Lecture 3: Glomerular and tubular function 1 Flashcards

1
Q

What occurs in the capillaries of the glomerulus?

A
  • 20% of plasma volume is filtered (about 125ml/min for both kidneys)
  • GHP about 60mmHg, capsular oncotic pressure about 32mmHg, Caspsular hydrostatic pressure about 18mmHg
  • It is located between 2 arterioles
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2
Q

How does GFR alter with blood pressure?

A
  • As mean arterial pressure varies within relastic normals GFR stays constant due to renal Autoregulation
  • At very low MABP (<80) it begins to decline and at very high MABP (>160) it begins to increase
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3
Q

How does GFR alter with constriction and dilation of afferent and efferent arterioles?

A
  • Increase GFR
    • Constriction of efferent
    • Dilation of Afferent
  • Decreased GFR
    • Constriction of afferent
    • Not sure there is a way to dilate the efferent
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4
Q

What are the mechanisms of renal autoregulation?

A
  • Extrinsic methods:
    • Renin-angiotensin II: Constriction of efferent arteriole (inc. GFR)
    • ANP and BNP: Dilation of afferent arteriole (inc. GFR)
    • Sympathetic nervous system: Constriction of afferent (dec. GFR)
  • Intrinsic methods:
    • Myogenic: Increasing arterial pressure stretches the afferent arteriole inducing it to constrict which offsets pressure increase and keeps GFR stable
    • Tubuloglomerular feedback: Macula densa cells detect NaCl levels in DCT, if high they signal to afferent to constrict
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5
Q

How does the tubuloglomerular feedback mechanism work?

A
  • High GFR –> more NaCl passes macula densa cells –> paracrine signals –> afferent arteriole contriction –> decreased GFR
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6
Q

How does the RAAS work?

A
  • Low GFR –> less NaCl passes macula densa cells –> paracrine signals released –> JG cells release Renin which converts Angiotensinogen (from liver) –> Angiotensin I which –> angiotensin II (by ACE):
    • –> constriction of efferent arteriole
    • –> Increased aldosterone –> increased Na+ uptake from distal nephron –> increased BV
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7
Q

Overall what happens in the proximal tubule?

A
  • Major site of filtarte reabsorption:
    • 66% of water and inorganic ions reabsorbed
    • 100% of glucose and A.A’s absorbed
    • 90% of bicarbonate reabsorbed
  • pH drops to 6.7 from 7.4
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8
Q

Describe the process of active transport in the early proximal tubule

A
  • Na/K pump on basal membrane pumps out Na+, which down its conc. gradient diffuses into the peritubular capillaries
  • On the apical surface (lumenal surface) Na+ diffuses down its concentration gradient as it is in high concentration in the lumen, but low conc in the cell due to the Na/K pump.
  • As Na+ moves into the cell it co-transports other solutes in such as glucose
  • These diffuse down their gradients out of the basal surface and into the blood
  • Water follows the Na+ paracellularly via osmosis through leaky tight junctions
    • Some solvents follow via this pathway which is solvent drag
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9
Q

How does bicarbonate reabsorption occur in the proximal tubule?

A
  • HCO3- cant diffuse accross the apical membrane hence:
  • HCO3- + H+ ⇔ H2CO3 ⇔ (conversion done by carbonic anhydrase) H20 + CO2
  • CO2 can diffuse in freely, where in the cell it is converted back into H2CO3 and then into HCO3- which diffuses into the blood, and H+ which is pumped back into the lumen in exchange for a Na+ (NHE)
  • 90% of filtered HCO3- is reabsorbed
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10
Q

How does the proximal tubule generate new bicarbonate?

How will an increase in ECF [H+] effect this?

A
  • Metabolises glutamine to ammonium ion and bicarbonate
  • Ammonium is secreted into the lumen by a Na+/NH4+ exchanger
  • Bicarbonate is transported into the blood
  • Increase in ECF [H+] –> increased glutamine metabolism
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11
Q

Why cant H+ be secreted into the lumen to maintain pH?

A
  • All H+ excreted by NHE used to make bicarbonate
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12
Q

What is fanconi syndrome?

A
  • Impaired ability for PCT to reabsorb HCO3-, Pi, amino acids, glucose, and low-MW proteins resulting in increased urinary excretion
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13
Q

What occurs in the late proximal tubule?

A
  • Chloride becomes concentrated in lumen –> paracellular movement –> blood
  • This causes lumen to become electropositive which induces Na+ to move paracellularly into blood
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14
Q
A
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15
Q

What is secreted in the proximal tubule?

A
  • Organic anions and organic cations
  • Anions move out through antiporter in exchange for Cl-, urate or OH-
  • Cations move out through antiporter with H+
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