Tubular fluid-to-plasma ratios along the nephron
- Measurement
- Exam priority: medium
Comparing a solute's level in the tubule with its level in plasma shows, segment by segment, what the nephron removes or adds.
Also called: TF/P ratio, TF/P inulin
See it in 3D: Kidneys
What it is
The TF/P ratio is the concentration of a solute in tubular fluid divided by its plasma concentration. Because filtrate starts with TF/P ≈ 1 for small solutes, any change shows net reabsorption of the solute relative to water, or secretion. Inulin, which is neither reabsorbed nor secreted, rises only as water is removed, so TF/P inulin measures water reabsorption and is the reference for the other curves.
How it works, step by step
- Filtrate enters Bowman's space with the same small-solute concentrations as plasma (TF/P ≈ 1).
- Early PT: Na⁺ enters with glucose, amino acids and via NHE3; HCO₃⁻ is reclaimed, so these solutes fall fast.
- Water follows by osmosis through aquaporin-1 and leaky tight junctions; small gradients are enough because permeability is high.
- Water moving through junctions drags dissolved solutes with it (solvent drag), adding to K⁺, Ca²⁺ and Cl⁻ reabsorption.
- Late PT: luminal Cl⁻ is now higher than plasma; Cl⁻ diffuses out paracellularly, pulling Na⁺ and creating a lumen-positive voltage.
- Descending limb removes water (osmolality rises); thick ascending limb removes salt without water, so DCT fluid is hypo-osmotic.
- Collecting duct: tight epithelium; final concentration depends on ADH and the medullary gradient.
The nephron: segments, transporters and diuretics
Na⁺ reabsorbed ≈ 65–70%Water reabsorbed ≈ 65–70%
Proximal tubule
- What it does: Reabsorbs ≈ 65–70% of filtered Na⁺ and water (isosmotically), nearly all glucose and amino acids, ≈ 80% of HCO₃⁻ and most K⁺ and phosphate. Secretes organic acids and bases (many drugs) and makes NH₄⁺ from glutamine.
- Transporters: NHE3 (Na⁺/H⁺) with carbonic anhydrase; SGLT2 (early segment, ≈ 90% of glucose) and SGLT1; NaPi-2a (phosphate); basolateral Na⁺/K⁺-ATPase.
- Hormones: Ang II stimulates NHE3 (more Na⁺ and HCO₃⁻: contraction alkalosis); PTH inhibits NaPi-2a (phosphaturia) and activates 1α-hydroxylase (active vitamin D).
- Drugs: Carbonic anhydrase inhibitors (1), SGLT2 inhibitors (2) and osmotic diuretics (3, with the thin descending limb).
- Disorders: Fanconi syndrome (loses glucose, amino acids, phosphate and HCO₃⁻); type 2 (proximal) renal tubular acidosis; familial renal glucosuria (SGLT2).
- Glomerulus and arterioles
- Proximal tubule
- Thin descending limb
- Thin ascending limb
- Thick ascending limb
- Distal convoluted tubule
- Collecting tubule and duct
- Juxtaglomerular apparatus (macula densa)
- Cortex
- Medulla
- Interstitial osmolality (≈ 300 → 1200 mOsm/kg)
Equations
- (TF/P)inulin
- tubular fluid / plasma inulin concentration
TF/P inulin = 3 means one third of filtered water remains.
- x
- the solute studied
Corrects the solute ratio for water removal.
- Px
- free plasma concentration of x
Na⁺: 180 L/day × 142 mEq/L ≈ 25 000 mEq/day.
Osmolality of tubular fluid (mOsm/kg)
| Parameter | Value | Note |
|---|---|---|
| Bowman's space and end of proximal tubule | ≈ 300 | isosmotic |
| Tip of long loop | ≈ 1200 | juxtamedullary nephrons |
| Early distal tubule | ≈ 100 | diluting segment |
| Final urine with ADH | up to ≈ 1200 | |
| Final urine without ADH | ≈ 50 |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Typical TF/P at the end of the proximal tubule
| Substance | TF/P | Meaning |
|---|---|---|
| Inulin | ≈ 3 | two thirds of water reabsorbed |
| Na⁺, osmolality | ≈ 1 | reabsorbed in proportion to water (isosmotic) |
| Cl⁻ | ≈ 1.2–1.4 | lags early, then reabsorbed late (paracellular) |
| Urea | ≈ 1.5 | about half reabsorbed, less than water |
| HCO₃⁻ | ≈ 0.2–0.3 | most reclaimed early |
| Glucose, amino acids | ≈ 0 | almost all reabsorbed in the early segment |
| PAH | > 3 | secreted |
Clinical notes
- Proximal tubular diseases (Fanconi syndrome) show TF/P glucose, phosphate and amino acids staying high: these solutes appear in urine.
- Leaky proximal versus tight distal epithelia explain why proximal diuretics are weak (later segments recover the salt) while collecting-duct drugs act on a small but final fraction.
Test yourself
Think of the answer, then tap the question to see it.
At the end of the proximal tubule TF/P inulin is 3 and TF/P Na⁺ is 1. What fraction of filtered Na⁺ has been reabsorbed?
Fraction remaining = 1 ÷ 3 ≈ 33%, so about two thirds (≈ 67%) of the filtered Na⁺ has been reabsorbed, with water in proportion.
Why does TF/P osmolality stay near 1 along the proximal tubule though most of the fluid is removed?
The epithelium is very permeable to water, so water follows solute almost instantly; reabsorption is isosmotic.
Which solute's TF/P rises above that of inulin along the proximal tubule, and why?
PAH: it is secreted into the lumen, so its concentration rises faster than water removal alone would explain.
What is meant by gradient–time transport, and which solute is the classic example?
Reabsorption that has no transport maximum and depends on the gradient, the epithelial leakiness and contact time; Na⁺ in the proximal tubule.
Connections
See it in 3D1
The structures of Durgical's 3D anatomy where it happens.
In Biochemistry1
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Physiology, Renal (StatPearls (NCBI Bookshelf), 2025); Osmolality Tests: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025); Fanconi syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.