1. The Urinary System: Renal Function

2. I. Functions of the Urinary System
Regulate plasma ionic composition
Regulate plasma volume
Regulate plasma osmolarity
Regulate plasma pH
Remove metabolic waste products and foreign substances from plasma
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3. Other Functions of the Kidneys
Secrete erythropoietin
Secrete renin
Activate vitamin D3 to calcitriol
Gluconeogenesis
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4. Structures of the Urinary System
Figure 18.1
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5. Blood Supply Renal arteries enter kidney at hilus
Receive 20% of cardiac output at rest
Less than 1% of body weight
Utilize 16% of ATP usage by body
Function is to filter blood
Renal veins exit at hilus
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6. Anatomy of the Kidney Figure 18.2
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7. Anatomy of the Nephron Renal corpuscle
Glomerulus = capillary network for filtration
Bowman’s capsule
Receives the filtrate
Inflow to renal tubules
Figure 18.3
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8. Cortical/Juxtamedullary Nephrons
Figure 18.4
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9. Juxtaglomerular Apparatus
Figure 18.5
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10. Nephron Blood Supply Figure 18.6b
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11. Renal Exchange Processes
Figure 18.7
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12. Glomerular Filtration
Movement of protein-free plasma from glomerulus to Bowman’s capsule
GFR = 125 mL/min or 180 liters/day
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13. Renal Corpuscle Figure 18.8 (1 of 3)
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14. Renal Corpuscle Figure 18.8 (2 of 3)
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15. Starling Forces Favoring Filtration Across Glomerulus
Glomerular capillary hydrostatic pressure
60 mm Hg
High due to resistance of efferent arteriole
Bowman’s capsule oncotic pressure
0 mm Hg
Low due to lack of protein in filtrate
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16. Starling Forces Opposing Filtration Across the Glomerulus
Bowman’s capsule hydrostatic pressure
15 mm Hg
Relatively high (compared to systemic capillaries) due to large volume of filtrate in closed space
Glomerular oncotic pressure
29 mm Hg
Higher than in systemic capillaries due to plasma proteins in smaller volume of plasma
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17. Glomerular Filtration Pressure
Figure 18.9a
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18. Glomerular Filtration Rate
Filtration pressure = (PGC + BC) – (PBC + GC) = (60 + 0) – ( ) = 16 mm Hg
Renal plasma flow = 625 mL/minute
GFR =125 mL/min = 180 liters/day
Compared to systemic capillaries
Filtration pressure = 2 mm Hg
Filtration rate = 3 liters/day
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19. Filtration Fraction 625 mL plasma enters kidneys per minute
125 mL filtered into Bowman’s capsule
125/625 = filtration fraction = 20%
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20. Glomerular Filtration Rate
Figure 18.9b
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21. Filtered Load of Glucose
GFR = 125 mL/min
Plasma [glucose] =100 mg/dL = 1 mg/mL
Filtered load of glucose = (125 mL/min) x (1 mg/mL) = 125 mg/min
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22. Regulation of GFR 180 liters fluid filtered/day
Only 1.5 liters urine excreted/day (<1%)
>99% of filtered fluid is reabsorbed
Small increase in GFR  large increase volume fluid filtered and excreted
GFR highly regulated
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23. Effect of MAP on GFR Figure 18.10
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24. GFR: Myogenic Regulation
Figure 18.11a
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25. GFR: Tubuloglomerular Regulation
Figure 18.11b
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26. Extrinsic Control of GFR
Figure 18.12
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27. Normal Rates of Filtration and Reabsorption
Table 18.1
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28. Solute Reabsorption Most occurs in proximal convoluted tubule
Some in distal convoluted tubule
Barrier for reabsorption
Epithelial cells of renal tubules
Endothelial cells of capillary (minimal)
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29. Reabsorption Barrier Figure 18.13
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30. Active Solute Reabsorption
Figure 18.14a
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31. Water Reabsorption Figure 18.14b
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32. Passive Solute Reabsorption
Figure 18.14c
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33. Renal Threshold For a solute which is normally 100% reabsorbed
If solute in filtrate saturates carriers, then some solute excreted in urine
Solute in plasma that causes solute in filtrate to saturate carriers and spillover into urine = renal threshold
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34. Example: Glucose Reabsorption
Freely filtered at glomerulus
Normally 100% actively reabsorbed in proximal tubule
Normally, no glucose appears in urine
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35. Glucose Reabsorption Carrier proteins for glucose reabsorption
Apical membrane: secondary active transport
Basolateral membrane: facilitated diffusion
Figure 18.15
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36. Glucose Renal Curve Figure 18.16
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37. Renal Handling of Glucose
Plasma [glucose] = 100 mg/dL
Filtered load glucose = 125 mg/min
Transport maximum for glucose reabsorption = 375 mg/min
Theoretical renal threshold = 300 mg/dL
(GFR x Renal Threshold = Transport Maximum)
Actual renal threshold = 160–180 mg/dL
Filtered load = 225 mg/min
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38. Secretion Solute moves from peritubular capillaries into tubules
Barriers same as for reabsorption
Transport mechanisms same but opposite direction
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39. Secretion Secreted substances Potassium Hydrogen ions Choline
Creatinine
Penicillin
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40. Proximal Tubule Reabsorption
Proximal tubule is the mass reabsorber
70% water and sodium
100% glucose
Brush border provides for large surface area
Figure 18.17a
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41. Proximal Tubule Reabsorption
Non-regulated reabsorption
Leaky tight junctions allow paracellular transport
Figure 18.17a
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42. Distal Tubule and Collecting Duct
Transport is regulated across epithelium
Tight junctions limit paracellular transport
Figure 18.17b
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43. Water Conservation due to Loop of Henle
Loop of Henle establishes conditions necessary to concentrate urine
Minimizes water loss
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44. Tubule Regional Specialization
Table 18.2
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45. Excretion Rate
Amount of substance excreted = amount filtered + amount secreted – amount reabsorbed
Amount excreted depends on
Filtered load
Secretion rate
Reabsorption rate
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46. Basic Renal Processes Figure 18.18
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47. Renal Handling of Solute
If amount of solute excreted per minute is less than filtered load  solute was reabsorbed
If amount of solute excreted per minute is greater than filtered load  solute was secreted
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48. Clearance excretion rate Ux x V
Volume of plasma from which a substance has been removed by kidneys per unit time (Volume of plasma that contains the amount of a substance that has been excreted per unit time)
excretion rate Ux x V
Clearance = =
plasma concentration Px
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49. Clearance of Inulin Figure 18.19
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50. Clearance of Glucose Figure 18.20a
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51. Clearance of PAH Figure 18.20b
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52. Renal Blood Flow PAH clearance = measure of renal plasma flow
Convert plasma flow to blood flow
Blood is 55% plasma
clearance PAH mL/min
Renal blood flow = = = 1136 mL/min
1-hematocrit
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53. Clearance of Common Substances
Table 18.3
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