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The Urinary System: Part B
25 The Urinary System: Part B
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Most of tubular contents reabsorbed to blood Selective process
Tubular Reabsorption Most of tubular contents reabsorbed to blood Selective process ~ All organic nutrients reabsorbed Water and ion reabsorption hormonally regulated and adjusted Includes active and passive tubular reabsorption © 2013 Pearson Education, Inc.
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Tubule cell Peri- tubular capillary
Figure Transcellular and paracellular routes of tubular reabsorption. Slide 1 Transport across the basolateral membrane. (Often involves the lateral intercellular spaces because membrane transporters transport ions into these spaces.) The paracellular route involves: The transcellular route involves: 3 • Movement through leaky tight junctions, particularly in the PCT. • Movement through the inter- stitial fluid and into the capillary. Transport across the apical membrane. 1 Diffusion through the cytosol. 2 Movement through the inter- stitial fluid and into the capillary. 4 Filtrate in tubule lumen Tubule cell Interstitial fluid Peri- tubular capillary Tight junction Lateral intercellular space 3 4 1 2 3 4 H2O and solutes Transcellular route Capillary endothelial cell Apical membrane Paracellular route H2O and solutes Basolateral membranes © 2013 Pearson Education, Inc.
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Reabsorption of Nutrients, Water, and Ions
Na+ reabsorption by primary active transport provides energy and means for reabsorbing most other substances Creates electrical gradient passive reabsorption of anions Organic nutrients reabsorbed by secondary active transport Glucose, amino acids, some ions, vitamins © 2013 Pearson Education, Inc.
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Figure 25.14 Reabsorption by PCT cells.
Slide 1 At the basolateral membrane, Na+ is pumped into the interstitial space by the Na+-K+ ATPase. Active Na+ transport creates concentration gradients that drive: 1 2 “Downhill” Na+ entry at the apical membrane. Reabsorption of organic nutrients and certain ions by cotransport at the apical membrane. 3 Filtrate in tubule lumen Nucleus Interstitial fluid Peri- tubular capillary Tubule cell 4 Reabsorption of water by osmosis through aquaporins. Water reabsorption increases the concentration of the solutes that are left behind. These solutes can then be reabsorbed as they move down their gradients: 2 Glucose Amino acids Some ions Vitamins 1 3 4 5 Lipid-soluble substances diffuse by the transcellular route. Lipid- soluble substances 5 Various Ions and urea 6 6 Various ions (e.g., Cl−, Ca2+, K+) and urea diffuse by the paracellular route. Tight junction Paracellular route Primary active transport Transport protein Secondary active transport Ion channel Passive transport (diffusion) Aquaporin © 2013 Pearson Education, Inc.
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Reabsorptive Capabilities of Renal Tubules and Collecting Ducts
PCT Site of most reabsorption All nutrients, e.g., glucose and amino acids 65% of Na+ and water Many ions ~ All uric acid; ½ urea (later secreted back into filtrate) © 2013 Pearson Education, Inc.
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Reabsorptive Capabilities of Renal Tubules and Collecting Ducts
Nephron loop Descending limb - H2O can leave; solutes cannot Ascending limb – H2O cannot leave; solutes can Thin segment – passive Na+ movement Thick segment – Na+-K+-2Cl- symporter and Na+-H+ antiporter; some passes by paracellular route © 2013 Pearson Education, Inc.
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Figure 25.15 Summary of tubular reabsorption and secretion.
Cortex 65% of filtrate volume reabsorbed • H2O • Na+, HCO3−, and many other ions • Glucose, amino acids, and other nutrients Regulated reabsorption • Na+ (by aldosterone; Cl− follows) • Ca2+ (by parathyroid hormone) • H+ and NH4+ • Some drugs Regulated secretion • K+ (by aldosterone) Outer medulla Regulated reabsorption • H2O (by ADH) • Na+ (by aldosterone; Cl− follows) • Urea (increased by ADH) • Urea Regulated secretion • K+ (by aldosterone) Inner medulla • Reabsorption or secretion to maintain blood pH described in Chapter 26; involves H+, HCO3−, and NH4+ Reabsorption Secretion © 2013 Pearson Education, Inc.
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The three key players and their orientation in the osmotic gradient:
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to produce urine of varying concentration. (1 of 4) The three key players and their orientation in the osmotic gradient: (c) The collecting ducts of all nephrons use the gradient to adjust urine osmolality. 300 300 400 (a) The long nephron loops of juxtamedullary nephrons create the gradient. They act as countercurrent multipliers. 600 The osmolality of the medullary interstitial fluid progressively increases from the 300 mOsm of normal body fluid to 1200 mOsm at the deepest part of the medulla. 900 (b) The vasa recta preserve the gradient. They act as countercurrent exchangers. 1200 © 2013 Pearson Education, Inc.
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Long nephron loops of juxtamedullary nephrons create the gradient.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to produce urine of varying concentration. (2 of 4) Long nephron loops of juxtamedullary nephrons create the gradient. The countercurrent multiplier depends on three properties of the nephron loop to establish the osmotic gradient. Fluid flows in the opposite direction (countercurrent) through two adjacent parallel sections of a nephron loop. The descending limb is permeable to water, but not to salt. The ascending limb is impermeable to water, and pumps out salt. © 2013 Pearson Education, Inc.
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Long nephron loops of juxtamedullary nephrons create the gradient.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to produce urine of varying concentration. (3 of 4) Long nephron loops of juxtamedullary nephrons create the gradient. These properties establish a positive feedback cycle that uses the flow of fluid to multiply the power of the salt pumps. Interstitial fluid osmolality Start here Water leaves the descending limb Salt is pumped out of the ascending limb Osmolality of filtrate in descending limb Osmolality of filtrate entering the ascending limb © 2013 Pearson Education, Inc.
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Osmolality of interstitial fluid (mOsm)
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to produce urine of varying concentration. (4 of 4) (continued) As water and solutes are reabsorbed, the loop first concentrates the filtrate, then dilutes it. Active transport Passive transport Water impermeable 300 300 100 Cortex 100 300 300 Filtrate entering the nephron loop is isosmotic to both blood plasma and cortical interstitial fluid. 1 5 Filtrate is at its most dilute as it leaves the nephron loop. At 100 mOsm, it is hypo-osmotic to the interstitial fluid. 400 400 200 Na+ and Cl- are pumped out of the filtrate. This increases the interstitial fluid osmolality. 4 Outer medulla Osmolality of interstitial fluid (mOsm) 600 600 400 Water moves out of the filtrate in the descending limb down its osmotic gradient. This concentrates the filtrate. 2 900 900 700 Filtrate reaches its highest concentration at the bend of the loop. 3 Inner medulla Nephron loop 1200 1200 © 2013 Pearson Education, Inc.
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Vasa recta preserve the gradient.
Figure 25.16b Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to produce urine of varying concentration. Vasa recta preserve the gradient. The entire length of the vasa recta is highly permeable to water and solutes. Due to countercurrent exchanges between each section of the vasa recta and its surrounding interstitial fluid, the blood within the vasa recta remains nearly isosmotic to the surrounding fluid. As a result, the vasa recta do not undo the osmotic gradient as they remove reabsorbed water and solutes. Blood from efferent arteriole To vein 300 325 300 400 The countercurrent flow of fluid moves through two adjacent parallel sections of the vasa recta. 400 600 600 900 900 1200 Vasa recta © 2013 Pearson Education, Inc.
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Formation of Dilute or Concentrated Urine
Osmotic gradient used to raise urine concentration > 300 mOsm to conserve water Overhydration large volume dilute urine ADH production ; urine ~ 100 mOsm If aldosterone present, additional ions removed ~ 50 mOsm Dehydration small volume concentrated urine Maximal ADH released; urine ~ 1200 mOsm Severe dehydration – 99% water reabsorbed © 2013 Pearson Education, Inc.
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Figure 25.17 Mechanism for forming dilute or concentrated urine.
If we were so overhydrated we had no ADH... If we were so dehydrated we had maximal ADH... Osmolality of extracellular fluids Osmolality of extracellular fluids ADH release from posterior pituitary ADH release from posterior pituitary Number of aquaporins (H2O channels) in collecting duct Number of aquaporins (H2O channels) in collecting duct H2O reabsorption from collecting duct H2O reabsorption from collecting duct Large volume of dilute urine Small volume of concentrated urine Collecting duct Collecting duct Descending limb of nephron loop 300 Descending limb of nephron loop 300 100 100 150 DCT 100 DCT 300 Cortex Cortex 300 100 300 300 100 300 300 400 600 400 Osmolality of interstitial fluid (mOsm) 600 600 400 Osmolality of interstitial fluid (mOsm) 600 600 100 Outer medulla Outer medulla Urea 900 700 900 900 700 900 900 Urea Urea Inner medulla Inner medulla 1200 100 1200 1200 1200 1200 Large volume of dilute urine Small volume of concentrated urine Urea contributes to the osmotic gradient. ADH increases its recycling. Active transport Passive transport © 2013 Pearson Education, Inc.
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Physical Characteristics of Urine
Color and transparency Clear Cloudy may indicate urinary tract infection Pale to deep yellow from urochrome Pigment from hemoglobin breakdown; more concentrated urine deeper color Abnormal color (pink, brown, smoky) Food ingestion, bile pigments, blood, drugs © 2013 Pearson Education, Inc.
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Physical Characteristics of Urine
Odor Slightly aromatic when fresh Develops ammonia odor upon standing As bacteria metabolize solutes May be altered by some drugs and vegetables © 2013 Pearson Education, Inc.
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Chemical Composition of Urine
95% water and 5% solutes Nitrogenous wastes Urea (from amino acid breakdown) – largest solute component Uric acid (from nucleic acid metabolism) Creatinine (metabolite of creatine phosphate) © 2013 Pearson Education, Inc.
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Collapses when empty; rugae appear
Urinary Bladder Collapses when empty; rugae appear Expands and rises superiorly during filling without significant rise in internal pressure ~ Full bladder 12 cm long; holds ~ 500 ml Can hold ~ twice that if necessary Can burst if overdistended © 2013 Pearson Education, Inc.
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Kidney Renal pelvis Ureter Urinary bladder
Figure Pyelogram. Kidney Renal pelvis Ureter Urinary bladder © 2013 Pearson Education, Inc.
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Figure 25.20a Structure of the urinary bladder and urethra.
Peritoneum Ureter Rugae Detrusor Adventitia Ureteric orifices Trigone of bladder Bladder neck Internal urethral sphincter Prostate Prostatic urethra Intermediate part of the urethra External urethral sphincter Urogenital diaphragm Spongy urethra Erectile tissue of penis External urethral orifice Male. The long male urethra has three regions: prostatic, intermediate, and spongy. © 2013 Pearson Education, Inc.
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Figure 25.20b Structure of the urinary bladder and urethra.
Peritoneum Ureter Rugae Detrusor Ureteric orifices Bladder neck Internal urethral sphincter Trigone External urethral sphincter Urogenital diaphragm Urethra External urethral orifice Female. © 2013 Pearson Education, Inc.
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Muscular tube draining urinary bladder
Urethra Muscular tube draining urinary bladder Lining epithelium © 2013 Pearson Education, Inc.
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Urethra Sphincters Internal urethral sphincter
Involuntary (smooth muscle) at bladder-urethra junction Contracts to open External urethral sphincter Voluntary (skeletal) muscle surrounding urethra as it passes through pelvic floor © 2013 Pearson Education, Inc.
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Urethra Female urethra (3–4 cm) Tightly bound to anterior vaginal wall
External urethral orifice Anterior to vaginal opening; posterior to clitoris © 2013 Pearson Education, Inc.
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Figure 25.20b Structure of the urinary bladder and urethra.
Peritoneum Ureter Rugae Detrusor Ureteric orifices Bladder neck Internal urethral sphincter Trigone External urethral sphincter Urogenital diaphragm Urethra External urethral orifice Female. © 2013 Pearson Education, Inc.
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Male urethra carries semen and urine
Three named regions Prostatic urethra (2.5 cm)—within prostate Intermediate part of the urethra (membranous urethra) (2 cm)—passes through urogenital diaphragm from prostate to beginning of penis Spongy urethra (15 cm)—passes through penis; opens via external urethral orifice © 2013 Pearson Education, Inc.
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Figure 25.20a Structure of the urinary bladder and urethra.
Peritoneum Ureter Rugae Detrusor Adventitia Ureteric orifices Trigone of bladder Bladder neck Internal urethral sphincter Prostate Prostatic urethra Intermediate part of the urethra External urethral sphincter Urogenital diaphragm Spongy urethra Erectile tissue of penis External urethral orifice Male. The long male urethra has three regions: prostatic, intermediate, and spongy. © 2013 Pearson Education, Inc.
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Three simultaneous events must occur
Micturition Urination or voiding Three simultaneous events must occur Contraction of detrusor by ANS Opening of internal urethral sphincter by ANS Opening of external urethral sphincter by somatic nervous system © 2013 Pearson Education, Inc.
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Figure 25.21 Control of micturition.
Brain Higher brain centers Urinary bladder fills, stretching bladder wall Allow or inhibit micturition as appropriate Afferent impulses from stretch receptors Pontine micturition center Pontine storage center Simple spinal reflex Promotes micturition by acting on all three spinal efferents Inhibits micturition by acting on all three Spinal efferents Spinal cord Spinal cord Parasympathetic activity Sympathetic activity Somatic motor nerve activity Parasympathetic activity Sympathetic activity Somatic motor nerve activity Detrusor contracts; internal urethral sphincter opens External urethral sphincter opens Inhibits Micturition © 2013 Pearson Education, Inc.
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