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Unformatted text preview: Urine Production  Four processes  Filtration i Filtrate of blood formed at glomerulus  Reabsorption  Specific molecules in the filtrate removed i Secretion  Specific molecules added to the filtrate  Excretion  Urine is excreted from body Filtration  Liquid components of blood filtered into Bowman’s capsule  Water and small solutes cross glomerular wall  Blood cells and large macromolecules not filtered  Glomerular capillaries very leaky  Podocytes with foot processes form filtration structure  Mesangial cells control blood pressure and filtration within glomerulus  Filtrate flows from Bowman’s capsule into proximal tubule The Filtration Membrane • endothelial-capsular membrane - filtering unit of a nephron – glomerular endothelium – glomerular basement membrane – slit membranes between pedicles of podocytes • Filtered substances move from the blood stream through the 3 barriers • principle of filtration - to force fluids and solutes through a membrane by pressure  Same in capillaries elsewhere Net Filtration Pressure • Glomerular filtration depends on 3 main pressures, 1 that promotes and 2 that oppose filtration. • Filtration of blood is promoted by blood glomerular hydrostatic press (BGHP) and opposed by capsular hydrostatic pressure (CHP) and blood colloid osmotic pressure (BCOP). – The net filtration pressure (NFP) is about 10 mm Hg. Net Filtration Pressure • NFP = total pressure that promotes filtration • NFP = GBHP - (CHP + BCOP) = 10mm Hg Glomerular Filtration Rate • Homeostasis requires GFR that is constant – too high => useful substance lost due to speed of fluid passage through nephron – too low => sufficient waste products may not be removed from body • Changes in net filtration pressure affects GFR – filtration stops if GBHP drops to 45 mm Hg – functions normally with mean arterial pressures 80-180mmHg Regulation of GFR • The mechanisms that regulate GFR adjust blood flow into and out of the glomerulus and alter the glomerular capillary surface area available for filtration. • 3 principal mechanisms: renal autoregulation, neural regulation, and hormonal regulation Renal Autoregulation of GFR • Mechanisms that maintain a constant GFR despite changes in arterial BP – Myogenic mechanism • systemic increases in BP, stretch afferent arteriole • smooth muscle contraction reduces diameter of arteriole returning GFR to its previous level in seconds – Tubuloglomerular feedback • elevated systemic BP raises GFR so that fluid flows too rapidly through renal tubule; Na + , Cl- and water are not reabsorbed...
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This note was uploaded on 04/19/2008 for the course PCB 3713 taught by Professor Sikorski during the Spring '08 term at University of South Florida - Tampa.
- Spring '08