The Urinary SystemRenalPhysiology

Glomerular FiltrationTubular ReabsorptionTubular SecretionFunctions of Nephron & Collecting Duct

Glomerular FiltrationObjectivesDescribe filtration membraneDiscuss pressures involves in filtration productionDiscuss factors which control glomerular filtration rate

Glomerular Filtrate (Ultrafiltrate)Fluid that enter capsular spaceFiltration fraction - % of blood plasma in afferent arteriole that becomes filtrate (16-20%)Vol. in adults daily 150L 180Lhowever, >99% returned to blood stream hence 1-2 L excreted in urine

Filtration Membrane

Filtration Membrane

Filtration MembraneGlomerular endothelial cellsLeaky, large fenestrations (70-100 nm)Permits all solutes of blood plasma (but not blood cells & platelets)Basal lamina (basement membrane)Layer b/w endothelium & podocytesPrevent larger plasma proteins

Filtration MembraneFiltration slitsFormed by pedicels of podocytesAllow passage of molecules < 6-7 nm(e.g. H2O, glucose, vitamin, amino acids, small proteins, ammonia, urea & ions)Albumin 7.1nm hence

Principles of FiltrationHigh filtration surface area long & extensive glomerular capillariesMesangial cells when relaxed - surface area, gl. filtration & vice versaThin & porous membrane total thickness 0.1 mHigh BP of glomerular capillariesefferent arteriole < in diameter than afferent arteriole

Net Filtration Pressure (NFP)Glomerular blood hydrostatic pressure (GBHP)Force H2O and solutes in blood plasma through filtration membrane; 55 mmHgCapsular hydrostatic pressure (CHP)Pressure exerted by fluid in capsule space & renal tubule; 15 mmHgBlood colloid osmotic pressure (BCOP)[Protein] in plasma > [protein] in tubular fluid hence promote osmotic return of filtered H2O; 30 mmHg

Net Filtration Pressure (NFP)

Glomerular Filtration Rate (GFR)Amount of filtrate formed in all renal corpuscle of both kidneysAdults - 125 ml/min; 105 ml/minRelate to -NFP-blood flow in glomerular capillaries -diameter of both afferent & efferent arterioles-surface area available for filtration

GFR ControlRenal autoregulationMyogenic mechanism- BP stretching of smooth muscle fibres of afferent arterioles results in contraction of the fibres lumen of afferent arteriole; GBHP and GFR vice versa

GFR ControlRenal autoregulationTubuloglomerular feedback upon delivery of Na+, Cl- & H2O during BP & GFR detected by macula densa causing inhibition of NO release from JGA hence contraction of afferent arterioles blood flow &GFR vice versa

GFR ControlNeural regulation activity of renal sympathetic nerves release noradrenaline hence activation of 1 receptors of afferent arteriole contraction & GFR conserve blood volume & allows blood flow to other tissues

GFR ControlHormonal regulation angiotensin II (potent vasoconstrictor) GFR atrial natriuretic peptide (ANP) is released by heart atria cells during blood volume relax mesangial cells hence glomerular capillary surface area & GFR