Chapter 19b
The Kidneys
Reabsorption
• Principles governing the tubular reabsorption of solutes and water
Figure 19-11
Na+ is reabsorbed by active transport.
Electrochemical gradient drives anionreabsorption.
Water moves by osmosis, followingsolute reabsorption.
Concentrations of other solutesincrease as fluid volume in lumendecreases. Permeable solutes arereabsorbed by diffusion.
Na+
Anions
H2O
K+, Ca2+,urea
Tubularepithelium Extracellular fluidTubule lumen
Filtrate is similar tointerstitial fluid.
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2
3
4
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2
3
4
Reabsorption
• Transepithelial transport• Substances cross both apical (lumen side) and
basolateral membrane• Paracellular pathway• Substances pass through the junction between
two adjacent cells
Reabsorption
Figure 19-12
[Na+] high [Na+] high[Na+] low
K+Na+ ATP
Proximal tubule cellInterstitial
fluidTubulelumen
Na+ reabsorbed Na+ enters cell through membrane proteins,moving down its electrochemical gradient.
Na+ is pumped out the basolateral sideof cell by the Na+-K+-ATPase.
ATP = Active transporter= Membrane protein
KEY
Na+
Filtrate is similar tointerstitial fluid.
1 2
1
2
• Sodium reabsorption in the proximal tubule
Reabsorption
• Sodium-linked glucose reabsorption in the proximal tubule
Figure 19-13
= SGLT secondary active transporter
= GLUT facilitated diffusion carrier
[Na+] high[glu] low
Na+ Na+
[Na+] high
[glu] low
glu glu
[Na+] low[glu] high
K+ATP
Glucose and Na+
reabsorbed+
Na+ moving down its electrochemical gradientusing the SGLT protein pulls glucose into thecell against its concentration gradient.
Glucose diffuses out the basolateral side ofthe cell using the GLUT protein.
Na+ is pumped out by Na+-K+-ATPase.
Proximal tubule cell Interstitial fluidTubule lumen
ATP = Active transporterKEY
Filtrate is similar tointerstitial fluid.
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Reabsorption
• Urea • Passive reabsorption
• Plasma proteins • Transcytosis
Reabsorption
• Saturation of mediated transport
Figure 19-14
Renal threshold isplasma concentrationat which saturationoccurs.
Transport maximum (Tm) is transportrate at saturation.
Saturation occurs.
Plasma [substrate] (mg/mL)
Tran
spor
t rat
e of
sub
stra
te (m
g/m
in)
Reabsorption
• Glucose handling by the nephron
Figure 19-15a
Reabsorption
Figure 19-15b
Reabsorption
Figure 19-15c
Reabsorption
Figure 19-15d
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
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
Inulin Clearance
• Inulin clearance is equal to GFR
Figure 19-16
Glomerulus
Peritubularcapillaries
Afferentarteriole
Efferentarteriole
Nephron
Filtration(100 mL/min)
= 100 mL ofplasma or filtrate
Inulin concentrationis 4/100 mL.
GFR = 100 mL /min
100 mL plasma isreabsorbed. No inulinis reabsorbed.100% of inulin isexcreted so inulinclearance = 100 mL/min.
Inulinmolecules
100 mL,0% inulin
reabsorbed
Inulin clearance = 100 mL/min100% inulin
excreted
KEY
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4
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2
3
4
Inulin Clearance
Figure 19-16, steps 1–4
Glomerulus
Peritubularcapillaries
Afferentarteriole
Efferentarteriole
Nephron
Filtration(100 mL/min)
= 100 mL ofplasma or filtrate
Inulin concentrationis 4/100 mL.
GFR = 100 mL /min
100 mL plasma isreabsorbed. No inulinis reabsorbed.100% of inulin isexcreted so inulinclearance = 100 mL/min.
Inulinmolecules
100 mL,0% inulin
reabsorbed
Inulin clearance = 100 mL/min100% inulin
excreted
KEY
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2
3
4
1
2
3
4
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
Excretion
Table 19-2
Filtration(100 mL/min)
Glucosemolecules
100 mL,100% glucose
reabsorbed
Glucoseclearance
= 0 mL/min
(a) Glucose clearance
No glucoseexcreted
Plasma concentrationis 4/100 mL.GFR = 100 mL /min
100 mL plasma isreabsorbed.Clearance depends onrenal handling of solute.
KEY= 100 mL ofplasma or filtrate
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4
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Excretion
• The relationship between clearance and excretion
Figure 19-17a
Excretion
Figure 19-17b(b) Urea clearance
Plasma concentrationis 4/100 mL.GFR = 100 mL /min
100 mL plasma isreabsorbed.Clearance depends onrenal handling of solute.
KEYFiltration
(100 mL/min)
Ureamolecules
100 mL,50% of ureareabsorbed
50% of ureaexcreted
Ureaclearance
= 50 mL/min
= 100 mL ofplasma or filtrate
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4
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4
Excretion
Figure 19-17c(c) Penicillin clearance
Plasma concentrationis 4/100 mL.GFR = 100 mL /min
100 mL plasma isreabsorbed.Clearance depends onrenal handling of solute.
KEY
Filtration(100 mL/min)
100 mL,0 penicillinreabsorbed
Someadditionalpenicillinsecreted.
Penicillinclearance =150 mL/min
More penicillinis excreted than
was filtered.
Penicillinmolecules
= 100 mL ofplasma or filtrate
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4
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
Micturition
• The storage of urine and the micturition reflex
Figure 19-18a
Bladder(smooth muscle)
Internal sphincter (smoothmuscle) passively contracted
External sphincter (skeletal muscle) stays contracted
(a) Bladder at rest
Tonicdischarge
Relaxed(filling)state
HigherCNSinput
Incontinence
Micturition
Figure 19-18b
Stretch receptors fire.
Parasympathetic neurons fire.Motor neurons stop firing.
Smooth muscle contracts.Internal sphincter passivelypulled open. External sphincterrelaxes.
(b) Micturition
Internal sphincter
External sphincter
Tonicdischargeinhibited
Higher CNSinput may
facilitate orinhibit reflex
Sensory neuron
Parasympatheticneuron
Motor neuron
Stretchreceptors
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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
Summary
• Reabsorption• How solutes are transported• Transport maximum and renal threshold
• Secretion• Excretion• Clearance, inulin, and creatinine
• Micturition