L32 GFR 2011

L32 GFR 2011 - Mammalian Physiology BIOAP 4580 Spring 2011...

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-1/11- Mammalian Physiology BIOAP 4580 Prof. Beyenbach Spring 2011 COPYRIGHT GLOMERULAR FILTRATION AND RENAL CLEARANCES A. Balance of forces involved in glomerular filtration (Table 1). 1) Force supporting filtration: glomerular capillary pressure. The hydrostatic pressure in the glomerular capillary derives from the pressure generated by the heart. 2) Forces opposing filtration: plasma oncotic pressure and hydrostatic pressure in Bowman’s capsule. The plasma oncotic pressure stems from the presence of albumin in the plasma. The hydrostatic pressure in the Bowman’s capsule is the pressure of the tubular fluid on the other side of the filtration barrier. It drives the flow of tubular fluid down the renal tubule. From collecting ducts on, peritubular smooth muscle provides the peristaltic movement of urine into the urinary bladder that may burst if it is not emptied. Table 1. Forces in an amphibian ( Necturus ) and two mammals Necturus Rat Man Mean arterial pressure (mm Hg) 25.0 110 100 Glomerular capillary pressure (mm Hg) 17.7 45 90 - Intracapsular pressure (mm Hg) 1.5 10 15 - Plasma oncotic pressure (mm Hg) 10.4 18 30 Net filtration pressure (mm Hg) 5.8 17 45 eq. 1 where P GLOM is the permeability of the glomerulus. The glomerular permeability is high for water and small molecules, but low for proteins (albumin) and other macromolecules. In renal disease, the permeability may be 1) excessively high, allowing proteins to be filtered (a protein-losing kidney causing edema), or 2) excessively low as in auto-immune disease which may reduce the number and size of filtration pores. B. The anatomy of the filtration barrier: capillary endothelium and epithelial cells (Fig. 1) Three sites come under consideration as glomerular filtration barrier: 1) the capillary endothelium with a pore size between 500 Å and 1000Å, 2) basement membrane consisting of 3 layers, and 3) slits between epithelial cells with a diameter of 250 Å. An anatomical pore has not been observed yet, but a physiological pore size has been measured between 75 Å and 100 Å.
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-2/11- Fig. 1. The glomerular filtration complex consists of 1) the capillary endothelium, 2) the basement membrane of epithelial cells at the basolateral side (glomerular mesangium), and 3) slits between "foot processes" at the apical side of epithelial cells. 4) Diagram of the filtration barrier (Fig.2). Electron-micrographs of the glomerulus reveal pores in the capillary endothelium (fenestrations) as well as openings (slits) between the foot processes of epithelial cells. However, fenestrations and slits are not the 'ultrafilter'. The 'ultrafilter' is the basement membrane. Fig. 2. Diagram of filtration barrier. The tri-layered basement membrane is
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L32 GFR 2011 - Mammalian Physiology BIOAP 4580 Spring 2011...

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