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# 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.

Updated Nov 13, 2026

Also called: TF/P ratio, TF/P inulin

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- TF/P inulin ≈ 3 at the end of the proximal tubule → two thirds of filtered water reabsorbed; it rises to > 100 in final urine.
- Proximal tubule is isosmotic: TF/P Na⁺ and TF/P osmolality stay ≈ 1 while two thirds of the volume is reabsorbed.
- Glucose, amino acids and HCO₃⁻ fall early (TF/P ≪ 1); Cl⁻ rises slightly above 1 in the late proximal tubule; PAH and urea rise.
- (TF/P)x ÷ (TF/P)inulin = fraction of the filtered load of x still in the tubule at that point.
- Na⁺ reabsorption is gradient–time limited (no Tm): the fraction taken depends on how long fluid stays and how leaky the epithelium is.
- Osmolality: ≈ 300 (PT) → up to ≈ 1200 at the hairpin (juxtamedullary) → ≈ 100 in early DCT → 50–1200 in urine, set by ADH.

## 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)

Illustrative schematic drawn by Durgical (a long-loop juxtamedullary nephron); proportions and percentages are approximate and not for clinical decisions.

## Equations

**Fraction of filtered water remaining**1 ÷ (TF/P)inulin

**(TF/P) inulin**
tubular fluid / plasma inulin concentration

TF/P inulin = 3 means one third of filtered water remains.

**Fraction of filtered load remaining**(TF/P)x ÷ (TF/P)inulin

**x**
the solute studied

Corrects the solute ratio for water removal.

**Filtered load**Filtered load = GFR × Px

**P x**
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.

## Memory aid

Inulin is the ruler: divide by it to know how much of x is still on the train.

## 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.

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## Connections

## See it in 3D 1

The structures of Durgical's 3D anatomy where it happens.

- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related topics 5

- [Proximal tubule transport](https://durgical.com/en/physiology/proximal-tubule-transport/)
- [Glucose transport maximum and renal threshold](https://durgical.com/en/physiology/glucose-transport-maximum-and-renal-threshold/)
- [Renal clearance](https://durgical.com/en/physiology/renal-clearance/)
- [Transport in the loop of Henle](https://durgical.com/en/physiology/transport-in-the-loop-of-henle/)
- [The countercurrent mechanism, urea recycling and urine concentration](https://durgical.com/en/physiology/the-countercurrent-mechanism-urea-recycling-and-urine-concentration/)

## In Biochemistry 1

- [Tests of renal tubular function](https://durgical.com/en/biochemistry/topics/tests-of-renal-tubular-function/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://www.ncbi.nlm.nih.gov/books/NBK538339/) (StatPearls (NCBI Bookshelf), 2025); [Osmolality Tests: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/osmolality-tests/) (MedlinePlus, US National Library of Medicine, 2025); [Fanconi syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000333.htm) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/tubular-fluid-to-plasma-ratios-along-the-nephron/