Watch a nephron filter blood and form urine

How to use this simulation

1. Watch first without changing anything: blue water, orange glucose and green salt particles cross the glomerulus into the tubule, but red proteins/blood cells never do.
2. Follow the green arrow beside the PCT: glucose particles leave the fluid and rejoin the blood — at a normal level, almost all of them do.
3. Drag blood glucose above 200: some glucose particles stop turning back and head towards the urine instead — the "glucose in urine" reading flips to present.
4. Drag water intake to its minimum: watch more water turn back at the collecting duct, and the urine-per-day reading drop. Now push it to maximum and watch the reverse.
5. Compare the green arrows beside the loop of Henle's two limbs: water leaves on the way down, salt leaves on the way up — two different jobs.

Why drinking a lot of water sends you to the bathroom so often

Drink a lot of water on a hot day and you are back in the bathroom within the hour; drink little, or sweat a lot, and your urine turns dark yellow and scarce. Somehow the body always seems to know exactly how much water to keep and how much to let go.

That whole calculation is run by roughly a million tiny filtering tubes packed into each kidney: nephrons. Every nephron filters a huge share of the blood passing through it each day, then takes almost everything useful straight back, leaving only genuine waste behind. This simulation lets you watch that filter-and-reclaim process happen in real time.

A surprising fact: the volume of fluid a kidney filters every day is many times larger than the body's entire blood plasma — because the same blood cycles through the kidneys again and again.

Starting from zero: the structure of a nephron

Every nephron starts at the glomerulus — a tangled tuft of blood capillaries fed by the afferent arteriole, sitting inside a cup-shaped sac called Bowman's capsule. The first filtration step happens here.

The filtered fluid then passes, in order, through the proximal convoluted tubule (PCT), the loop of Henle (a descending limb going down, an ascending limb coming back up), the distal convoluted tubule (DCT), and finally the collecting duct, where whatever is left drains as urine into the renal pelvis.

The whole tubule is wrapped by the peritubular capillaries — blood leaving through the efferent arteriole flows alongside it, and this is where reabsorbed substances rejoin the bloodstream. The upper parts (glomerulus, PCT, DCT) sit in the renal cortex; the loop and collecting duct dip down into the renal medulla.

Not every nephron's loop of Henle is the same length. Most nephrons (cortical nephrons) have a short loop that stays close to the cortex. But roughly a fifth of nephrons (juxtamedullary nephrons) have a much longer loop that plunges deep into the medulla — it is mainly these long loops that build the medulla's concentration gradient, the thing that actually lets urine become genuinely concentrated.

Key terms in the nephron

Get the vocabulary straight before the formulas.

Filtration: what crosses the glomerulus, and what never does

The capillary wall at the glomerulus acts like a sieve. Blood pressure here is unusually high (the afferent arteriole is wider than the efferent one, so pressure builds up), and that pressure alone pushes water, glucose, salt (Na⁺/Cl⁻) and urea across the wall into Bowman's capsule. This is ultrafiltration — fast and pressure-driven, unlike an ordinary filter.

Blood proteins (such as albumin) and blood cells are simply too large to cross that wall — they stay in the blood and leave through the efferent arteriole. This size-based sorting is what makes filtration selective, and it is why a healthy person's urine should contain no protein or blood cells at all; finding either is a sign something is wrong with the kidney.

Not all the plasma reaching the glomerulus is filtered — only a share of it, called the filtration fraction, typically around 20%. The rest continues through the efferent arteriole into the peritubular capillaries.

Reabsorption: why glucose is fully reclaimed at the PCT

The filtrate carries useful substances too, so the tubule's job is to selectively take them back — selective reabsorption. Most of it happens at the PCT: almost all glucose, and most of the water and salt, are reabsorbed here by active transport and osmosis.

Normally, blood glucose is low enough that the PCT's carrier proteins (transporters) can reclaim every filtered glucose molecule — none reaches the urine. But if blood glucose rises past the renal threshold, roughly 180 mg/dL (as in uncontrolled diabetes), the transporters become saturated, and the extra glucose that cannot be reabsorbed spills into the urine — a condition called glucosuria.

Raise the blood-glucose slider in this simulation and you see exactly that saturation effect: below threshold every glucose particle turns back; above it, some particles fail to turn back and head towards the urine instead.

The loop of Henle: two limbs, two different jobs

The loop of Henle's trick is neat: the descending limb is permeable to water but not to salt, so water leaves into the salty medulla and the fluid grows more concentrated. The ascending limb is almost the opposite — nearly impermeable to water, but it actively pumps Na⁺ and Cl⁻ out, so the fluid becomes more dilute again.

Together, the two limbs' opposite behaviour builds a salt gradient in the medulla (mild near the top, very concentrated deep down) — the counter-current multiplier. That gradient is exactly what lets the collecting duct later pull water out and concentrate urine well beyond blood strength; without this trick, a mammal could never produce urine more concentrated than its own blood.

DCT, the collecting duct and ADH: the final water decision

The DCT does a little more reabsorption plus some secretion (pushing extra K⁺ or H⁺ from the blood into the tubule, which helps keep blood pH steady). But the final call belongs to the collecting duct, and that call is controlled by ADH (antidiuretic hormone).

When the body is short on water (low intake, heavy sweating), the pituitary gland releases more ADH; ADH makes the collecting duct wall more permeable to water, so more water is reabsorbed and urine becomes scant and concentrated. Drink plenty of water and ADH falls, the duct stays less permeable, and more water leaves as pale, plentiful urine.

The water-intake slider here controls exactly this ADH effect: drag it down (less intake, more ADH) and you will see more water turning back at the collecting duct; drag it up and the reverse happens.

The formulas: from GFR to litres of urine a day

GFR is the rate fluid is filtered at the glomerulus each minute, about 125 mL/min in a healthy adult. With 60 × 24 = 1440 minutes in a day, that scales up to a strikingly large daily total.

Try this in the simulation

Predict each outcome before you press play, then compare.

• Slowly raise blood glucose from 60 to 400: at what point does the first glucose particle start heading for the urine instead of turning back?
• Take water intake to its minimum and note urine-per-day, then to its maximum — how many times larger is the reading?
• Track the red protein/blood-cell particles for the whole run: do any of them ever cross the glomerulus into the tubule?
• Watch where the green salt particles leave the tubule — on the descending limb, or the ascending one?
• Put the filtrate-per-day and urine-per-day readings side by side and divide them yourself to check the per cent reabsorbed reading.

Solved problems

Each solution states the formula first, then substitutes the numbers.

Common mistakes

Clearing these up is worth marks in every exam that touches this topic.

• Thinking urine is simply "the extra blood filtered out". 180 litres are filtered daily, but about 99% is reclaimed; urine is only what was never reabsorbed.
• Assuming a little glucose in urine is normal. It should not be there at all in a healthy person; its presence signals blood glucose above the renal threshold.
• Confusing filtration with reabsorption. Filtration happens at the glomerulus (sorting by size); reabsorption happens along the tubule (reclaiming what is useful) — two separate steps.
• Swapping the two limbs of the loop of Henle. The descending limb loses water; the ascending limb loses salt — not the other way round.
• Thinking more ADH means more urine. More ADH means more water reabsorbed in the collecting duct, so urine actually decreases and becomes more concentrated.

Real life and medicine

The same filter-and-reclaim principle underlies diagnosis, medication and even the artificial kidney.

• In diabetes mellitus, blood glucose above the renal threshold shows up as glucose in the urine — the principle behind the earliest urine-glucose tests.
• When kidneys fail, a dialysis machine does the nephron's job from outside the body, filtering waste from the blood and returning cleaned blood.
• In the rare condition diabetes insipidus, ADH does not work properly, so a person passes many litres of dilute urine a day and feels constantly thirsty.
• Diuretic drugs reduce reabsorption in the tubule to remove excess water and salt — a common treatment for high blood pressure.
• Routine urine tests in pregnancy check for glucose and protein, either of which can be an early sign of gestational diabetes or kidney trouble.
• Marathon runners and people working in extreme heat lose a lot of water as sweat, and their ADH level rises well above normal in response — urine output all but stops, the body's last line of defence against dehydration.

Exam corner

The nephron and urine formation come up across school biology — structure and function questions, definition and short-answer questions on GFR, the renal threshold and ADH, and longer questions asking for the filtration-reabsorption-secretion sequence with reasons.

Revision: the one-screen summary

This list plus the two formulas above are all you need the night before an exam.

• GFR ≈ 125 mL/min ⇒ filtrate per day ≈ 180 L; urine per day ≈ 1–2 L (about 99% reabsorbed).
• Filtration happens at the glomerulus: small solutes (water, glucose, salt, urea) cross; large ones (proteins, blood cells) never do.
• Almost all glucose and most water/salt are reabsorbed at the PCT; above the renal threshold, excess glucose spills into urine (glucosuria).
• Loop of Henle: descending limb loses water, ascending limb loses salt — building the counter-current multiplier.
• ADH at the collecting duct sets the final water reabsorption — more ADH means less, more concentrated urine.
• The DCT also secretes a little (K⁺, H⁺), helping keep blood pH and ion balance steady.

Frequently asked questions