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### Normal Glucose Homeostasis- Normal blood g ...
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Flashcards in this deck (11)

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  • What is the normal blood glucose range?

    • 60–120 mg/dL
    physiology glucose

  • What terms describe blood glucose above and below the normal range?

    • Above 120 mg/dL: hyperglycemia
    • Below 60 mg/dL: hypoglycemia
    glucose definitions

  • Where does glucose filtration begin in the nephron?

    • Bowman's capsule (glomerulus → Bowman's space)
    renal filtration

  • Where does the kidney primarily reabsorb filtered glucose?

    • Proximal convoluted tubule (PCT)
    renal reabsorption

  • Which transporter facilitates glucose reabsorption in the PCT and how?

    • Sodium-glucose co-transporter 2 (SGLT2): transports sodium and glucose from tubular lumen back into the bloodstream
    transporters sglt2

  • What is the normal rate of renal glucose secretion?

    • 0 mL/min (no active glucose secretion under normal conditions)
    renal secretion

  • What happens to filtered glucose if it is not reabsorbed in the nephron?

    • It passes through the collecting duct and is excreted in urine (glucosuria)
    glucosuria excretion

  • What is the renal threshold for glucose and what does it mean?

    • ~200 mg/dL: plasma concentration above which glucose is no longer fully reabsorbed and urinary excretion begins
    renal threshold

  • What is the tubular transport maximum for glucose and its significance?

    • \(T_m \approx 375\;\text{mg/min}\): maximum PCT reabsorption capacity; once reached transporters are saturated and excess glucose is excreted
    tm transport

  • What causes the splay phenomenon in glucose clearance curves?

    • Heterogeneity among nephrons: individual nephrons reach their \(T_m\) at different filtered loads, producing a gradual transition from complete reabsorption to excretion
    splay nephrons

  • How does pregnancy affect renal glucose handling?

    • Increased blood volume and GFR → increased glucose filtration; can cause glucosuria without hyperglycemia
    pregnancy renal
Study Notes

Overview

  • Concise summary: how the kidney filters and reabsorbs glucose, key limits (renal threshold, \(T_m\)), the splay phenomenon, and changes in pregnancy.

Normal glucose homeostasis

  • Normal plasma glucose: \(60\)–\(120\ \text{mg/dL}\).
  • Definitions: Hyperglycemia > upper range; hypoglycemia < lower range.
  • Glucose is essential for energy and must be kept within a narrow range.

Where filtration and reabsorption happen

  • Filtration: begins at Bowman's capsule; all plasma glucose is freely filtered at the glomerulus.
  • Reabsorption site: mainly the proximal convoluted tubule (PCT).
  • Transporter: sodium–glucose cotransporter SGLT2 moves glucose back into the blood together with Na+.
  • Under normal conditions, the kidney does not secrete glucose; urine glucose is essentially zero.

Key quantitative concepts

  • Filtered load (amount filtered per minute):

\(\(\text{Filtered load} = GFR \times [Glucose]_{plasma}\)\)

  • Units must match so result is in mg/min (convert mL to dL when needed).

  • Example: if \(GFR=125\ \text{mL/min}\) and \([Glucose]=100\ \text{mg/dL}\) then

    \[\text{Filtered load} = 125\ \text{mL/min} \times 100\ \text{mg/dL} \times \frac{1\ \text{dL}}{100\ \text{mL}} = 125\ \text{mg/min}.\]

  • Renal threshold: plasma concentration at which glucose begins to appear in urine, approximately \(\approx 200\ \text{mg/dL}\).

  • Tubular transport maximum (\(T_m\)): maximum reabsorptive capacity, approximately \(T_m \approx 375\ \text{mg/min}\).
  • Below the threshold, nearly all filtered glucose is reabsorbed.
  • When filtered load exceeds \(T_m\), excess glucose is excreted.

The splay phenomenon

  • The shift from complete reabsorption to excretion is gradual, not abrupt.
  • Caused by nephron heterogeneity: different nephrons saturate at slightly different loads.
  • On clearance curves this creates a rounded transition (splay) between full reabsorption and saturation.

Effects of pregnancy

  • Pregnancy increases plasma volume and GFR.
  • Higher GFR raises the filtered load of glucose even when plasma glucose is normal.
  • Result: glucosuria (glucose in urine) can occur with normal plasma glucose — a common physiologic finding in pregnancy.

Clinical implications & quick checks

  • Finding glucose in urine usually suggests plasma glucose above the renal threshold, except in pregnancy or when GFR is increased.
  • Calculating filtered load helps predict whether reabsorption capacity will be exceeded.
  • Brief note: drugs that inhibit SGLT2 (not detailed here) intentionally increase urinary glucose by lowering reabsorption.

Quick reference (values)

  • Normal plasma glucose: \(60\)–\(120\ \text{mg/dL}\)
  • Renal threshold: \(\approx 200\ \text{mg/dL}\)
  • Tubular transport maximum: \(T_m \approx 375\ \text{mg/min}\)

Study tips

  • Memorize the normal range, renal threshold, and \(T_m\) values.
  • Practice filtered load calculations using different GFR and glucose values.
  • Visualize the clearance curve: flat (no excretion) → splay (gradual rise) → steep (excretion increases after \(T_m\)).