An Introduction to the Urinary SystemThe urinary system removes most physiological wastes26-1 Urinary System FunctionsThree Functions of the Urinary System:1. Excretion - Removal of organic wastes from body fluids2. Elimination - Discharge of waste products3. Homeostatic regulation of blood plasma volume and solute concentrationStructures and Functions of the Urinary System:Kidneys — organs that produce urineUrinary tract — organs that eliminate urineUreters (paired tubes)Urinary bladder (muscular sac)Urethra (exit tube)Urination or micturition — process of eliminating urineContraction of muscular urinary bladder forces urine through urethra, and out of bodyHomeostatic Functions of the Urinary System:1. Regulates blood volume and blood pressureBy adjusting volume of water lost in urineReleasing erythropoietin and renin2. Regulates plasma ion concentrationsSodium, potassium, and chloride ions (by controlling quantities lost in urine)Calcium ion levels (through synthesis of calcitriol)3. Helps stabilize blood pHBy controlling loss of hydrogen ions and bicarbonate ions in urine4. Conserves valuable nutrientsBy preventing excretion while excreting organic waste products5. Assists liverIn detoxifying poisons. Also during starvation, deaminates amino acids so other tissues can metabolize them for energy26-2 The Kidneys:The Kidneys:Located on either side of vertebral columnLeft kidney lies superior to right kidneySuperior surface capped by adrenal glandPosition is maintained by:Overlying peritoneumContact with adjacent visceral organsSupporting connective tissuesFloating Kidney:Suspensory fibers damaged, kidney displaced, can cause pain, distortion, or ureter/blood vessels during movementEach Kidney Is Protected and StabilizedBy three concentric layers of connective tissue1. Fibrous capsule (innermost layer)A layer of collagen fibersCovers outer surface of entire organ2. Perinephric fat capsuleA thick layer of adipose tissueSurrounds renal capsule3. Renal fascia (outermost layer)A dense, fibrous outer layerAnchors kidney to surrounding structuresTypical Adult KidneyIs about 10 cm long, 50.5 cm wide, and 3 cm thick (4 in. ´ 2.2 in. ´ 1.2 in.)Weighs about 150 g (5.25 oz)HilumPoint of entry for renal artery and renal nervesPoint of exit for renal vein and ureterRenal CortexSuperficial portion of kidney in contact with renal capsuleReddish brown and granularRenal Pyramids6 to 18 distinct conical or triangular structures in renal medullaBase abuts cortexTip (renal papilla) projects into renal sinusRenal ColumnsBands of cortical tissue separate adjacent renal pyramidsExtend into medullaHave distinct granular textureRenal LobeConsists of:Renal pyramidOverlying area of renal cortexAdjacent tissues of renal columnsProduces urineRenal PapillaDucts discharge urine into minor calyx, a cup-shaped drainMajor CalyxFormed by four or five minor calycesRenal PelvisLarge, funnel-shaped chamberConsists of two or three major calycesFills most of renal sinusConnected to ureter, which drains kidneyBlood Supply to the Kidneys:Kidneys receive 20%–25% of total cardiac output1200 mL of blood flows through kidneys each minuteKidney receives blood through renal arteryBlood flow:From renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate (interlobular) arteries → afferent arterioles → glomerulus efferent arteriole → peritubular capillaries → venules → cortical radiate (interlobular) veins → arcuate veins → interlobar veins → renal veinRenal Nerves - Innervate kidneys and uretersEnter each kidney at hilumFollow tributaries of renal arteries to individual nephronsSympathetic InnervationAdjusts rate of urine formation by changing blood flow and blood pressure at nephronStimulates release of reninRestricts losses of water and salt in urine by stimulating reabsorption at nephron (renin-angiotensin system stimulates aldosterone release from adrenal cortex)Nephrons – functional unit of the kidneyMicroscopic, tubular structures in cortex of each renal lobeEach kidney has approximately 1.25 million nephronsWhere urine production beginsThe Nephron - Consists of renal tubule and renal corpuscleRenal corpuscle - Spherical structure consisting of:Glomerular capsule (Bowman’s capsule)Cup-shaped chamberCapillary network (glomerulus)Renal tubule - Long tubular passagewayBegins at renal corpuscleGlomerulus - Consists of 50 intertwining capillariesBlood delivered via afferent arterioleBlood leaves in efferent arterioleFlows into peritubular capillariesWhich drain into small venulesAnd return blood to venous systemNote: blood leaves the capillary network through the efferent arteriole and NOT through a venuleFiltration:Occurs in renal corpuscleBlood pressureForces water and dissolved solutes out of glomerular capillaries into capsular spaceProduces protein-free solution (filtrate), similar to blood plasma except lacks proteinsThree Functions of the Renal Tubule:1. Reabsorb useful organic nutrients that enter filtrate2. Reabsorb more than 90% of water in filtrate3. Secrete waste products that failed to enter renal corpuscle through filtration at glomerulusSegments of the Renal Tubule:Located in cortexProximal convoluted tubule (PCT)Nephron loop (loop of Henle)U-shaped tubeExtends partially into medullaIn juxtamedullary nephrons, the nephron loop extends deeper into the medulla (important for their function!)Distal convoluted tubule (DCT)Organization of the NephronTraveling along tubule, filtrate (tubular fluid) gradually changes compositionDue to reabsorption and secretion of substancesChanges vary with activities in each segment of nephronEach Nephron:Empties into the collecting systemA series of tubes that carries the filtrate away from nephronCollecting ductsReceive fluid from many nephronsEach collecting duct:Begins in cortexDescends into medullaCarries fluid to papillary duct that drains into a minor calyxCortical Nephrons:85% of all nephronsLocated mostly within superficial cortex of kidneyNephron loop (Loop of Henle) is relatively shortEfferent arteriole delivers blood to a network of peritubular capillariesJuxtamedullary Nephrons:15% of nephronsNephron loops extend deep into medullaPeritubular capillaries connect to vasa recta (capillary network of juxtamedullary nephrons - found surrounding the loop of Henle)The Renal Corpuscle:Each renal corpuscle is 150–250 µm in diameter, consists of:Glomerular capsule - Forms outer wall of renal corpuscle,
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