Blood (red) flows into each kidney and cleaned blood (blue) flows out. Urine (yellow) is squeezed down the ureters and slowly fills the bladder; here 1 second is 10 minutes of body time. Press “Follow a drop of blood” to trace one drop all the way into urine.
Rotate the body and click any part, or use the coloured legend, to see its job. Blood enters each kidney through a renal artery; about a million nephrons filter it; urine collects in the renal pelvis, runs down the ureter to the bladder and leaves through the urethra. Tick “See inside” to cut the kidneys open.
Your cells are always making waste. The biggest one is urea, which the liver makes when it breaks down spare protein. Your body also gains extra water and salt from food and drink. If these built up in the blood, cells would stop working. The urinary system removes them: the kidneys clean the blood and turn the waste into urine, and the rest of the system stores the urine and lets it out when convenient. This removal of wastes made by the body is called excretion.
Blood enters each kidney through a renal artery branching straight off the aorta. Inside, about a million tiny filtering units called nephrons make urine. Their filters sit in the outer cortex, and their long tubes dip into the cone-shaped pyramids of the medulla. Urine collects in the funnel-shaped renal pelvis and flows down a ureter to the bladder. The bladder stores it until it leaves through the urethra. The cleaned blood returns to the heart through the renal vein.
The kidneys do not pick out the waste one molecule at a time. First, blood pressure pushes water and all small molecules out of the capillaries of each glomerulus into the nephron: this is filtration. Blood cells and large proteins stay in the blood. It adds up to about 180 litres of filtrate a day, the whole blood plasma filtered about 60 times. Then the tubules take back what the body needs: all the glucose, most of the salt, and about 99% of the water. This is selective reabsorption. They also actively add some extra wastes and acid, which is called secretion. Only about 1–2 litres a day is left as urine.
When you are short of water, your blood gets saltier and the brain’s hypothalamus notices. The pituitary then releases ADH (antidiuretic hormone). ADH makes the collecting ducts more permeable to water. Water moves out into the very salty medulla and back into the blood, leaving a small amount of dark, concentrated urine. After a lot of drinking, ADH falls and you make plenty of pale, dilute urine. This is negative feedback that keeps the blood’s water level steady (homeostasis). Emptying the bladder is controlled by nerves. Stretch sensors report how full it is. When you decide to go, the bladder wall (the detrusor muscle) contracts and two sphincters relax. The inner one is automatic; the outer one is under your control.
Press Follow a drop of blood and watch the drop change colour when it is filtered in the cortex. Switch between Drank plenty and Dehydrated and compare the urine colour, the ADH level and how fast the bladder fills. Let the bladder pass 150 mL, 400 mL and 500 mL and read the Urge readout, then press Empty bladder and watch the sphincters turn green. Tick See inside the kidneys to see the pyramids pointing at the renal pelvis.
| Part | What it is | Main job | Key fact |
|---|---|---|---|
| Kidneys | Two bean-shaped organs against the back wall of the abdomen | Filter blood, make urine, balance water, salt and pH | Right kidney sits a little lower (liver above it) |
| Renal cortex | Outer layer of the kidney | Filtration in the glomeruli | About 180 L of filtrate a day |
| Renal medulla | Inner layer, 8–18 cone-shaped pyramids | Concentrates urine (loops of Henle, collecting ducts) | Very salty inside, which pulls water out |
| Renal pelvis | Funnel in the centre of the kidney | Collects urine and passes it to the ureter | Kidney stones often lodge here |
| Renal artery | Branch of the aorta | Brings blood into the kidney | About 1.2 L/min for both kidneys |
| Renal vein | Vessel into the inferior vena cava | Takes cleaned blood away | Less urea, balanced salt and water |
| Ureters | Two muscular tubes | Carry urine to the bladder by peristalsis | About 25–30 cm long |
| Bladder | Stretchy muscular bag in the pelvis | Stores urine | Holds about 400–600 mL |
| Urethra & sphincters | Tube to the outside with two muscle rings | Lets urine out when you choose | About 4 cm (female), 20 cm (male) |
| Substance | In the filtrate | In the urine | Why |
|---|---|---|---|
| Water | ~180 L/day | ~1.5 L/day | ~99% reabsorbed; ADH fine-tunes the last part |
| Glucose | Yes | Normally none | All reabsorbed (unless blood sugar is very high, as in untreated diabetes) |
| Salt (Na+, Cl−) | Yes | Some | Most reabsorbed; aldosterone adjusts it |
| Urea | Yes | Yes, concentrated | About half is reabsorbed, the rest leaves |
| Proteins | Almost none | Only traces | Too big to pass the glomerular filter |
| Blood cells | No | No | Too big to be filtered |
| You have… | ADH | Urine |
|---|---|---|
| Drunk a lot | Low | Lots of pale, dilute urine |
| A normal day | Medium | About 1–2 L, yellow |
| Sweated a lot or drunk little | High | A little dark, concentrated urine |
The yellow colour comes from urochrome, a pigment from the breakdown of old red blood cells. Some foods (beetroot), vitamins (B2 turns urine bright yellow) and medicines can change the colour too.
They keep the blood’s water and salt balanced, adjust its pH by getting rid of acid, and help control blood pressure by releasing the enzyme renin. They also make the hormone erythropoietin (EPO), which tells the bone marrow to make red blood cells, and they turn vitamin D into its active form for strong bones.
The urinary system is made of the two kidneys, two ureters, the bladder and the urethra. The kidneys filter the blood to remove wastes such as urea, extra water and extra salt, and turn them into urine. The ureters carry urine to the bladder, which stores it until it leaves through the urethra. The kidneys also help control blood pressure, blood pH and red blood cell production.
Key takeaway: kidneys clean the blood and make urine; the rest stores and removes it.Urine is made in the nephrons of the kidney and flows through the collecting ducts into the renal pelvis. It then goes down the ureter to the bladder, where it is stored, and finally leaves through the urethra.
Key takeaway: kidney (nephrons → renal pelvis) → ureter → bladder → urethra.In each glomerulus, a knot of capillaries in the kidney cortex, blood pressure pushes water and small molecules such as glucose, salts and urea into the nephron. Blood cells and proteins are too big and stay behind. The tubules then take back useful substances (all the glucose, most of the water and salt) and add some extra wastes. What remains is urine.
Key takeaway: filtration, then selective reabsorption, plus secretion.About 1.2 litres of blood flows through the kidneys every minute, roughly 1,700 litres a day. From it they filter about 180 litres of fluid a day, but they return about 99% of it to the blood, so only about 1 to 2 litres leaves as urine.
Key takeaway: about 180 litres filtered, about 1.5 litres of urine.When you are short of water, your brain releases more ADH (antidiuretic hormone). ADH makes the collecting ducts in the kidneys put more water back into the blood. The same amount of waste is then dissolved in less water, so the urine is darker and more concentrated. When you drink a lot, ADH falls and the urine becomes pale and plentiful.
Key takeaway: more ADH = less water in the urine = darker urine.Not by gravity alone. The walls of the ureters contain smooth muscle that contracts in waves, called peristalsis, a few times a minute. These waves squeeze urine down in small spurts, so it reaches the bladder even when you are lying down. The ureters enter the bladder at a slant, which stops urine flowing back up when the bladder squeezes.
Key takeaway: peristalsis pushes urine down the ureters.An adult bladder usually holds about 400 to 600 mL. Stretch sensors in the wall start sending signals as it fills. Most people first notice the urge at about 150 to 250 mL and feel a strong urge at about 400 to 500 mL. The outer sphincter is under voluntary control, so you can usually wait until it is convenient.
Key takeaway: holds ~400–600 mL; the first urge comes at ~150–250 mL.The liver sits in the upper right of the abdomen, right above the right kidney, and pushes it down by a centimetre or two. Both kidneys lie against the back muscles, behind the lining of the abdomen, protected by the lowest ribs.
Key takeaway: the liver pushes the right kidney down.Alcohol reduces the amount of ADH the pituitary gland releases. With less ADH, the collecting ducts keep less water, so the kidneys make more, and more dilute, urine. Strong drinks can make you lose more water than they contain, which is one reason heavy drinking can leave you dehydrated.
Key takeaway: alcohol lowers ADH, so more water is lost in urine.