Urinary System
The body's editorSystema urinarium
A filtration and reclamation plant that decides, litre by litre, what the blood keeps and what it loses.
- Primary role
- Filters blood, balances fluid and electrolytes
- Scale
- ~2 million nephrons
- Every day
- Filters about 180 L of fluid
Organs in this atlas 1
How it works as a whole
The kidneys work by a counterintuitive strategy: filter almost everything out of the blood indiscriminately, then carefully take back what is worth keeping. About 180 litres are filtered a day and roughly 99% is reabsorbed, which sounds wasteful but gives exquisitely fine control. Along the way the same machinery sets blood pressure, holds pH steady, decides sodium and potassium balance, and signals for more red blood cells when oxygen runs low.
Other key structures
- NephronThe functional unit — filter, then selective reabsorption
- UretersMuscular tubes that milk urine downward by wave
- BladderExpandable reservoir, holding roughly half a litre
- UrethraThe final outflow, under both voluntary and involuntary control
Connected systems
- Cardiovascular SystemSets long-term blood pressure and blood volume
- Endocrine SystemResponds to antidiuretic hormone and aldosterone minute by minute
- Respiratory SystemHandles the slow half of acid-base balance the lungs cannot
Common conditions
- Chronic kidney disease
- Kidney stones
- Urinary tract infection
- Incontinence
- Renal hypertension
- Bladder cancer
The through-lineIt is an endocrine organ as much as a filter: renin to raise blood pressure, erythropoietin to make red cells, and the final activation step of vitamin D all happen here.
Did you knowUrine is sterile on the way out of a healthy kidney; its familiar smell develops only once bacteria reach it.
The path through this system
0 of 10 steps done — read it, walk it in 3D, then be asked about it.
Start: Kidneys: the lesson
ReadKidneys: the lessonStructure and story of the kidneys, layer by layer.
3 min
Guided walk-throughGuided dissection: kidneysWalk the kidneys structure by structure, then cut inward layer by layer.
6 min
Memo cardsKidneys memo cardsTurn the cards on the kidneys — structures, layers, key facts — and grade yourself.
4 min
Multiple choiceKidneys quizA multiple-choice round on the kidneys, reshuffled every time.
4 min
Click on the 3D modelFind the kidneys structuresClick the named structures directly on the live 3D kidneys.
5 minLabel the diagramLabelled diagram: KidneysThe kidneys plate with its labels taken off: put each one back on its number, or draw it yourself first.
5 min
Match layersMatch kidneys layersPair each layer of the kidneys, outside in, with what it does.
4 min
Match pairsMatch kidneys structuresPair each structure in the kidneys with its role.
4 min
Multiple choiceUrinary System quizA quiz pulling from every organ in the urinary system.
3 minReadKidneys in the clinicWhere the kidneys shows up in the clinic, and what goes wrong.
3 min