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Module 11-Renal System.pdf

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Physiology 2130
Anita Woods

Module 11 - Renal System • Renal system includes kidneys, ureters, bladder and urethra • Kidneys regulate water balance, electrolyte levels, pH of blood and long-term regulation of arterial pressure Kidney Functions • Basic function to remove nonessential substances form plasma (waste metabolites, excess water, electrolytes) and recover essential substances (ie. glucose) • Major role regulating water levels, chemical concentration of body fluid compartments and pH of blood. • Do not produce water or electrolytes only conserve them by reducing amount removed • Elimination of waste or foreign substances important (drugs, food additives and vitamins excreted in urine) • Act as endocrine gland, producing hormones or components of hormonal systems i.e. erythropoietin, renin, vitamin D and stanniocalcin. Kidney Anatomy • Roughly the size of a fist • Consist of outer renal cortex, middle renal medulla, and inner calyces that drain into central renal pelvis • Renal pelvis then drains into ureter • Within renal pyramids are functional units of kidneys (nephrons) • Each nephron drains through a collecting duct into a calyx. Kidney Blood Supply • Blood flows to kidneys through renal artery, which branches into several interlobar arteries that also branch into arcuate arteries. • Blood in arcuate arteries flows through interlobular arteries to supply nephron which drains into interlobular vein, the arcuate vein, the interlobar vein and then renal vein. Nephron Anatomy • Functional unit of the kidneys, over 1 million in each kidney • Function is to filter blood, reabsorb essential substances, and excrete nonessential molecules and waste • Composed of highly coiled hollow tube surrounded by complex blood supply • Glomerular (Bowman’s) capsule surrounds very small, highly permeable capillary bed called glomerulus • Collectively referred to as renal corpuscle • Tubular portion consists of: proximal convoluted tubule (highly coiled portion), descending and ascending limb of loop of Henle, distal convoluted tube and collecting duct. Nephron Blood Supply • Blood from renal artery eventually reaches interlobular artery that drains into afferent arteriole. • Afferent arteriole gives rise to glomerulus • Blood from glomerulus enters efferent arteriole, then enters peritubular capillaries, which then drains into interlobular vein and back to renal vein Renal Corpuscle • Made up of glomerular capsule and glomerulus • Site where blood is filtered, called glomerular filtration • Fluid filtered from blood enters glomerular capsule (capsular space) called filtrate • Glomerular filtration facilitated by highly permeable capillary endothelium surrounded by podocytes • Larger diameter afferent arteriole and smaller diameter efferent arteriole also enhance glomerular filtration Processes along Nephron - Terms Filtration: movement of fluid through glomerular capillary due to hydrostatic pressures Filtrate: solution created by filtration. Generally composed of water plus all dissolved solutes in blood (except proteins too large to be filtered) Reabsorption: movement of a substance from lumen of nephron back into blood Secretion: movement of substance from blood into lumen of nephron Excretion: removal of substance form body Excretion = filtration + secretion - reabsorption Glomerular Filtration • Bulk flow of fluid from blood into glomerular capsule. Filtrate contains same substances as plasma with exception of large proteins & rbcs • Affected by extremely permeable capillaries which make up glomerulus and Starling Forces • Podocytes (epithelial cells) surround the capillaries. Have large filtration slits formed between pedicles. This increases filtration at glomerulus. Starling Forces • Causes bulk movement of fluid across capillaries due to hydrostatic and colloid osmotic forces • Blood hydrostatic force ~ 60 mmHg (diameter difference, afferent and efferent) • Colloid osmotic pressure ~ -32 mmHg (plasma proteins) • Capsular hydrostatic pressure -18 mmHg • Net filtration pressure is 10 mmHg out of glomerulus into capsular space Glomerular Filtration Rate (GFR): volume of fluid filtered by glomerulus during a certain time period • Kidneys filter ~ 180 L/day • Filtered load: amount of substances filtered by kidneys each day Filtered load = GFR x Plasma Concentration of Substance • Important to calculate urine concentration and amount of solute excreted • Urine concentration: amount of solute excreted per unit volume of urine (g/L) • Amount of solute excreted: actual amount (grams) of solute excreted in urine Amount Excreted = Urine Concentration x Amount of water excreted/ Day (1.8 L/day) • Amount reabsorbed: amount of filtered substance taken back up by kidneys Amount Reabsorbed = Filtered Load - Amount Excreted Fraction Excreted = (Amount Excreted / Filtered Load) x 100% Example: GFR = 180 L/day Na+ plasma concentration = 5 g/L Amount of Na+ reabsorbed = 626 g/day Amount of water excreted = 1.8 L 1. Amt. Excreted = Amt. Filtered (Filtered Load) + Amt. Secreted - Amt. Reabsorbed Na+ not secreted in this example 2. Amt. Excreted = Filtered Load - Amt. Reabsorbed Need to calculate Filtered load 3. Filtered Load - GFR x Plasma Concentration 180 L/day x 5 g/L = 900 g (Insert into equation 2) Amt. Na+ Excreted = 900g - 626 g = 274 g 4. Urine Concentration of Na+ = Amt. of Na+ excreted/ Amt. of Water excreted 274 g / 1.8 L = 152.2 g of Na+/L of Urine Reabsorption - 99% substances filtered get reabsorbed - Two Transport routes: 1) Paracellular transport: Diffusion between leaky parts of tight junctions in tubular cells. Generally non-regulated. 2) Transcellular transport: Transported across tubular membrane from lumen into cell and into interstitial fluid and blood. Can be regulated by hormones, mostly non- regulated. Na/K+ Pump - Active Transport - 3 Na+ out and 2 K+ in - Powers other transport mechanisms through secondary active transport Secondary Active Transport - As Na+ moves into cell other substances either move in with it or move out to exchange with the Na+ coming in - Includes - Na+/glucose transporter: - Located on luminal side of tubule cells - Each Na+ diffusing in carries a glucose molecule with it - Na+/H+ exchanger: - Located on luminal side - Na+ diffuses in, H+ out Secretion - Generally hormonally regulated - Can occur through regulation - Most secreted substances eventually excreted in urine. - Includes: H+ and K+ (reliant on pump presence) Tubular Transport Mechanisms - Na+ Reabsorption: in proximal tubule, ascending loop of Henle and early distal tubule - Mostly through non-regulated mechanisms - Can be regulated through angiotensin II and aldosterone in late distal tubule and collecting duct (In healthy humans, all glucose filtered is reabsorbed in proximal tubule, aa also reabsorbed in proximal tubule) - Water reabsorption: in proximal tubule, descending limb of loop of Henle - Non-regulated - Regulated by ADH in late distal tubule and collecting duct - K+ reabsorption: in proximal tubule and ascending limb of loop of Henle - K+ secretion: in small amounts in ascending loop of Henle. 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