1. Chapter 19b
The Kidneys

2. Principles governing the tubular reabsorption of solutes and water
Filtrate is similar to interstitial fluid. 1 1
Na+
Na+ is reabsorbed by active transport. 2
Anions 2
Electrochemical gradient drives anion reabsorption. 3 3
H2O
Water moves by osmosis, following solute reabsorption. 4
K+, Ca2+, urea 4
Concentrations of other solutes increase as fluid volume in lumen decreases. Permeable solutes are reabsorbed by diffusion.
Tubular epithelium
Tubule lumen
Extracellular fluid
Figure 19-11

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

4. Sodium reabsorption in the proximal tubule
Filtrate is similar to interstitial fluid. 1
Na+ reabsorbed
Na+ enters cell through membrane proteins, moving down its electrochemical gradient. 2
Na+ is pumped out the basolateral side of cell by the Na+-K+-ATPase.
[Na+] high
[Na+] low
[Na+] high 2 1
Na+
Na+
ATP K+
Tubule lumen
Interstitial fluid
KEY
Proximal tubule cell
= Membrane protein
ATP
= Active transporter
Figure 19-12

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

6. Reabsorption
Urea
Passive reabsorption
Plasma proteins
Transcytosis

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

8. Glucose handling by the nephron
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. Important in homeostatic regulation
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 = 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 concentration is 4/100 mL.
Inulin Clearance
Inulin clearance is equal to GFR
Efferent arteriole
Filtration (100 mL/min)
Peritubular capillaries
Glomerulus 2
Afferent arteriole 1
Inulin molecules
Nephron
KEY
= 100 mL of plasma or filtrate 1
Inulin concentration is 4/100 mL. 3
100 mL, 0% inulin reabsorbed 2
GFR = 100 mL /min 3
100 mL plasma is reabsorbed. No inulin is reabsorbed. 4
Inulin clearance = 100 mL/min 4
100% of inulin is excreted so inulin clearance = 100 mL/min.
100% inulin excreted
Figure 19-16

15. Inulin concentration is 4/100 mL.
Inulin Clearance
Efferent arteriole
Filtration (100 mL/min)
Peritubular capillaries
Glomerulus 2
Afferent arteriole 1
Inulin molecules
Nephron
KEY
= 100 mL of plasma or filtrate 1
Inulin concentration is 4/100 mL. 3
100 mL, 0% inulin reabsorbed 2
GFR = 100 mL /min 3
100 mL plasma is reabsorbed. No inulin is reabsorbed. 4
Inulin clearance = 100 mL/min 4
100% of inulin is excreted so inulin clearance = 100 mL/min.
100% inulin excreted
Figure 19-16, steps 1–4

16. 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. 100 mL, 100% glucose reabsorbed Glucose clearance = 0 mL/min
Excretion
The relationship between clearance and excretion
KEY
Filtration (100 mL/min)
= 100 mL of plasma or filtrate 1
Plasma concentration is 4/100 mL. 2 2
GFR = 100 mL /min 1
Glucose molecules 3
100 mL plasma is reabsorbed. 4
Clearance depends on renal handling of solute. 3
100 mL, 100% glucose reabsorbed 4
Glucose clearance = 0 mL/min
No glucose excreted
(a) Glucose clearance
Figure 19-17a

19. Excretion Filtration (100 mL/min) = 100 mL of plasma or filtrate 1
KEY
Filtration (100 mL/min)
= 100 mL of plasma or filtrate 1
Plasma concentration is 4/100 mL. 2 2
GFR = 100 mL /min 1 3
100 mL plasma is reabsorbed.
Urea molecules 4
Clearance depends on renal handling of solute. 3
100 mL, 50% of urea reabsorbed 4
Urea clearance = 50 mL/min
50% of urea excreted
(b) Urea clearance
Figure 19-17b

20. Excretion Filtration (100 mL/min) = 100 mL of plasma or filtrate 1
KEY
Filtration (100 mL/min)
= 100 mL of plasma or filtrate 1
Plasma concentration is 4/100 mL. 2
Some additional penicillin secreted. 2
GFR = 100 mL /min 1
Penicillin molecules 3
100 mL plasma is reabsorbed. 4
Clearance depends on renal handling of solute. 3
100 mL, 0 penicillin reabsorbed 4
Penicillin clearance = 150 mL/min
More penicillin is excreted than was 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. Relaxed (filling) state
Micturition
The storage of urine and the micturition reflex
Higher CNS input
Relaxed (filling) state
Bladder (smooth muscle)
Internal sphincter (smooth muscle) passively contracted
Tonic discharge
External sphincter (skeletal muscle) stays contracted
(a) Bladder at rest
Incontinence
Figure 19-18a

23. Micturition
Stretch receptors
Higher CNS input may facilitate or inhibit reflex
Sensory neuron 1
Parasympathetic neuron 1
Stretch receptors fire. 2 3 2
Parasympathetic neurons fire. Motor neurons stop firing.
Motor neuron
Tonic discharge inhibited 3
Smooth muscle contracts. Internal sphincter passively pulled open. External sphincter relaxes.
Internal sphincter 2 3
External sphincter
(b) Micturition
Figure 19-18b

24. Functions of the kidneys Anatomy
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. Reabsorption Secretion Excretion Micturition Summary
How solutes are transported
Transport maximum and renal threshold
Secretion
Excretion
Clearance, inulin, and creatinine
Micturition