Each kidney is packed with about a million tiny filters called nephrons. Follow one of them: blood is filtered under pressure, most of the water is taken back, and the rest becomes urine.
A nephron is the kidney's basic working unit, and each kidney holds about a million of them. Every nephron is a filter plus a long coiled tube: blood is pushed through the filter at one end, and as the fluid travels the tube the body takes back what it still needs and adds what it wants to lose. What is left at the far end is urine.
The kidney is a quiet, hard-working cleaning system with no obvious moving parts. Unfolding a single nephron shows the whole logic in one strip: filter first, reclaim next, top up the waste, then drain — which is far clearer than the organ viewed from outside.
The afferent arteriole brings blood into the glomerulus, a knot of leaky capillaries inside the Bowman's capsule. Because the vessel leaving is narrower than the one arriving, the pressure here stays high.
That pressure forces water and small dissolved waste out of the blood and into the capsule. Big things — blood cells and large proteins — cannot pass, so they stay in the blood. The fluid that crosses over is the filtrate.
The filtrate then flows into the tubule. Along the proximal tubule especially, most of the useful water, salts and nutrients are pulled back into the peritubular capillaries — so the body does not lose them.
At the same time, the tubule does the reverse: substances the body wants gone are moved from the blood into the tubule. That is secretion, and it tops up the waste in the fluid.
The filtrate dips down and back up the hairpin loop of Henle, which builds up a concentration gradient in the surrounding tissue. That gradient is what lets water be reclaimed and the final urine be concentrated.
After the distal tubule makes final adjustments, the remaining fluid enters the collecting duct. What leaves is urine, ready to be stored in the bladder and passed out.
It does more than filter. The nephron filters a lot of fluid first, then reabsorbs almost all of the useful water and salts back into the blood. What is finally left — a small concentrated remainder — becomes urine.
They move substances in opposite directions. Reabsorption takes useful things out of the tubule back into the blood; secretion takes waste from the blood into the tubule. Both help set what leaves the body.
Filtration is driven by pressure, not by any pump inside the nephron. If the pressure in the glomerulus is too low, less fluid is filtered — which is one reason kidney function depends on good blood pressure.
It builds up a concentration gradient in the surrounding tissue, so that water can later be drawn back out of the fluid. That is what lets the kidney produce concentrated urine instead of losing all the water.
No. It is a heavily simplified educational model of the nephron. It is not medical diagnosis or clinical guidance — seek a professional physician for any health-related question.