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Intermediate Physics for Medicine and Biology Chapter 5: Transport Through Neutral MembranesProfessor Yasser M. Kadah

Web: http://www.k-space.org

TextbookRussell K. Hobbie and Bradley J. Roth, Intermediate Physics for Medicine and Biology, 4th ed., Springer-Verlag, New York, 2007. (Hardcopy)Textbook's official web site: http://www.oakland.edu/~roth/hobbie.htm

MembranesCells are surrounded by a membrane 7-10 nm thickPermeable to a substanceSubstance can pass freely through itSemipermeableOnly certain substances can get through it PermeantSubstance that can pass through

MembranesExamplesStraight poresTortuous poresNo pores; molecules dissolveWater motion: bulk flowSolute motion: random walkEffective motion: diffusion superimposed on bulk flow

Osmotic PressureGas law

N1* : # of molecules, n1* : # of molesDenote,

Osmotic PressureImagine volume V* divided into two subvolumes V,VPressure remains p1 in both partitionsAverage number of molecules remain unchanged

Osmotic PressureSecond speciesCannot pass through

Osmotic PressureTotal partial pressure for all species that cannot pass through the membrane is called osmotic pressure and denoted by

Osmotic PressureTotal pressure = driving pressure + osmotic pressure

There is no flow if the driving pressure is the same between the two sides of the membrane

Clinical ExamplesCapillary modelArterial SideVenous Sidepipdiipopdoo

EdemaAbnormal collection of fluid inside tissue

Edema due to Heart FailureRight heart failureSwelling of legsLeft heart failurePulmonary edemaRoot cause:Rising venous pressure

Nephrotic Syndrome, Liver Disease and AscitisLower protein in bloodHypoproteinemiaSeveral causesNephrons leaking proteinsLiver malfunctionAscitis (blocking of veins in the liver)

Edema of Inflammatory Reactions3 StepsVasodilationFluid exodation (plasma)Cellular migrationRise in osmotic pressure in extracellular space

Headaches in Renal DialysisCapillary-brain barrierLow permeability to ureaPlasma urea , temporary urea osmotic pressure inside brain Water flows into brain causing cerebral edema, which can cause severe headache.Converse: inject into blood urea/manitolWater flows from brain to bloodEmergency treatment for cerebral edema

Osmotic DiuresisWater and many solutes pass into nephron from the bloodNet reabsorption occurs through the rest of the nephronMost of water and variable for solutesMedium-weight molecules are not reabsorbed at all (e.g., manitol, glucose)If they are present, water reabsorption is lessIncrease in urine volume

The Artificial KidneyNo solvent drag

where

The Artificial Kidney: ExampleLet: RT= 510-6 m/s S= 2 m2V= 40 LThen,= 1.1 hDialysis typically takes hoursA number of must elapseLarger molecules are slowerNot to cause cerebral edema and headache

Countercurrent Transport

Problem AssignmentsInformation posted on web site

Web: http://www.k-space.org