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Human Physiology II · Systems Physiology

Tubular Reabsorption and Secretion

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

After filtration, the renal tubule reclaims useful solutes and water () and pumps selected wastes from blood into the tubule (). The performs the bulk, non-regulated reabsorption—about two-thirds of sodium and water and essentially all filtered glucose and amino acids. Reabsorption has a capacity limit (transport maximum); glucose above its spills into urine. The separates water and salt handling, while the distal tubule and collecting duct fine-tune final composition using (sodium and water) and (acid–base).

Why this matters

Glucose in the urine (glucosuria) is a classic sign that plasma glucose has exceeded the renal threshold—used historically to screen for diabetes mellitus and still a principle behind some glucose-lowering drugs that block tubular glucose reabsorption (lowering the effective threshold). Urine dipsticks also detect substances that are normally fully reabsorbed, indicating either overwhelming load or tubular dysfunction. Clinical values, diagnostic thresholds, and drug dosing vary by institution and jurisdiction; these notes support education and do not replace clinical instruction or supervision.

The college version

1. The Proximal Convoluted Tubule: Obligatory Reabsorption

The proximal convoluted tubule (PCT) reclaims about 65% of filtered sodium and water and nearly 100% of filtered glucose and amino acids, plus most filtered bicarbonate. Sodium is reabsorbed by the basolateral Na⁺/K⁺-ATPase, which pumps sodium out of the cell into the blood, keeping intracellular sodium low so filtered sodium enters down its gradient through apical cotransporters—glucose and amino acids ride in with sodium (secondary active transport), and water follows the reabsorbed solutes osmotically (obligatory water reabsorption). Bicarbonate reabsorption is coupled to hydrogen ion secretion: secreted H⁺ combines with filtered HCO₃⁻ to form CO₂ and water, and the CO₂ diffuses into the cell to be regenerated as bicarbonate and returned to blood.

2. Transport Maximum and Renal Threshold

Carrier-mediated reabsorption saturates. The is the maximum rate at which a substance can be reabsorbed—once all carriers are occupied, additional filtered substance is excreted. The renal threshold is the plasma concentration at which a substance first appears in the urine. For glucose, Tm is about 375 mg/min and the renal threshold roughly 180–200 mg/dL: below threshold all glucose is reclaimed; above it, glucose "spills" into urine.

3. Loop of Henle and the Distal Nephron

The loop of Henle has two limbs with opposite permeabilities. The is highly permeable to water but not to solutes, so water leaves and the fluid becomes concentrated. The is impermeable to water but actively reabsorbs sodium, potassium, and chloride (via the Na⁺-K⁺-2Cl⁻ cotransporter in the thick segment), so the fluid becomes dilute. The and early collecting duct reabsorb more sodium and calcium under hormonal control. The late collecting duct has two cell types: principal cells, which reabsorb sodium and secrete potassium and respond to aldosterone and ADH, and intercalated cells, which secrete hydrogen ions (or bicarbonate) to regulate acid–base balance.

How it works

  1. The PCT reclaims the bulk of filtered water, sodium, glucose, amino acids, and bicarbonate.
  2. Carrier proteins reabsorb glucose and amino acids until their Tm is reached.
  3. The loop of Henle makes tubular fluid first concentrated (descending) then dilute (ascending).
  4. The distal tubule and collecting duct adjust sodium, potassium, water, and acid under hormone control.
  5. Whatever remains after reabsorption plus whatever was secreted becomes urine.

Common confusions

Do not confuseWithDifference
ReabsorptionSecretionReabsorption = tubule → blood; secretion = blood → tubule
Obligatory reabsorptionHormone-regulated reabsorptionObligatory (PCT) is automatic bulk; regulated (distal/collecting) is adjustable
Transport maximumRenal thresholdTm is a rate (mg/min); threshold is a plasma concentration
Descending limbAscending limbDescending loses water; ascending loses salt but not water
Principal cellsIntercalated cellsPrincipal = Na⁺/K⁺/water; intercalated = acid–base

Memory aids

"PCT = Packed, Conveyor-belt Takeback." In the Proximal tubule, sodium, water, glucose, amino acids, and bicarbonate are all Picked back up. For the loop: "Down with water, Up with salt" (descending loses water, ascending loses salt).

Quick review

Topic Recap

The proximal tubule does the heavy lifting of reclamation, reabsorbing most filtered sodium, water, glucose, amino acids, and bicarbonate; saturation of its carriers (Tm) explains the renal threshold and glucosuria. The loop of Henle splits water and salt handling, and the distal tubule plus collecting duct—via principal and intercalated cells—finish the job under hormonal and acid–base control. Net excretion is the sum of filtration minus reabsorption plus secretion.

Knowledge Check

  1. Roughly what fraction of filtered sodium and water does the proximal tubule reabsorb?
  2. What is the difference between reabsorption and secretion?
  3. What happens to glucose in urine when plasma glucose exceeds the renal threshold, and why?
  4. Which limb of the loop of Henle is permeable to water but not to solutes?
  5. Which collecting-duct cell type secretes hydrogen ions for acid–base balance?

Answers and Rationales

  1. About 65% (two-thirds)—the PCT performs bulk, .
  2. Reabsorption moves substances from tubule into blood; secretion moves them from blood into tubule.
  3. Glucose appears in the urine because carriers have reached their transport maximum (Tm), so excess filtered glucose cannot be reabsorbed and is excreted.
  4. The descending limb—it lets water leave while trapping solutes, concentrating the fluid.
  5. Intercalated cells—they secrete H⁺ (or HCO₃⁻) to regulate pH.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Filtration dumps a lot of useful stuff into the tubule—water, salt, sugar, and even bicarbonate. The tubule's job is to "grab back" what the body wants and "kick out" extra waste that was missed. The first stretch of tubule (the proximal tubule) is a greedy recycler: it takes back almost everything without asking, because it's always a good idea to reclaim salt, water, and sugar. Farther down, more selective stations decide the final amounts.

This is like a conveyor belt sorting machine that first reclaims every reusable item, then later hand-picks specific ones. It stops being exact because the early reclaiming is not truly "all" of everything—there is a limit (a maximum conveyor speed) past which sugar starts falling off the end into the trash, and later stations actively fine-tune using hormones.

Simple Example

Normally no glucose appears in urine because the proximal tubule reabsorbs every filtered glucose molecule. But if blood glucose rises very high, the reabsorption machinery saturates, and the excess glucose spills into the urine—the sugar you can detect with a urine dipstick.

Worked example

  1. Filtrate enters the PCT; Na⁺/K⁺-ATPase establishes the sodium gradient that drives apical sodium entry.
  2. Sodium cotransport pulls glucose and amino acids across the apical membrane; they exit the basolateral side by facilitated diffusion into the blood.
  3. Water follows the reabsorbed solutes osmotically through aquaporins and between cells, so filtrate volume shrinks while its composition stays roughly isotonic.
  4. In the descending limb, water exits into the hypertonic medulla; in the ascending limb, salt is pumped out while water is trapped, diluting the tubular fluid.
  5. In the late distal tubule and collecting duct, principal cells reabsorb sodium (in exchange for potassium) under aldosterone, and intercalated cells secrete H⁺ or HCO₃⁻ to tune pH; ADH then controls how much water is finally reabsorbed.

The governing concept is mass balance: Excreted = Filtered − Reabsorbed + Secreted. For a freely filtered substance, excretion rate = (GFR × plasma concentration) − reabsorption + secretion. When reabsorption is saturated (Tm reached), any further increase in filtered load passes straight into the urine.

Key takeaways

  • High yield: The PCT reabsorbs ~65% of filtered Na⁺ and water and ~100% of glucose and amino acids.
  • High yield: Glucose's Tm (~375 mg/min) and renal threshold (~180–200 mg/dL) explain glucosuria in uncontrolled hyperglycemia.
  • High yield: Descending limb = water out (concentrating); ascending limb = salt out, water trapped (diluting).
  • Principal cells handle Na⁺ (reabsorb) and K⁺ (secrete) under aldosterone; intercalated cells handle H⁺/HCO₃⁻.
  • Reabsorption = tubule → blood; secretion = blood → tubule (opposite directions).
  • Excreted = Filtered − Reabsorbed + Secreted.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Distinguish tubular reabsorption from tubular secretion and state where most reabsorption occurs.
  • Describe the proximal convoluted tubule's bulk ("obligatory") reabsorption of sodium, water, glucose, amino acids, and bicarbonate.
  • Define transport maximum (Tm) and renal threshold, and use glucose to explain when a substance appears in urine.
  • Contrast the descending and ascending limbs of the loop of Henle and the roles of principal and intercalated cells in the late collecting duct.

Key vocabulary

Reabsorption
Movement of solutes/water from tubule back to blood
Secretion
Movement of solutes from blood into tubule
Obligatory reabsorption
Bulk, non-regulated reclaiming in the PCT
Transport maximum (Tm)
Maximum reabsorption rate of a carrier system
Renal threshold
Plasma concentration where a solute appears in urine
Proximal convoluted tubule
First coiled segment after Bowman's capsule
Loop of Henle
Hairpin segment with descending and ascending limbs
Descending limb
Water-permeable, solute-impermeable limb
Ascending limb
Water-impermeable, salt-reabsorbing limb
Distal convoluted tubule
Segment after the loop
Principal cells
Collecting-duct cells for Na⁺/K⁺ and water
Intercalated cells
Collecting-duct cells for H⁺/HCO₃⁻

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