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 Electrochemical gradient drives anion
reabsorption.
Anions
3 H2O
3 Water moves by osmosis, following
solute reabsorption.
4
4 Concentrations of other solutes
increase as fluid volume in lumen
decreases. Permeable solutes are
reabsorbed by diffusion.
K+, Ca2+,
urea
Tubule lumen
Tubular
epithelium
Extracellular fluid
Figure 19-11
Reabsorption
• Transepithelial transport
• Substances cross both apical (lumen side) and
basolateral membrane
• Paracellular pathway
• Substances pass through the junction between
two adjacent cells
Reabsorption
• Sodium reabsorption in the proximal tubule
Filtrate is similar to
interstitial fluid.
Na+ reabsorbed
1 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
1
Na+
Na+
Tubule
lumen
Proximal tubule cell
2
ATP
[Na+] high
K+
Interstitial
fluid
KEY
= Membrane protein
ATP = Active transporter
Figure 19-12
Reabsorption
• Sodium-linked glucose reabsorption in the
proximal tubule
Filtrate is similar to
interstitial fluid.
Glucose and Na++
reabsorbed
[Na+]
[Na+]
low
[glu] high
high
[glu] low
2
[glu] low
1
glu
glu
Na+
Na+
3
[Na+] high
ATP
K+
1 Na+ moving down its electrochemical gradient
using the SGLT protein pulls glucose into the
cell against its concentration gradient.
2 Glucose diffuses out the basolateral side of
the cell using the GLUT protein.
3 Na+ is pumped out by Na+-K+-ATPase.
KEY
ATP = Active transporter
= SGLT secondary active transporter
Tubule lumen
Proximal tubule cell
Interstitial fluid
= GLUT facilitated diffusion carrier
Figure 19-13
Reabsorption
• Urea
• Passive reabsorption
• Plasma proteins
• Transcytosis
Reabsorption
Transport rate of substrate (mg/min)
• Saturation of mediated transport
Transport maximum (Tm) is transport
rate at saturation.
Saturation occurs.
Renal threshold is
plasma concentration
at which saturation
occurs.
Plasma [substrate] (mg/mL)
Figure 19-14
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
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
2 GFR = 100 mL /min
3 100 mL plasma is
reabsorbed. No inulin
is reabsorbed.
4 100% of inulin is
excreted so inulin
clearance = 100 mL/min.
100% inulin
excreted
100 mL,
0% inulin
reabsorbed
4 Inulin clearance
= 100 mL/min
Figure 19-16
Inulin Clearance
Efferent
arteriole
Filtration
(100 mL/min)
Peritubular
capillaries
Glomerulus
2
Afferent
arteriole
1
Nephron
Inulin
molecules
KEY
= 100 mL of
plasma or filtrate
1 Inulin concentration
is 4/100 mL.
3
2 GFR = 100 mL /min
3 100 mL plasma is
reabsorbed. No inulin
is reabsorbed.
4 100% of inulin is
excreted so inulin
clearance = 100 mL/min.
100% inulin
excreted
100 mL,
0% inulin
reabsorbed
4 Inulin clearance
= 100 mL/min
Figure 19-16, steps 1–4
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
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
3 100 mL plasma is
reabsorbed.
Glucose
molecules
4 Clearance depends on
renal handling of solute.
3
No glucose
excreted
100 mL,
100% glucose
reabsorbed
4 Glucose
clearance
= 0 mL/min
(a) Glucose clearance
Figure 19-17a
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
3 100 mL plasma is
reabsorbed.
Urea
molecules
4 Clearance depends on
renal handling of solute.
3
100 mL,
50% of urea
reabsorbed
4
50% of urea
excreted
Urea
clearance
= 50 mL/min
(b) Urea clearance
Figure 19-17b
Excretion
KEY
Filtration
(100 mL/min)
= 100 mL of
plasma or filtrate
1 Plasma concentration
is 4/100 mL.
2
Some
additional
penicillin
secreted.
1
Penicillin
molecules
2 GFR = 100 mL /min
3 100 mL plasma is
reabsorbed.
4 Clearance depends on
renal handling of solute.
3
More penicillin
is excreted than
was filtered.
100 mL,
0 penicillin
reabsorbed
4 Penicillin
clearance =
150 mL/min
(c) Penicillin clearance
Figure 19-17c
Gout
•
Limit animal protein. Avoid or severely limit highpurine 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
Higher
CNS
input
Relaxed
(filling)
state
Bladder
(smooth muscle)
Internal sphincter (smooth
muscle) passively contracted
External sphincter (skeletal
muscle) stays contracted
Tonic
discharge
(a) Bladder at rest
Incontinence
Figure 19-18a
Micturition
Stretch
receptors
Sensory neuron
1
Parasympathetic
neuron
2
Higher CNS
input may
facilitate or
inhibit reflex
3
2 Parasympathetic neurons fire.
Motor neurons stop firing.
Motor neuron
Internal sphincter
3
External sphincter
1 Stretch receptors fire.
2
Tonic
discharge
inhibited
3 Smooth muscle contracts.
Internal sphincter passively
pulled open. External sphincter
relaxes.
(b) Micturition
Figure 19-18b
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