What it does
Each kidney holds about a million tiny tubes called nephrons. Every nephron does two jobs. First, filtration: at the glomerulus, a knot of capillaries, blood pressure pushes water and small molecules out of the blood into a cup called Bowman’s capsule. Then reabsorption: as this filtrate flows along the tubule, everything the body still needs (all the glucose and amino acids, most of the salt and nearly all the water) is taken back into the blood. What is left, mostly water, urea and extra salts, is urine. This page draws a nephron you can tap part by part, and works out a day’s balance for both kidneys. Water moves back into the blood by osmosis, which you can watch in the osmosis lab; to see the double circulation that brings blood to the kidneys, open double circulation.
Using it
- Pick a Level. Class 10 gives the main idea of each part. Class 11 adds the filtration rate, the share of water taken back in each part, the counter-current loop of Henle, and the hormones ADH and aldosterone.
- Tap a part of the nephron: the glomerulus and capsule, the proximal tubule, the loop of Henle, the distal tubule or the collecting duct.
- Move water you drink in a day. More water means more, paler urine; less water means the body keeps more and the urine is small and concentrated.
- Move blood glucose. Above 180 mg/dL (in this model) the tubules cannot take all the glucose back, and sugar spills into the urine.
Worked example
With 2 L drunk a day, this model gives 1.5 L of urine. The kidneys filter 125 mL a minute × 1440 minutes = 180 L a day, so the urine is 1.5 ÷ 180 = 0.83 % of the filtrate: 99.17 % of the water goes back into the blood. The filtrate carries 54 g of urea; half is taken back, so 27 g leaves in 1.5 L, a concentration of 18 g per litre. At a blood glucose of 90 mg/dL, 162 g of glucose is filtered and all of it is taken back. Raise it to 300 mg/dL and 540 g is filtered, but only 324 g can be taken back, so 216 g a day ends up in the urine.
Where this fits in school
Class 10 science (life processes) covers excretion in humans, the nephron as the filtering unit, and reabsorption. Class 11 biology (excretory products and their elimination) adds the filtration rate, what each part of the tubule does, the counter-current mechanism and the hormones that control urine. This page is an independent learning aid. It is not made by or connected with NCERT, and it does not copy any textbook.
Questions
Why filter so much and take most of it back? Filtering everything small and then keeping what is needed lets the kidney get rid of any unwanted small substance, even ones the body has never met before.
Is this how dialysis works? Dialysis also uses a filter that lets small molecules through, but a machine does it. The eGFR calculator calculator is for science work with real formulas; this page is a teaching model only.
Is anything saved? No. Nothing you choose is stored or sent anywhere.
Where the numbers come from
- The parts of the nephron and what each does: standard school biology, written by fibub in its own words.
- Round teaching values: about 125 mL a minute (180 L a day) filtered, half the urea taken back, 99.4 % of sodium taken back, and a single glucose threshold of 180 mg/dL. Real kidneys vary; this is not medical.
- Each number on the page was checked by hand in the tests (for example 1.5 L of urine is 0.83 % of 180 L, so 99.17 % of the water is taken back).
- Drawings are fibub's own simple shapes. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.