Urine is not simply blood with the red cells removed. A kidney first filters water and small substances into microscopic tubes, then carefully returns useful substances to the blood. Travel from the renal artery into one nephron to follow both streams.
How can we remove waste without losing glucose and all our water?
The kidney contains many nephrons. Each has a capillary knot (glomerulus), a collecting cup (Bowman’s capsule) and a long tubule. Select “Enter a nephron” to separate the blood route from the filtrate route. The enlarged nephron is a diagram of the process, not to anatomical scale. Open tube ends mark where blood or urine enters and leaves the view. Small substances cross the walls during filtration and reabsorption; red cells remain inside the blood vessels.
Anatomical view
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Click the model or choose a name below to read its function.
Red cell / stays in bloodLarge protein / retainedWaterGlucoseIonsUrea
Filtration is not reabsorption
Filtration: blood → capsule, driven by pressure. Reabsorption: tubule → blood, by selective transport and osmosis. Secretion: blood → tubule. Excretion: urine leaves the body. These four words describe different processes.
Try the water-saving control
More ADH makes the collecting duct more permeable to water. In a functioning medullary gradient, more water returns to the blood, leaving a smaller volume of more concentrated urine. This animation illustrates the direction, not a clinical urine-volume prediction.
Check your understanding · Does glucose in the initial filtrate mean it must appear in the urine?
No. In normal conditions, proximal tubules reclaim essentially all of it before the fluid becomes urine.