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Chapter 26: The Urinary SystemChapter ObjectivesOVERVIEW OF KIDNEY FUNCTIONOVERVIEW OF RENAL PHYSIOLOGYTUBULAR REABSPOPTION AND SECRETIONChapter Lecture NotesOverview of Kidney FunctionsRegulation of blood ionic compositionNa+, K+, Ca+2, Cl- and phosphate ionsRegulation of blood pH, osmolarity & glucoseRegulation of blood volumeconserving or eliminating waterRegulation of blood pressuresecreting the enzyme reninadjusting renal resistanceRelease of erythropoietin & calcitriolExcretion of wastes & foreign substancesNephronsThe nephron is the functional unit of the kidney. (Fig 26.5)A nephron consists of aRenal corpuscleglomerulus is a capillary ballglomerular (Bowman’s) capsule is double-walled epithelial cupRenal tubuleproximal convoluted tubuleloop of Henle (nephron loop)descending limb – permeable to water, but impermeable to solutesthin ascending limbthick ascending limb - impermeable to water and solutesdistal convoluted tubule – variable permeability to watercollecting duct – variable permeability to waterdistal convoluted tubules of several nephrons drain into to a single collecting ductmany collecting ducts drain into a small number of papillary ductspapillary ducts drain urine to the renal pelvis and ureter.Blood Vessels around the Nephron (Fig 26.5)Glomerular capillaries are formed between the afferent & efferent arteriolesEfferent arterioles give rise to the peritubular capillaries and vasa rectaThere are two types of nephrons that have differing structure and function.A cortical nephron usually has its glomerulus in the outer portion of the cortex and a short loop of Henle that penetrates only into the outer region of the medulla (Fig 26.5a)80-85% of nephrons are cortical nephronsA juxtamedullary nephron usually has its glomerulus deep in the cortex close to the medulla; its long loop of Henle stretches through the medulla and almost reaches the renal papilla (Fig 26.5b)15-20% of nephrons are juxtamedullary nephronsAllow excretion of dilute or concentrated urineHistology of the Glomerular CapsuleGlomerular (Bowman’s) capsuleThe glomerular capsule consists of visceral and parietal layers (Fig 26.6)The visceral layer consists of modified simple squamous epithelial cells called podocytesThe parietal layer consists of simple squamous epithelium and forms the outer wall of the capsuleFluid filtered from the glomerular capillaries enters the capsular space, the space between the two layers of the glomerular capsule.Juxtaglomerular ApparatusStructure where afferent arteriole makes contact with ascending limb of loop of Henle (Fig 26.6)macula densa is thickened part of ascending limbjuxtaglomerular cells are modified muscle cells in arteriolethe JGA helps regulate blood pressure and the rate of blood filtration by the kidneysOverview of Renal PhysiologyNephrons and collecting ducts perform 3 basic processes (Fig 26.7)glomerular filtrationa portion of the blood plasma is filtered into the glomerular capsuleLocation - renal corpuscletubular reabsorptionwater & useful substances are reabsorbed into the bloodLocation – renal tubules and collecting ducttubular secretionwastes are removed from the blood & secreted into urineLocation – renal tubules and collecting ductGlomerular Filtrationglomerular endotheliumstops all cells and plateletsglomerular basement membranestops large plasma proteinsslit membranes between pedicels of podocytesstops medium plasma proteinsThe principle of filtration - force fluids and solutes through a membrane by pressureis similar in glomerular capillaries as in capillaries elsewhere in the body.Net Filtration PressureGlomerular Filtration RateGlomerular Filtration Rate (GFR) = Amount of filtrate formed in all renal corpuscles of both kidneys / minuteaverage adult male rate is 125 mL/minChanges in net filtration pressure affects GFRfiltration stops if GBHP drops to 45mm Hgfunctions normally with mean arterial pressures 80-180Regulation of GFRThe mechanisms that regulate GFR adjust blood flow into and out of the glomerulus and alter the glomerular capillary surface area available for filtration. (Table 26.2)The three principal mechanisms that control GFR areRenal autoregulationMechanisms that maintain a constant GFR despite changes in arterial BPmyogenic mechanismsystemic increases in BP, stretch the afferent arteriolesmooth muscle contraction reduces the diameter of the arteriole returning the GFR to its previous level in secondstubuloglomerular feedback (Fig 26.10)elevated systemic BP raises the GFR so that fluid flows too rapidly through the renal tubule & Na+, Cl- and water are not reabsorbedmacula densa detects that difference & releases a vasoconstrictor from the juxtaglomerular apparatusafferent arterioles constrict & reduce GFRNeural regulationBlood vessels of the kidney are supplied by sympathetic fibers that cause vasoconstriction of afferent arteriolesAt rest, renal blood vessels are maximally dilated because sympathetic activity is minimalrenal autoregulation prevailsWith moderate sympathetic stimulation, both afferent & efferent arterioles constrict equallydecreasing GFR equallyWith extreme sympathetic stimulation (exercise or hemorrhage), vasoconstriction of afferent arterioles reduces GFRlowers urine output & permits blood flow to other tissuesHormonal regulationAtrial natriuretic peptide (ANP) increases GFRstretching of the atria that occurs with an increase in blood volume causes ANP releaserelaxes glomerular mesangial cells, cells between the glomerular capillaries, increasing capillary surface area and increasing GFRAngiotensin II reduces GFRpotent vasoconstrictor that narrows both afferent & efferent arterioles reducing GFRTubular Reabsorption & SecretionNormal GFR is so high that volume of filtrate in capsular space in half an hour is greater than the total plasma volumeNephron must reabsorb 99% of the filtrate (Table 26.3)Another important function of nephrons is tubular secretionReabsorption RoutesA substance being reabsorbed can move between adjacent tubule cells or through an individual tubule cell before entering a peritubular capillary. (Fig 26.11)Paracellular reabsorption - 50% of reabsorbed material moves between cells by diffusion in some parts of tubuleTranscellular reabsorption - material moves through both the apical and basal membranes of the tubule cell by passive and active transportTransport MechanismsTransport across membranes can be either active or passive.Secondary active transport - energy stored in an


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MCC BIO 202 - The Urinary System

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