Chapter 26: Urinary System PDF

Chapter 26: Urinary system Main function of kidney: 1) filtration, 2) reabsorption, 3) secretion Primary renal function: Filters 200 L bld/d Removes toxins, metabolic waste & excess ions Regulates volume & chemical makeup of blood Maintains proper water/salt/acid/base balance in bod...

Chapter 26: Urinary system Main function of kidney: 1) filtration, 2) reabsorption, 3) secretion Primary renal function: Filters 200 L bld/d Removes toxins, metabolic waste & excess ions Regulates volume & chemical makeup of blood Maintains proper water/salt/acid/base balance in body Gluconeogenesis during prolonged fasting Production of renin (regulates blood pressure) Production of erythropoietin (stimulates RBC formation) Activates Vit D Kidney: retroperitoneal (located behind abdominal lining); renal hilus: entry & exit point for blood vessels, lymphatics, nerves 3 layers of kidney - supportive tissue Renal capsule: fibrous capsule prevents infection Adipose capsule: fatty mass cushions kidney & attaches to body wall Renal fascia: outer layer of dense fibrous anchors kidney Internal anatomy of kidney Renal cortex: light colored, granular superficial region glomeruli located here Renal medulla ○ Renal columns inward extensions of cortical tissue, separate the…. Pyramids parallel bundles of collecting tubules Papillae base of pyramid, drainage point Renal pelvis ○ Minor & major calyces collect urine from papillae empty into pelvis Renal cortex (filtration begins in the nephrons) → renal medulla (contains structure like the loops of Henle & collecting ducts, concentrating the urine) → renal pelvis (collects urine from the medulla & funnels it to ureters) → ureter (tubes that transport urine to the bladder) → bladder (store urine until it’s excreted) Anatomy of the nephron (located between renal cortex & medulla) Cortical nephrons (general filtration): mostly in the outer part of the kidney (renal cortex); handle most of kidney’s filtering work Juxtamedullary nephrons (urine concentration): near border of cortex & medulla (cortex/medulla junction) w/ long loops extending deep into medulla; concentrate urine by reabsorbing more water ○ Juxta: Latin for "next to" or "near." ○ Medullary: Refers to the renal medulla, the inner part of the kidney. ○ Nephrons: Functional units of the kidney responsible for filtering blood and forming urine. Mechanism of urine formation (occurs in juxtamedullary nephrons & cortical nephrons) ○ Kidneys filter body’s entire plasma volume about 60 d/day ○ Filtrate contains all plasma components except protein & blood cells ○ Become urine, filtrate loses water, nutrient, essential ions  ○ Urine contains metabolic wastes & unneeded substances ○ Urine formation/blood composition adjustment (glomerular filtration → tubular reabsorption → tubular secretion) Removal of filtrate from blood (glomerular filtration) Removal of keepers from filtrate (tubular reabsorption) Secretion of waste into filtrate (tubular secretion) Glomerular capillaries ○ Afferent arteriole (larger diameter; bring blood into glomerulus = more blood flow) → glomerulus (filtration happens) → efferent arteriole (smaller diameter; take blood away from glomerulus → smaller size creates resistance) ○ Arterioles are high resistance; efferent resistance higher (blood has harder time to leave glomerulus); causes high BP in glomerulus (aids in formation of filtrate along entire length of glomerular capillaries) Renal corpuscle (found in renal cortex) Glomerulus (protective layer of kidney) ○ Glomerular epithelium: fenestrated (small pores or openings), allows solute rich, protein free filtrate to pass from blood into Bowman's capsule (no blood cells to pass though) ○ Glomerular filtration - Very efficient because of…. Fenestrations in the capillary wall, permeable membrane increase surface area High glomerular BP High net filtration pressure ○ Net Filtration Pressure (NFP) Pressure responsible for filtrate formation NFP = HPg - (OPg + HPc) HPg = glomerular hydrostatic pressure OPg = osmotic pressure of glomerular blood HPc = capsular hydrostatic pressure ○ Glomerular filtration rate (GFR): total amount of filtrate formed/minute tightly controlled by 2 types of mechanisms Factors governing GFR Total surface area availability for filtration Filtration membrane permeability Net filtration pressure (directly proportional) Too high GFR → needed substances CANNOT be reabsorbed & lost Too low GFR → everything is reabsorbed, including waste Three regulatory mechanisms Renal autoregulation (intrinsic controls)  ○ Normal conditions = maintain nearly constant GFR when MAP is in the range of 80-180 mm Hg Myogenic control Increase BP → constriction of afferent arterioles = maintain normal GFR; protect glomeruli from damaging high BP Decrease BP → dilation of afferent arterioles = maintain normal GFR Flow-dependent tubuloglomerular feedback Directed by macula densa cells Increase GFR (filtrate flow rate increase in tubule) → filtrate (NaCl) is high because no time for reabsorption; macula densa cells (JGA) respond to increase NaCl → release; vasoconstriction chem. (acts on afferent arteriole → decrease GFR) Opposite occur if GFR decrease Neural controls (extrinsic controls) ○ Resting conditions → max dilation of renal blood vessel → intrinsic control prevail ○ Stress conditions → S-ANS trigger → NE & E release → afferent arterioles constrict → inhibit filtration → renin release Hormonal mechanism - renin-angiotensin (stimulate by S-ANS) Bowman’s capsule ○ Parietal layer ○ Visceral layer (podocytes): support cells, have end process w/ spaces called filtration slits; regulate filtration Filtration membrane: porous membrane between blood & filtrate  Filter between blood & interior glomerular capsule ○ Fenestrated endothelium ○ Visceral membrane of glomerular capsule w/ foot processes ○ Gel-like basement membrane (negatively charged) Juxtaglomerular apparatus (located near glomerulus or renal corpuscle; regulates BP & filtration rate) DCT lies against the afferent (sometimes efferent) arteriole JG cells on arteriole wall, mechanoreceptors, secretory granules w/ renin Mesangial cells: interconnected w/ gap junctions; may pass signals between macula densa & granular (JG) Macula densa of ascending limb of Loop of Henle: chemoreceptors (NaCl) Peritubular capillaries ○ Low pressure, adapted for absorption, arise from efferent arterioles, cling to renal tubules, empty into renal venous system, vasa recta: long, straight efferent arterioles of JM nephrons ○ Tubular reabsorption: process of selectively reclaiming desirable substances Main site: PCT - transported substances through 3 membranes (luminal membrane, capillary endothelial cell, basolateral membrane) Substances reabsorbed Ions: Ca, Mg, K, Na All organic nutrients, water H2O & ion reabsorption - hormonally controlled ○ Reabsorption: active or passive Na reabsorption is always: almost always active transport Na transport drives reabsorption of other solutes & H2O ○ Dilute vs. concentrated urine Production of dilute urine is the default; collecting duct impermeable to water Production of concentrated urine is stimulated by ADH; ADH targets collecting ducts → increase H2O permeability & reabsorbed ○ Renal tubule (part of nephron forms urine; absorption) Proximal Convoluted Tubule (PCT) - found in cortex 65% of Na+ & H2O, nutrients, ions, small proteins Loop of Henle (partly medulla): Thin segment: simple squamous cells = permeable to H2O Thick segment: simple cuboidal/columnar cells (impermeable to water but allows passive movement of ions like sodium & chloride) Descending limb: H2O (primarily thin segment)  Ascending limb: Na+, Cl-, K+ (may include thin segment but is predominantly the thick segment) Distal Convoluted Tubule (DCT) - found in cortex: cell cuboidal, no microvilli; more in secretion than absorption, confined to cortex DCT & collecting duct ○ Reabsorption is hormonally regulated Ca2+ (parathyroid hormone; PTH = calcium reabsorption in kidneys, increase calcium reabsorption when low levels) H2O (antidiuretic hormone; ADH = water reabsorption, increase permeability of collecting ducts in kidneys) Na+ (aldosterone, produce by adrenal glands promote sodium reabsorption in kidney, & atrial natriuretic peptide or ANP = oppose aldosterone by inhibiting sodium reabsorption, decrease blood volume & pressure) Sympathetic nervous system Extreme stress → ep/norep release → vasoconstriction of afferent arterioles → diverts blood away from kidney temporarily → activates renin-angiotensin system → increased BP → maintain blood flow to vital organs Tubular secretion: substances move from peritubular capillaries to tubules Important for: disposing of substances not already in the filtrate, eliminating undesirable substances, ridding body of excess potassium ions, controlling blood pH, disposes of substances that are bound to plasma proteins Blood supply of kidney Aorta → renal artery → segmental artery → lobar artery → interlobar artery → arcuate artery → interlobular artery → afferent arteriole → glomerulus (capillaries) → efferent arteriole → peritubular capillaries and vasa recta → interlobular vein → arcuate vein → Interlobar vein → renal vein → inferior vena cava Blood pressure declines along renal circulation; relatively high in glomerulus, strong outward force pushing fluid out of capillaries in glomerulus Ureters Entry to bladder: base of bladder through posterior wall As bladder pressure increases: distal ends of the ureters close, prevents backflow Trilayered wall 1) Transitional epithelium mucosa (stretch) 2) Smooth muscle muscularis 3) Fibrous connective tissue adventitia Propulsion: active, respond to stretch by contracting Urinary bladder Normally holds 500 mL Retroperitoneal Three openings: two ureters, urethra  Trigone: area between 3 openings (frequent site of bladder infection) Layers: ○ Mucosa: transitional epithelium ○ Muscularis: circular, longitudinal, oblique → form detrusor muscle compress bladder) ○ Adventitia Urethra Function: drains urine from bladder Female vs. male urethra ○ Male: longer & serves both urinary and reproductive function Male urethra: prostatic, membranous, spongy ○ Female: shorter & only carries urine ○ Two sphincters: Internal (involuntary): controls release of urine from bladder (in males, prevents backflow of semen into the bladder during ejaculation) External (voluntary): allows conscious control over urination Developmental aspects Embryonic: 3 sets kidney develop, but only 1 kept and develop by 5th week Fetal: urine produced by 3rd month Infants: small bladders, kidneys CAN’T concentrate urine Childhood: control of voluntary urethral sphincter develops w/ nervous system (2 y.o) Urinary tract infections: 80% caused by E. coli Aging: kidney function declines, many elderly people become incontinent Congenital: ○ Horseshoe: Kidneys are fused together, happens in 1 in 600 people (higher risk of infections) ○ Hypospadias (under; abnormal opening): urethral orifice not at distal end of penis  ○ Polycystic: urine-filled cyst due to abnormal develop of collecting ducts → renal failure Diuretics (Increase Urine Output) Osmotic diuretics: Unabsorbed substances (e.g., high glucose in diabetes) increase urine. ADH inhibitors: Alcohol/caffeine block ADH, reducing water retention. Na+ inhibitors: Caffeine/drugs block sodium and water reabsorption. Effects of aging Kidney size and blood flow decrease; fewer glomeruli function (ability to secrete & absorb decrease) Reduced ability to concentrate urine and respond to ADH/aldosterone Less vitamin D production → calcium deficiency, osteoporosis, fractures Counter-Current Mechanism Interaction between filtrate flow through loop of Henle & blood flow through vasa recta → concentrate urine Thin loop of Henle: impermeable to solutes, permeable to H2O Thick loop of Henle: permeable to solutes, impermeable to H2O  Filtrate moves down → descending loop of Henle → H2O removed to area of greater solute Filtrate moves up → ascending loop of Henle → solutes removed to area of lesser solute