Topic 43 - Transport processes in the proximal tubule Flashcards

1
Q

Words to include

A
  • Not permable for:
    • Creatinine
    • Urea
    • HCO3-
    • Proteins
  • Reabsorption
    • Actively to interstitium
    • Passively to the peritubular capillaries
  • Lumen
  • ISF
  • Sodium (Na+)
    • Na+/K+-ATPase pump
      • 3Na out/2K in
  • Hydrogen (H+)
    • Na+ entry = H+ secretion
    • Amiloride
    • Carbonic anhydrase
  • Bicarbonate (HCO3-)
    • Impermable cells for HCO3-
      • Indirect transport of HCO3-
    • CO2 diffusing into cell
      • H+
      • HCO3-
      • IC carbonic anhydrase
  • Chloride (Cl-)
    • Luminal membrane
      • Chloride-acidic anion antiporter protein
        • Cl- → cell
        • Acidic anion → lumen
          • Binds H+
          • Free acid → diffuses back to cell
  • Water (H2O)
    • Type 1:
      • Increased peritubular oncotic pressure
      • Paracellulary migrates
        • Lumen → interstitium
    • Type 2:
      • Aquaphorin-1
  • Glucose, amino acids
    • Secondary acitve symport
      • Na+/K+-ATPase pump
      • Glucose + Na+
      • Amino acid + Na+
    • Specific carrier
  • Urea
    • Passively reabsorbed through cells
    • Paracellular pathways
    • Stays in interstitium
      • Formation of osmotic layering
  • Proteins
    • Lume
    • Back to tubular cells by pinocytosis
      • From tubular cells as amino acids to interstitium
  • Organic anions and cations
    • Secretion
      • Na+/K+-ATPase pump
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2
Q

Topics to include in the essay

A
  1. Characteristic of the proximal tubule
  2. Sodium
  3. Hydrogen
  4. Bicarbonate ion
  5. Chloride
  6. Water
  7. Glucose & amino acids
  8. Urea
  9. Proteins
  10. Organic anions & cations
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3
Q

Give the general characteristics of the transport processes in the proximal tubule

Give the substances transported in the proximal tubule

A
  • High transport capacity
  • No high gradient will be created
  • 2/3 of water
  • 95% of the filtered bicarbonate reabsorbed
  • 70% of the filtrate is reabsorbed in the proximal tubule
  • Hormonal regulation is not significant
  • The wall of the tubule is not permeable for:
    • Creatinine
    • Urea
    • HCO3-
    • Proteins
  • Reabsorption takes place in two phases:
    1. Actively to the intersitium
    2. Passively to the peritubular capillaries
  • Substances transported in the proximal tubule:
    1. Sodium (Na+)
    2. Hydrogen (H+)
    3. Bicarbonate ion (HCO3-)
    4. Chloride (Cl-)
    5. Water (H2O)
    6. Glucose
    7. Amino acids
    8. Urea
    9. Proteins
    10. Organic cations & anions
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4
Q

Transport of sodium in the proximal tubule

A
  • Main motor: Na+/K+-ATPase pump
    • ​3 Na+ out / 2 K+ in
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5
Q

Transport of hyrdogen in the proximal tubule

A
  • Na+ entry = H+ secretion
  • Inhibited by amilloride
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6
Q

Transport of bicarbonate ion in the proximal tubule

A
  • The cell is impermable for HCO3-
    • Indirect HCO3- transport
  • CO2 diffusing into the cell transforms into H+ and HCO3- with the help of IC carbonic anhydrase
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7
Q

Transport of chloride in the proximal tubule

A
  1. The prerequisite for the transcellular chloride transport is the lower pH in the lumen than in the cell
  2. Then the chloride-acidic anion antiporter protein of the luminal membrane transports chloride to the cell, and acidic anion from the cell to the lumen
  3. In the lumen the acidic anion binds H+ and as free acid easily diffuses back to the cell
  4. Here it dissociated and another chloride transport can be carried out
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8
Q

Transport of water in the proximal tubule

A
  • Water can be transportet from the lume to the ISF in two different ways:
    1. Increased peritubular oncotic pressure makes the water paracellularly migrate from the lume → interstitium
    2. Facilitated by aquaporin-1, a special water channel
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9
Q

Transport of glucose & amino acids in the proximal tubule

A
  • Glucose and amino acids are 100% withdrawn from the proximal tubule together with Na+, via secondary active symport maintained by the Na+/K+-ATPase pump
  • Glucose and amino acids have their own specific carriers
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10
Q

Transport of urea in the proximal tubule

A
  • Half of the urea are either:
    • Passively resorbed through the cells
    • Paracellular pathways
  • Rest of the urea stays in the interstitium
    • Contribues to the formation of osmotic layering in the kidney
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11
Q

Transport of proteins in the proximal tubule

A
  • Some proteins can get into the lumen
    • The majority of these get back to the tubular cells by pinocytosis, then form here as amino acids to the interstitium
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12
Q

Transport of organic anions & cations in the proximal tubule

A
  • The removal of organic ions are not yet understood propperly
  • The basis of the secretion is presence of normal Na+/K+-ATPase pump
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