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Renal Summary.doc

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PHGY 210
Ann Wechsler

SUMMARY: RENAL PHYSIOLOGY Kidney: General concepts: - Functions: o Regulation of water and inorganic ion balance most important function o Removal of metabolic waste products from the blood and their excretion in the urine More relevant in terms of pathology o Removal of foreign chemicals from the blood. o Production of hormones/enzymes: Erythropoietin: controls erythrocyte production Renin: controls the formation of angiotensin and influences blood pressure and sodium balance. 1,25-dihydroxyvitamin D: active vitamin that influences calcium balance - Anatomy: o Paired organs o 150 grams each o Behind the peritoneum on either side of the vertebral column against the posterior abdominal wall. o Urine goes from kidney to ureter to bladder to urethra for excretion. o The renal medulla is the interior part of the kidney. It is formed of triangles called the renal pyramids o The renal cortex is on the exterior part of the kidney. It surrounds the medulla. o Blood supply: Renal artery branches to: Interlobar artery, which turns at 90 to become: Arcuate artery, which turns at 90 to become: Interlobular artery o Note that the venous structure is parallel to the arterial structure and that nephrons receive bood by the afferent arterioles o Nephron (1 million/kidney): Renal corpuscle: Glomerulus (capillary loops) Note that the terms corpuscle and glomerulus can be used interchangeably. Bowmans capsule Tubule o Notes about the anatomy of the nephron: The tubule is surrounded by a monolayer of epithelial cells All the convoluted parts of the nephrons are in the cortex, while the parts in the medulla are straight. o The renal corpuscle: The glomerulus is surrounded by a kind of balloon with a monolayer of epithelial cells called Bowmans capsule. The exterior part of Bowmans capsule is made of the parietal layer, and the interior part is made of the visceral layer. o Glomerular capillary wall: Capillary loops are surrounded by podocytes (foot processes) The foot processes are interdigitating in adjacent capillaries, with narrow spaces in between called the filtration slits. The fenestrae and filtration slits in the glomerular basement membrane (GBM) allow the passage of the filtrate from the capillary lumen to the capsular space. The capillary wall is composed of: Fenestrated endothelial cells GBM Visceral epithelial cells with podocytes. o Vascular supply of the nephron: The efferent arteriole branches into peritubular capillaries, which provide blood to the tubules, but also receive material from the tubules. Sections of the nephron and their functions: - Glomerulus: o Location: always in the cortex - Proximal tubule: o Location: convoluted: cortex Straight: half cortex, half medulla o Functions: Reabsorbs most of the filtered water and solutes (2/3 of all filtered water and sodium) It is also a major site of secretion for various solutes, except K+ Reabsorbs all amino acids and glucose Water reabsorption: Not regulated Water reabsorption in the proximal tubule depends on Na+ reabsorption 1. Na+ is reabsorbed from the tubular lumen to the interstitial fluid across the epithelial cells 2. The local osmolarity in the lumen decreases, while the local osmolarity in the interstitium increases. 3. The difference in osmolarity causes net diffusion of water from the lumen into the interstitial fluid a. Via tubular cells plasma membrane b. Via tight junctions 4. From the interstitium, water, sodium and everything else dissolved in the ISF move together by bulk flow into peritubular capillaries o Transport mechanisms: Diffusion (ex: urea) Sodium reabsorbed by Na+/H+ antiporter or Na+-glucose contransporter - Descending thin limb of Henles loop: o Location: medulla o Functions: Reabsorbs water o Particularities: No sodium reabsorption! Highly permeable to water - Ascending thin limb of Henles loop: o Location: medulla o Function: Not really important physiologically We will ignore it and consider the ascending part of the loop of Henle as being composed of the thick limb only. - Ascending thick limb of Henles loop: o Location: half medulla, half cortex o Functions: Reabsorbs sodium to create the countercurrent multiplier system. o Properties: Reabsorbs NaCl Impermeable to water - The juxtaglomerular apparatus: o Juxtaglomerular cells: Location: Function: secrete renin Regulation: receives information from tubular fluid via macula densa cells and afferent arterioles. o Macula Densa: Location: cortex, at the end of the thick ascending limb of Henles loop, between the afferent and efferent arterioles. - Distal convoluted tubule: o Location: cortex o Functions: Fine tuning Homeostatic control Adjusts rates of reabsorption and secretion according to physiological needs. Hormonal control of water and sodium reabsorption - Cortical collecting duct: o Location: cortex o Functions: Fine tuning Homeostatic control Adjusts rates of reabsorption and secretion according to physiological needs. Hormonal control of water and sodium reabsorption Secretion of potassium o Transport mechanisms: Na+ enters epithelial cells by diffusion via Na+ channels o Two critical components that allow regulation of water reabsorption in the CD: High osmolarity of the medullary interstitium Permeability of CD to water - Medullary collecting duct: o Location: medulla Regulation of the GFR: - The GFR is regulated by: o Net filtration pressure o Membrane permeability o Surface area available for filtration - The glomerular capillary blood pressure can be influenced by constricting or dilating the afferent arteriole (AA) or efferent arteriole (EA). This has a direct effect on the net filtration pressure (see equations): o Decrease GFR: Constrict AA Dilate EA o Increase GFR: Constrict EA Dilate AA Transport through peritubular cells: - Paracellular: passage through gap junctions between the epithelial cells - Transcellular: passage through the epithelial cells Important facts about tubular reabsorption: - Filtered loads are enormous, generally greater than the amounts of the substance in the body - Reabsorption of waste products is relatively incomplete - Reabsorption of most useful plasma components is relatively complete - Reabsorption of some substances is not regulated while others are highly regulated.
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