How Your Kidneys Filter Blood
Your two kidneys clean your entire blood supply about every 30 minutes. Roughly a million tiny units called nephrons do the work. Each one squeezes fluid out of the blood through a filter (the glomerulus), then carefully takes back the good stuff along a long tubule. Watch about 180 litres a day get filtered, follow a single molecule to see where it ends up, then use the Hydration and Diabetes controls to see how the final urine changes — from ~180 L filtered down to about 1.5 L of urine.
Live kidney readout
What's happening
The Science in Plain Language
1. Filtration — a size barrier. Blood enters each nephron through the afferent arteriole and swirls through the glomerulus, a tight ball of leaky capillaries. Because the vessel leaving (the efferent arteriole) is narrower, pressure builds and fluid is squeezed out through the filter into Bowman’s capsule. The filter’s pores are a very particular size: water, glucose, sodium, and the waste product urea are small enough to pass, but proteins (like albumin) and blood cells are too big and stay in the blood. The rate of this filtering is the glomerular filtration rate (GFR), normally about 125 mL/min — which adds up to roughly 180 litres a day.
2. An enormous volume is filtered — and almost all is taken back. Filtering 180 litres of your own fluid every day sounds reckless, and it would be if the kidney didn’t reclaim it. Obviously you don’t urinate 180 litres, so about 99% of that water (and nearly all the salt and sugar) is reabsorbed along the tubule — leaving only about 1.5 litres as urine.
3. The proximal tubule — the bulk reclaimer. As filtrate flows down the proximal tubule, the nephron pulls the valuable materials back into the surrounding peritubular capillaries. Normally 100% of the glucose (via SGLT2 transporters) and about 65% of the water and sodium are reabsorbed here, before the fluid even reaches the loop.
4. The loop of Henle — a countercurrent multiplier. The loop dives deep into the medulla and doubles back. The two limbs do opposite jobs: the descending limb is permeable to water (water leaves into the salty surroundings) while the thick ascending limb pumps sodium out but is water-tight. Working against each other, they stack up a steep salt gradient in the medulla — from about 300 mOsm/L near the cortex to as much as 1200 mOsm/L deep in the inner medulla. That gradient (the shaded band in the diagram) is the whole reason your kidney can make urine more concentrated than blood.
5. The distal tubule & collecting duct — the ADH dial. Final adjustments happen in the distal tubule and collecting duct, which run back down through that salty medulla. Here the hormone ADH (antidiuretic hormone / vasopressin) sets how leaky the duct is to water. When you are dehydrated, ADH is high: aquaporin channels open, water is drawn out into the medulla, and you make a small volume of very concentrated urine (up to ~1200 mOsm/L). When you are well hydrated, ADH is low: the duct stays water-tight, and you make a large volume of dilute urine (as low as ~50–90 mOsm/L). Toggle the Hydration control to watch the urine output, concentration, and reabsorption all shift.
6. Waste is left behind. Urea and other wastes are largely not reabsorbed, so they become concentrated and flow on through the collecting duct to make urine, which drains toward the bladder.
7. Why doctors test your urine. Because the healthy nephron keeps glucose and protein in the blood, finding either one in urine is a red flag:
- Glucose in the urine (glucosuria) usually means blood sugar is so high that the tubule’s transporters are overwhelmed. Every day the tubule can only reabsorb glucose up to a maximum rate (the transport maximum, Tm ~375 mg/min); above a blood level of about 180 mg/dL — the renal threshold — the excess spills into the urine. That is the classic diabetes warning sign. Turn on the Diabetes control to see green glucose particles escape into the urine.
- Protein or albumin in the urine means the glomerular filter itself is leaking — an early marker of kidney damage. This is exactly what the Microalbumin / ACR (albumin-to-creatinine ratio) urine test is designed to catch, often years before symptoms appear.