arterial end vs. -7mmHg at
the venous end) PROBLEM
Lymphatic system is the solution
Series of blind-ended channels
Drains excess interstitial fluid from tissues
Fluid moved along by pressure gradient; smooth muscle pumps & valves; contraction of
skeletal muscle (i.e. skeletal muscle pump)
Drain through larger and larger vessels, collects in lymph nodes, and then finally joins back up
with the venous network just before it re-enters the heart
Edema: swelling of tissue due to increased interstitial fluid volume
Caused by:
1. Decreased plasma proteins seen in severe malnutrition; decreases plasma osmotic
pressure, which causes less reabsorption
Explains distended stomachs seen in severe malnutrition (more filtration & high
2. Partial blockage of venous return to heart
Increases capillary hydrostatic pressure (Pc)
3. Blockage of lymphatic system
Caused by a worm; causes elephantiasis
4. Tissue damage
Burns damage capillaries & allergic/inflammatory reactions release histamine &
Increases Kf