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# What the kidneys do: functions and the three basic processes

- Concept
- Exam priority: medium

The kidneys clean the blood, keep its water, salts and acidity steady, help control blood pressure and make hormones.

Updated Nov 13, 2026

Also called: Renal functions, Filtration, reabsorption and secretion

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Vessels inside the kidney](https://durgical.com/en/anatomy/intrarenal-arteries-and-veins/)

## Key points

- Excretion = filtration − reabsorption + secretion.
- Four handling patterns: filtered only (creatinine, inulin), filtered and partly reabsorbed (Na⁺, Cl⁻, water), filtered and fully reabsorbed (glucose, amino acids), filtered and secreted (PAH, many organic acids and drugs).
- GFR is ≈ 180 L/day, so the whole plasma volume (≈ 3 L) is filtered about 60 times a day.
- A huge filtration with selective reabsorption lets the kidneys clear wastes fast and adjust each solute independently.
- Endocrine roles: erythropoietin (anemia in CKD), 1α-hydroxylation of vitamin D (calcitriol), renin; the kidney also makes glucose during long fasting.
- Failing kidneys cause fluid overload, hyperkalemia, acidosis, anemia, bone disease and hypertension: each maps to a lost function.

## What it is

The kidneys do far more than make urine. They excrete wastes and foreign chemicals, match the output of water and each electrolyte to intake, regulate blood pressure and acid–base balance, and act as endocrine organs (erythropoietin, active vitamin D, renin). Every substance in urine is the result of three processes: glomerular filtration, tubular reabsorption and tubular secretion.

## Key facts

- Wastes cleared: urea (from protein), creatinine (from muscle), uric acid (from purines), bilirubin end products, hormone metabolites, and many drugs and toxins.
- Balance: urine output of water, Na⁺, K⁺, Cl⁻, Ca²⁺, phosphate, Mg²⁺ and H⁺ is adjusted to intake within days.
- Blood pressure: long-term by excreting sodium and water (pressure natriuresis); short-term by releasing renin.
- Acid–base: excretes the daily fixed acid load and regenerates bicarbonate; the only route for sulfuric and phosphoric acids.
- Gluconeogenesis: in prolonged fasting the kidneys make a share of the body's glucose, close to that of the liver.

### The nephron: segments, transporters and diuretics

**GFR** ≈ 125 mL/min **Filtrate** ≈ 180 L/day **Urine** ≈ 1–2 L/day

Tap a nephron segment: what it reabsorbs, its transporters, the hormones, the diuretics that act there and the classic disorders.

- 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

**Urinary excretion**Excretion = Filtration − Reabsorption + Secretion

**Filtration**
GFR × plasma concentration (filtered load)

Applies to every freely filtered substance.

## Reference values (healthy young adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Renal blood flow | ≈ 1.1–1.2 L/min | ≈ 20–25% of cardiac output |
| GFR | ≈ 125 mL/min (≈ 180 L/day) | falls with age |
| Filtration fraction | ≈ 0.2 | GFR ÷ renal plasma flow |
| Urine volume | ≈ 1–2 L/day | |
| Nephrons per kidney | ≈ 0.8–1.2 million | new nephrons are not formed after birth |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## The four ways the kidney handles a filtered substance

| Pattern | Example | Clearance vs GFR |
| --- | --- | --- |
| Filtered, not reabsorbed or secreted | Inulin; creatinine (almost) | Equal to GFR |
| Filtered, partly reabsorbed | Na⁺, Cl⁻, water, urea | Less than GFR |
| Filtered, fully reabsorbed (normally) | Glucose, amino acids, HCO₃⁻ | ≈ 0 |
| Filtered and secreted | PAH, penicillin, many organic acids | More than GFR (PAH ≈ renal plasma flow) |

## Clinical notes

- CKD shows each lost function: oedema and hypertension (salt and water), hyperkalemia, metabolic acidosis, anemia (low EPO), hypocalcemia with hyperphosphatemia and bone disease (low calcitriol).
- Drugs cleared by filtration or secretion accumulate when GFR falls, which is why many are adjusted to the estimated GFR.

## Memory aid

The kidney is a sieve that throws everything out, then a picker that takes back what it wants, then a pusher that adds what must go.

## Test yourself

Think of the answer, then tap the question to see it.

## A substance has a clearance higher than the GFR. What does this say about how the kidney handles it?

It is both filtered and secreted by the tubules (net secretion); PAH is the classic example, cleared almost completely in one pass.

## Why is a GFR of about 180 L/day an advantage, when most of the filtrate is reabsorbed anyway?

Filtering the whole plasma many times a day removes wastes that are poorly reabsorbed quickly, and lets each needed solute be reclaimed and adjusted separately.

## Which three endocrine or metabolic functions of the kidney fail in chronic kidney disease?

Erythropoietin production (anemia), 1α-hydroxylation of vitamin D (low calcitriol, hypocalcemia, bone disease), and renin release; also gluconeogenesis in fasting.

## Glucose is freely filtered but is normally absent from urine. Which handling pattern is this?

Filtered and completely reabsorbed (by SGLT2 and SGLT1 in the proximal tubule), until the transport maximum is exceeded.

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## Connections

## See it in 3D 2

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

- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Vessels inside the kidney](https://durgical.com/en/anatomy/intrarenal-arteries-and-veins/)

## Related topics 5

- [Functional anatomy of the nephron](https://durgical.com/en/physiology/functional-anatomy-of-the-nephron/)
- [Renal clearance](https://durgical.com/en/physiology/renal-clearance/)
- [Glomerular filtration: the barrier and its determinants](https://durgical.com/en/physiology/glomerular-filtration-the-barrier-and-its-determinants/)
- [Erythropoiesis and erythropoietin](https://durgical.com/en/physiology/erythropoiesis-and-erythropoietin/)
- [Calcium regulation: PTH, vitamin D and calcitonin](https://durgical.com/en/physiology/calcium-regulation-pth-vitamin-d-and-calcitonin/)

## Related conditions 2

- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)

## In Biochemistry 2

- [Creatinine and estimated glomerular filtration rate (eGFR)](https://durgical.com/en/biochemistry/topics/creatinine-and-estimated-glomerular-filtration-rate/)
- [Urinalysis](https://durgical.com/en/biochemistry/topics/urinalysis/)

[← 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); [Anatomy, Abdomen and Pelvis: Kidneys](https://www.ncbi.nlm.nih.gov/books/NBK482385/) (StatPearls (NCBI Bookshelf), 2025); [Chronic Kidney Disease](https://www.ncbi.nlm.nih.gov/books/NBK535404/) (StatPearls (NCBI Bookshelf), 2025).

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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/what-the-kidneys-do-functions-and-the-three-basic-processes/