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 \(T_m\)).