All lessonsThe Lab · Lesson 2 · The kidney
The Nephron and Diuretics
Follow salt and water along a nephron, segment by segment, and see where each diuretic stops salt being taken back, and how much water follows it out.
Builds on Lesson 1: The Pressure–Volume Loop. Blood volume, met at the end here, is what fills the heart.
How the kidney handles salt and water
Meet the nephron
Each kidney is packed with tiny tubes called nephrons. Every nephron starts with a filter that strains fluid out of the blood, then runs it through one long, folded tube. Whatever is still in the tube at the end becomes urine.
How the kidney handles salt and water
The filter: where urine begins
The filter, the glomerulus, is a ball of tiny blood vessels. Blood pressure pushes water and small dissolved things, like salt, through it into the tube, while cells and proteins stay behind. That fluid is the filtrate.
How the kidney handles salt and water
Almost all of it goes back
The filters make a huge amount of filtrate every day, far more than all the blood in your body. Nearly all of it must be taken back into the blood. Taking it back is reabsorption. Only a sliver is left to become urine.
How the kidney handles salt and water
Water follows salt
Cells in the tube wall carry salt (strictly, sodium, the part of table salt the body counts) out to the blood. Where a wall lets water through, water then follows on its own, toward the side with more dissolved particles. This pull is osmosis.
Along the nephron · stop 1 of 5
Proximal tubule: the big take-back
The first stretch is the proximal tubule ("proximal" means nearest the filter). It does the bulk of the work, taking back most of the salt, and water follows in step, so the fluid left behind is just as salty as before.
Along the nephron · stop 2 of 5
Down the loop: water leaves
Next the tube dips into the kidney's deep layer, the medulla, which is kept very salty. This hairpin is the loop of Henle. On the way down its walls let water out but hold on to salt, so the salty surroundings draw water out.
Along the nephron · stop 3 of 5
Back up the loop: salt is pulled out
The upward thick ascending limb works the other way. Its walls block water, but a salt carrier (a protein that ferries salt across) pulls out a large share of the filtered salt. That keeps the deep layer salty; the fluid inside turns watery.
Along the nephron · stop 4 of 5
Early distal tubule: fine-tuning
Back near the filter, the early distal tubule ("distal" means further away) uses a different salt carrier to take back a small extra share of salt. Its walls also block water, so the fluid inside gets more watery still.
Along the nephron · stop 5 of 5
Collecting duct: salt in, potassium out
Many nephrons drain into one collecting duct. Here the hormone aldosterone, the body's salt-keeping signal, opens salt channels. Taking salt back makes the duct push potassium, another blood salt the heart and muscles need, out into the urine.
Along the nephron · stop 5 of 5
Collecting duct: the last chance for water
Running down through the salty deep layer, the duct can let water out too, but only when antidiuretic hormone (the water-keeping signal) opens water channels in its walls. The salty surroundings then pull water back, leaving concentrated urine.
Putting it together
The whole journey
Put together: the big take-backs come early, the later stretches fine-tune, and water follows salt wherever the walls let it. Of all the salt filtered, only a tiny sliver reaches the urine, and only a little water goes with it.
Putting it together · the key measures
Why it matters: blood volume
Salt kept in the body holds water, and that water makes up much of your blood volume: how much blood fills the vessels. A diuretic is a drug that makes the kidneys pass more urine. Most do it by passing more salt, which water follows.
What changes it · 1 of 5
Loop diuretics (furosemide)
normalwith a loop diureticblocked
A loop diuretic, such as furosemide, blocks the salt carrier in the thick ascending limb. A flood of salt rushes on; later stretches catch only some, and water follows the rest into the urine. Blood volume drops, and more potassium is lost.
What changes it · 2 of 5
Thiazide diuretics
normalwith a thiazideblocked
A thiazide diuretic blocks the salt carrier in the early distal tubule. That stretch normally takes back only a small share, so the extra salt lost is modest: a gentler diuretic. Water follows that salt out, blood volume dips and potassium loss rises.
What changes it · 3 of 5
Potassium-sparing diuretics
normalwith a potassium-sparing diureticblocked
A potassium-sparing diuretic acts in the collecting duct, either blocking aldosterone's signal or plugging the salt channels it opens. A little more salt and water leave, and with less salt taken up, less potassium is pushed into the urine, so potassium is kept.
What changes it · 4 of 5
Carbonic anhydrase inhibitors (acetazolamide)
normalwith acetazolamideblocked
The proximal tubule takes back much of its salt with bicarbonate, the blood's acid-soaking base, helped by an enzyme (a protein that speeds a chemical reaction) called carbonic anhydrase. Acetazolamide, an inhibitor (blocker) of it, leaves both in the tube; bicarbonate is lost.
What changes it · 5 of 5
Osmotic diuretics (mannitol)
normalwith mannitolmannitol in the tube
An osmotic diuretic such as mannitol is a sugar-like substance that passes the filter but is never taken back. Like salt, it holds water by osmosis, but it stays in the tube, so far more water than salt pours out.
Clinical values
Each bar shows a typical adult value from a standard textbook: a shaded band where the source gives a range, a short black tick where it gives one typical value, and a shaded run from zero where it gives only an upper limit. The red dot is the teaching model right now and moves as you change the drugs; the hollow circle is the model with no drug. Pick a drug to see which way the source says each value moves, and the model is set to match.
The banner above still applies: this is a teaching model with no hormones or reflexes. Its sodium, urine and potassium results come from each drug's direct transport effect; its bicarbonate rise with volume loss is only a crude stand-in for contraction alkalosis, which in the body follows later, through angiotensin II and aldosterone. Each drug gives a direction only for the values its cited passage states; every other value is marked "not listed in the source". Carbonic anhydrase inhibitors and osmotic diuretics are taught in the lesson and can be set with the sliders, but are not listed as states here because their source passages have not yet been checked against the cited edition.
Pick a drug
Adjust the model
Each slider is how strongly a drug's target is inhibited in the model, from none to the model's strongest setting, not an amount of drug.
- Loading reference values…
Sources
Explore freely
An illustrative teaching model. This is the model behind the lesson. Mix drugs, or set how strongly each one blocks its target, and watch the nephron, the salt and water left in the tube, and the outputs change at once; dashed outlines are the nephron with no drug. Amounts are shares of what was filtered. The numbers are outputs of a simplified model, not measurements or reference values; typical adult values are in the Clinical values tab. Less blood volume means less filling of the heart: the preload of Lesson 1.
Try a scenario
- Salt out
- –% filtered
- Water out
- –% filtered
- Urine
- –L/day
- Blood volume
- –index
- Potassium
- –mmol/L
- Bicarbonate
- –mmol/L
The nephron
Arrow width: amount taken back · dashed: no drug
Left in the tube
After each stop, as % of filtered (log scale) · dashed: no drug
Drugs
Each slider is how strongly a drug's target is inhibited in the model, from none to the model's strongest setting, not an amount of drug. Arrow keys nudge a slider; Page Up and Page Down take big steps.
Segment by segment
| Segment | Salt in | Salt back | Water in | Water back |
|---|
Percent of the filtered amount. "Back" means taken back into the blood.
About the model
What is simulated
One nephron at steady state, as a chain of segments. Everything is a share of the filtered amount (filtered = 100). Each segment takes back part of the sodium that reaches it, so blocking one segment sends more sodium on, and the later segments catch some of it. The early distal tubule and collecting duct have uptake that saturates, which is why they cannot catch everything a loop diuretic sends them.
Equations
Loop, Thz, Ksp, CA and Osm are the drug sliders, from 0 (none) to 1 (the model's strongest setting). Even the strongest setting leaves some transport working, as the equations show. The urine water line is the key idea: urine can only be so concentrated (at most 1 + 1.2·g times plasma strength), so every bit of solute left in the tube carries water out with it.
Blood, potassium and bicarbonate
- Blood volume (index, 100 with no drug) falls with the salt and water lost beyond the no-drug amounts, saturating at a 12-point drop: L = 0.6·ΔNa_out + 0.4·ΔW_out, index = 100 · (1 − 0.12·(1 − e−L/6)).
Simplifications
- Steady state only: no time course. Real diuretic responses build and fade over hours.
- No feedback. Filtration is fixed, and the hormones and reflexes that fight a fall in blood volume (renin, angiotensin, aldosterone, thirst, antidiuretic hormone) are left out, so the sodium, water and potassium results show each drug's direct transport effect. In the body, salt retention rebounds as each drug's effect wears off.
- Antidiuretic hormone is assumed present, so urine is concentrated as far as the medulla's salt gradient allows.
- The baseline is calibrated at load time from clinical-values.json: the textbook shares of sodium taken back in each segment, the typical daily urine output, and the normal plasma potassium and bicarbonate. The drug strengths are not fitted to any person or drug.
Sources for the structure
- Costanzo LS. Physiology. 6th edition. Philadelphia: Elsevier; 2018. Chapter 6, Renal Physiology.
- Hall JE. Guyton and Hall Textbook of Medical Physiology. 13th edition. Philadelphia: Elsevier; 2016. Chapters 28 to 32.