Download Reabsorption

Filtration and Reabsorption of Some Substances:
Substance
Water
Sodium
Glucose
Urea
Amount filtered/day
180 L
630 g
180 g
54 g
Amount excreted/day
1.8 L
3.2 g
0g
30 g
% reabsorbed
99%
99.5%
100%
44%
Note that most of the water and sodium, and all of the glucose, is reabsorbed.
Flow of Filtrate in Regions of the Kidney, and % Reabsorption by Each Region:
Region
Renal corpuscle/ glomerular capsule (GFR)
Proximal convoluted tubule (PCT)
End of PCT
Loop of Henle
End of Loop of Henle
Distal convoluted tubule (DCT)
End of DCT
Collecting ducts (CT)
End of CT
Passing into urine
Flow (ml/min)
125 (= GFR)
125
45
45
25
25
12
12
1
1
% reabsorption
65%
15%
10%
9.3%
= 0.7% of GFR
The majority of the reabsorption occurs in the PCT. Also note than only 0.7% of the
volume remains and forms the urine.
Extra info…
Functions of the Kidneys:
1. Maintaining H2O balance in the body.
2. Regulating quantity and concentration of most ECF ions, including Na+, Cl-,
K+, HCO3-, Ca2+, Mg2+, SO42-, PO43-, and H+. Even minor fluctuations in the ECF
concentrations of some of these electrolytes can have profound influences. For
example, changes in the ECF concentration of K+ can potentially lead to fatal
cardiac dysfunction.
3. Maintaining proper plasma volume, thereby contributing significantly to the
long-term regulation of arterial blood pressure. This function is accomplished
through the kidneys’ regulatory role in salt and water balance.
4. Helping maintain the proper acid-base balance of the body by adjusting
urinary output of H+ and HCO3-.
5. Maintaining the proper osmolarity (concentration of solutes) of body fluids,
primarily through regulation of H2O balance.
6. Excreting (eliminating) the end products (wastes) of bodily metabolism such
as urea, uric acid, and creatinine. If allowed to accumulate, these wastes are toxic,
especially to the brain.
7. Excreting many foreign compounds such as drugs, food additives, pesticides,
and other exogenous non-nutritive materials that have gained entrance to the body.
8. Secreting erythropoietin, a hormone that stimulates RBC production.
9. Secreting renin, an enzymatic hormone that triggers a chain reaction important in
the process of salt conservation by the kidneys.
10. Converting vitamin D into its active form (second hydroxylation).
Summary of Transport across Proximal and Distal Portions of the Nephron:
Proximal Tubule:
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Reabsorption
67% of filtered Na+ actively
reabsorbed
Not subject to control
Cl- follows passively
All filtered glucose and amino
acids reabsorbed by secondary
active transport
Not subject to control
Variable amounts of filtered
PO43- and other electrolytes
reabsorbed
Subject to control
65% of filtered H2O
osmotically reabsorbed
Not subject to control
50% of filtered urea passively
reabsorbed
Not subject to control
All filtered K+ reabsorbed
Not subject to control
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Secretion
Variable H+ secretion,
depending on acid-base
status of body
Organic-ion secretion
Not subject to control
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Distal Tubule and Collecting Duct:
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Reabsorption
Variable Na+ reabsorption
Controlled by aldosterone
Cl- follows passively
Variable H2O reabsorption
Controlled by ADH
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Secretion
Variable H+ secretion,
depending on acid-base
status of body
Variable K+ secretion
Controlled by aldosterone
Handling of Sodium and Water by Various Tubular Segments of the Nephron:
Tubular
segment
Proximal
tubule
%
67
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Loop of
Henle
25
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Distal and
Collecting
tubules
8
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Na+ reabsorption
Distinguishing features
Active
Uncontrolled
Plays a pivotal role in the
reabsorption of glucose, amino
acids, Cl-, H2O and urea
Active
Uncontrolled
Na+ along with Clreabsorption from the
ascending limb helps establish
the medullary interstitial
vertical osmotic gradient,
which is important in the
kidneys’ ability to produce
urine of varying concentrations
and volumes, depending on the
body’s needs
Active
Variable and subject to
aldosterone control
Important in the regulation of
ECF volume
Linked to K+ secretion and H+
secretion
%
65
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H2O reabsorption
Distinguishing features
Passive
Obligatory osmotic reabsorption
following active Na+ reabsorption
15
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Passive
Obligatory osmotic reabsorption from
the descending limb as the ascending
limb extrudes NaCl into the
interstitial fluid (that is, reabsorbs
NaCl)
20
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Passive
Not linked to solute reabsorption
Variable quantities of ‘free’ H2O
reabsorption subject to ADH control
Driving force is the vertical osmotic
gradient in the medullary interstitial
fluid established by the long loops of
Henle
Important in the regulation of ECF
osmolarity
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Reabsorption Capabilities of Different Segments of Renal Tubules/Collecting
Ducts:
Tubule segment
Proximal Convoluted
Tubule
Substance Reabsorbed
Na+
Virtually all nutrients
(glucose, amino acids,
vitamins)
Cations (K+, Mg2+, Ca2+,
others)
Anions (Cl-, HCO3-)
Water
Urea and lipid-soluble
solutes
Small proteins
Loop of Henle
Descending limb
Ascending limb
Water
Na+, Cl-, K+
Ca2+, Mg2+
Distal convoluted
tubule
Na+
Ca2+
Cl-
Water
Collecting duct
Na+, H+, K+, HCO3-, and Cl-
Water
Urea
Mechanism
Primary active transport via ATP-dependent Na+K+ carrier; sets up electrochemical gradient for
passive solute diffusion, osmosis, and secondary
active transport (cotransport) with Na +
Active transport; cotransport with Na +.
Active transport; cotransport with Na + for most;
K+ mainly by the paracellular route
Passive transport; paracellular diffusion driven by
electrochemical gradient for Cl-; active transport
(cotransport with Na+ for HCO3-)
Osmosis, driven by solute reabsorption
(obligatory)
Passive diffusion driven by the electrochemical
gradient created by osmotic movement of water
Endocytosed by tubule cells and digested to amino
acids within tubular cells.
Osmosis
Active transport of Cl-, Na+, and K+ via a Na+-K+2Cl- cotransport in thick portion; also paracellular
transport; Na+-H+ antitransport.
Passive transport driven by electrochemical
gradient; paracellular route
Primary active transport; requires aldosterone
PTH-mediated primary active transport via ATPdependent Ca2+ carrier
Diffusion; follow electrochemical gradient created
by active reabsorption of Na+; also cotransport
with Na+
Osmosis; facultative water reabsorption; depends
on ADH to increase porosity of tubule epithelium
in most distal portion
Aldosterone-mediated primary active transport of
Na+ and the medullary gradient to create the
conditions for passive transport of some HCO3and Cl-, and cotransport of Na+, H+, K+, HCO3-,
and Cl-.
Osmosis; facultative water reabsorption; depends
on ADH to increase porosity of tubule epithelium
Facilitated diffusion in response to concentration
gradient in the deep medulla region; most remains
in medullary interstitial space