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Chapter 19b

Chapter 19b. The Kidneys. Reabsorption. Principles governing the tubular reabsorption of solutes and water. Filtrate is similar to interstitial fluid. 1. Na + is reabsorbed by active transport. 1. Na +. 2. 2. Anions. Electrochemical gradient drives anion reabsorption. 3.

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Chapter 19b

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  1. Chapter 19b The Kidneys

  2. Reabsorption • Principles governing the tubular reabsorption of solutes and water Filtrate is similar tointerstitial fluid. 1 Na+ is reabsorbed by active transport. 1 Na+ 2 2 Anions Electrochemical gradient drives anionreabsorption. 3 Water moves by osmosis, followingsolute reabsorption. 3 H2O 4 4 Concentrations of other solutesincrease as fluid volume in lumendecreases. Permeable solutes arereabsorbed by diffusion. K+, Ca2+,urea Tubularepithelium Extracellular fluid Tubule lumen Figure 19-11

  3. Reabsorption • Transepithelialtransport • Substances cross both apical (lumen side) and basolateral membrane • Paracellular pathway • Substances pass through the junction between two adjacent cells

  4. Reabsorption • Sodium reabsorption in the proximal tubule Filtrate is similar tointerstitial fluid. 1 Na+ enters cell through membrane proteins,moving down its electrochemical gradient. Na+ reabsorbed 2 Na+ is pumped out the basolateral sideof cell by the Na+-K+-ATPase. [Na+] high [Na+] low [Na+] high 2 1 Na+ ATP Na+ K+ KEY Tubulelumen Interstitialfluid Proximal tubule cell = Membrane protein ATP = Active transporter Figure 19-12

  5. Reabsorption • Sodium-linked glucose reabsorption in the proximal tubule Filtrate is similar tointerstitial fluid. 1 Na+ moving down its electrochemical gradientusing the SGLT protein pulls glucose into thecell against its concentration gradient. + Glucose and Na+reabsorbed [Na+] low[glu] high [Na+] high[glu] low 2 [glu] low 2 Glucose diffuses out the basolateral side ofthe cell using the GLUT protein. glu glu 1 Na+ Na+ 3 3 Na+ is pumped out by Na+-K+-ATPase. [Na+] high ATP K+ KEY ATP = Active transporter = SGLT secondary active transporter Tubule lumen Interstitial fluid Proximal tubule cell = GLUT facilitated diffusion carrier Figure 19-13

  6. Reabsorption • Urea • Passive reabsorption • Plasma proteins • Transcytosis

  7. Reabsorption • Saturation of mediated transport Transport maximum (Tm) is transportrate at saturation. Saturation occurs. Transport rate of substrate (mg/min) Renal threshold isplasma concentrationat which saturationoccurs. Plasma [substrate] (mg/mL) Figure 19-14

  8. Reabsorption • Glucose handling by the nephron Figure 19-15a

  9. Reabsorption Figure 19-15b

  10. Reabsorption Figure 19-15c

  11. Reabsorption Figure 19-15d

  12. Secretion • Transfer of molecules from extracellular fluid into lumen of the nephron • Active process • Important in homeostatic regulation • K+ and H+ • Increasing secretion enhances nephron excretion • A competitive process • Penicillin and probenecid

  13. Excretion • Excretion = filtration – reabsorption + secretion • Clearance • Rate at which a solute disappears from the body by excretion or by metabolism • Non-invasive way to measure GFR • Inulin and creatinine used to measure GFR

  14. Inulin Clearance • Inulin clearance is equal to GFR Efferentarteriole Filtration(100 mL/min) Peritubularcapillaries Glomerulus 2 Afferentarteriole 1 Nephron Inulinmolecules KEY = 100 mL ofplasma or filtrate 1 3 Inulin concentrationis 4/100 mL. 100 mL,0% inulinreabsorbed 2 GFR = 100 mL /min 3 100 mL plasma isreabsorbed. No inulinis reabsorbed. Inulin clearance = 100 mL/min 4 100% inulinexcreted 4 100% of inulin isexcreted so inulinclearance = 100 mL/min. Figure 19-16

  15. Inulin Clearance Efferentarteriole Filtration(100 mL/min) Peritubularcapillaries Glomerulus 2 Afferentarteriole 1 Nephron Inulinmolecules KEY = 100 mL ofplasma or filtrate 1 3 Inulin concentrationis 4/100 mL. 100 mL,0% inulinreabsorbed 2 GFR = 100 mL /min 3 100 mL plasma isreabsorbed. No inulinis reabsorbed. Inulin clearance = 100 mL/min 4 100% inulinexcreted 100% of inulin isexcreted so inulinclearance = 100 mL/min. 4 Figure 19-16, steps 1–4

  16. GFR • Filtered load of X = [X]plasma GFR • Filtered load of inulin = excretion rate of inulin • GFR = excretion rate of inulin/[inulin]plasma = inulin clearance • GFR = inulin clearance

  17. Excretion Table 19-2

  18. Excretion • The relationship between clearance and excretion KEY Filtration(100 mL/min) = 100 mL ofplasma or filtrate 1 Plasma concentrationis 4/100 mL. 2 2 GFR = 100 mL /min 1 3 100 mL plasma isreabsorbed. Glucosemolecules 4 Clearance depends onrenal handling of solute. 3 100 mL,100% glucosereabsorbed 4 Glucoseclearance= 0 mL/min No glucoseexcreted (a) Glucose clearance Figure 19-17a

  19. Excretion KEY Filtration(100 mL/min) = 100 mL ofplasma or filtrate 1 Plasma concentrationis 4/100 mL. 2 2 GFR = 100 mL /min 1 100 mL plasma isreabsorbed. 3 Ureamolecules 4 Clearance depends onrenal handling of solute. 3 100 mL,50% of ureareabsorbed 4 Ureaclearance = 50 mL/min 50% of ureaexcreted (b) Urea clearance Figure 19-17b

  20. Excretion KEY Filtration(100 mL/min) = 100 mL ofplasma or filtrate 1 Plasma concentrationis 4/100 mL. 2 Someadditionalpenicillinsecreted. 2 GFR = 100 mL /min 1 100 mL plasma isreabsorbed. 3 Penicillinmolecules 4 Clearance depends onrenal handling of solute. 100 mL,0 penicillinreabsorbed 3 4 Penicillinclearance =150 mL/min More penicillinis excreted thanwas filtered. (c) Penicillin clearance Figure 19-17c

  21. Gout • Limit animal protein. Avoid or severely limit high-purine foods, including organ meats, such as liver, and herring, anchovies and mackerel. Red meat (beef, pork and lamb), fatty fish and seafood (tuna, shrimp, lobster and scallops) are associated with increased risk of gout. Because all animal protein contains purines, limit your intake. • Eat more plant-based proteins. You can increase your protein by including more plant-based sources, such as beans and legumes. This switch will also help you cut down on saturated fats, which may indirectly contribute to obesity and gout. • Limit or avoid alcohol. Alcohol interferes with the elimination of uric acid from your body. Drinking beer, in particular, has been linked to gout attacks

  22. Micturition • The storage of urine and the micturition reflex HigherCNSinput Relaxed(filling)state Bladder(smooth muscle) Internal sphincter (smoothmuscle) passively contracted Tonicdischarge External sphincter (skeletal muscle) stays contracted (a) Bladder at rest Incontinence Figure 19-18a

  23. Micturition Higher CNSinput mayfacilitate orinhibit reflex Stretchreceptors Sensory neuron 1 Parasympatheticneuron 1 Stretch receptors fire. 2 3 2 Parasympathetic neurons fire.Motor neurons stop firing. Motor neuron Smooth muscle contracts.Internal sphincter passivelypulled open. External sphincterrelaxes. Tonicdischargeinhibited 3 2 Internal sphincter 3 External sphincter (b) Micturition Figure 19-18b

  24. Summary • Functions of the kidneys • Anatomy • Kidney, nephron, cortex, and medulla • Renal blood flow and fluid flow from glomerulus to renal pelvis • Overview of kidney function • Filtration • Podocytes, filtration slits, and mesangial cells • Filtration fraction, GFR, and regulation of GFR

  25. Summary • Reabsorption • How solutes are transported • Transport maximum and renal threshold • Secretion • Excretion • Clearance, inulin, and creatinine • Micturition

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