Physiology and Pharmacology
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show | Solute exchange -nutrition of tissue ad hormone/drug delivery - uses diffusion along conc gradient obeying Fick's law
Fluid exchange - regulation of plasma and interstitial fluid volume - by bulk flow due to pressure gradients obeying starlings principle
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Classes of capillaries | show 🗑
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show | Provides a tortuous pathway for small hydrophobic solutes
Interactions of proteins embedded in membranes forms a tight barrier between cells
Not continuous - small gaps allow formation of narrow channels
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show | Blocks protein access to the intercellular cleft
Transmembrane proteins conjugated to sugar molecules
Water and small molecules can move
Proteins are repelled - 4-5nm gaps too small
Hyaluronidase (released by macrophages) destroys this in inflammation
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Vesicle transport | show 🗑
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Types of solutes | show 🗑
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show | Solute flux is proportional to diffusion coefficient, surface area and conc difference but inversely proportional to thickness
Permeability is the diffusion flux/(area of membrane x conc difference)
Diffusion flux = Permeability x area x conc difference
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show | Increased blood flow
Reduced interstitial concentration
Reduce capillary permeability e.g. in response to shear stress
Recruitment of capillaries
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show | Less steep decay in luminal concentration - faster replenishment
Lower interstitial conc
Maintains concentration difference
Increased flow increases shear stress that increases permeability
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Recruitment of capillaries | show 🗑
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show | A feature of lymphatic disorder, cardiac failure, renal failure, pleural effusions, peritoneal swelling and joint effusions
Dysregulation of fluid exchange leads to accumulation
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Fluid absorption | show 🗑
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show | Cannulated artery of dog leg
Injected saline or serum into the muscle
With saline dilution of blood was seen
Serum produced no change - proteins cannot be reabsorbed and maintain water in interstitial fluid
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What affects flow | show 🗑
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show | Flow = LpA[(Pc-Pi)-sigma(Pip-Pii)]
Lp = hydraulic conductance of endothelium
A = wall area
Sigma = osmotic reflection coefficient (capillary is leaky so only a fraction of osmotic pressure is exerted)
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Medical importance of sigma | show 🗑
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show | Usually favour filtration and formation of lymph
Increased imbalance causes oedema e.g. pulmonary oedema or mitral stenosis (due to increase in Pc or decrease in Pip)
If blood pressure falls e.g. haemorrhage these favour absorption (Pc reduced)
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Causes of excessive capillary filtration | show 🗑
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