How does the excretory system work?

Every cell in you is busy, and busy cells make waste: carbon dioxide from burning food, nitrogen left over from protein, old red blood cells, extra salt and water. Getting rid of it is called excretion, and your body has four ways out, run by five organs. We are facing the person, so their right side is on your left.

The best known is the urinary tract. Two kidneys high at the back clean the blood that the aorta brings them, about a litre every minute, and turn the waste into urine. Two thin tubes, the ureters, squeeze it down in little waves to the bladder, a stretchy bag low in the belly that holds about 400 to 600 mL. When you go, it leaves through the urethra. In women the urethra is short, about 4 cm; in men it is about 20 cm long and passes through the prostate gland. How the kidneys filter is its own box: see KidneyClear.

The others share the work. The lungs breathe out carbon dioxide and water vapour (LungsClear). The liver turns poisonous ammonia into urea and sends old-blood pigment out in bile (LiverClear). The large intestine carries that bile and the leftovers of food out as stool (IntestineClear). And the skin loses water and a little salt in sweat (SkinClear).

Every day your cells turn food and oxygen into energy, and that leaves waste. Each kind has its own way out.

Carbon dioxide is the biggest by far: about 1 kg a day for an active adult. It leaves every cell, rides in your blood to the lungs and is simply breathed out. Most of the weight you lose when you slim down actually leaves this way, as carbon in your breath.

Urea carries the nitrogen from the protein you eat, about 20 to 35 g a day. The liver makes it, the blood carries it (through the heart) to the kidneys, and they send it out in urine, with extra salt, acids and many medicines.

Bilirubin is the yellow pigment left when about 200 billion old red blood cells are broken down each day. The liver puts it into bile, bile flows into the gut, and bacteria turn it brown: that is what colours stool. A little comes back and colours urine yellow.

Sweat is mostly for cooling, not cleaning, but it takes water and a little salt with it. And water leaves by all four exits: about 2.5 litres in a cool day, and about the same comes in.

Protein is made of amino acids, and every amino acid carries nitrogen. Your body reuses most of them, but the extras are burned for energy or turned into sugar. First the liver pulls the nitrogen off, and it comes away as ammonia (NH₃).

Ammonia is toxic. Even a little too much in the blood upsets the brain, causing confusion and sleepiness. So the liver never lets it pile up. In the urea cycle, a loop of reactions that starts inside the cell's mitochondria, it joins two nitrogens and one carbon into urea: small, harmless and easy to dissolve. It costs energy, about 3 ATP per molecule, but it is worth it.

The blood carries urea to the kidneys, which send it out in urine (see KidneyClear). More protein means more urea: about 20 to 35 g a day on an ordinary diet. How the liver does its other cleaning is in LiverClear.

Animals solve the same problem in different ways. Fish live in water, so they just let ammonia out through their gills: it needs about 500 mL of water for every gram of nitrogen. Birds and reptiles make uric acid, which barely dissolves and leaves as a white paste, saving water and weight. We are in between: urea needs about 50 mL of water per gram.

The bladder is a bag of smooth muscle called the detrusor. As urine trickles in from the ureters, the muscle relaxes and stretches, so the pressure inside stays low. An adult bladder holds about 400 to 600 mL.

Stretch sensors in the wall report to the spinal cord and up to the brain. At about 150 to 300 mL you feel the first urge. A reflex in the spinal cord and brainstem would empty the bladder right then, but your brain holds the go signal until you are somewhere suitable. Two gates help: the internal sphincter, a ring of smooth muscle at the bladder's neck that works on its own, and the external sphincter, a ring you can squeeze on purpose.

When you decide to go, the brain lets go: the external sphincter relaxes, the detrusor squeezes and urine flows at about 20 mL a second. Babies empty by reflex. Children learn to hold the signal, usually by day between about 2 and 4 years, and at night a little later. Wetting the bed at 5 is common and normal.

Why do you go more when it is cold? Blood leaves the chilly skin for the warm core, the body senses extra fluid and makes more urine. Coffee and tea contain caffeine, which makes the kidneys pass a bit more for a few hours. And when you are nervous, the brain listens harder to the bladder, so you feel the urge sooner even though there is no more urine.

Your body keeps its water almost constant, so what goes out must match what comes in. The exits share the job, and the weather decides the split.

On a hot day the skin sweats to cool you, and sweat can take 1.5 litres or more. On the cricket field in May it can be more than 1 litre an hour. The kidneys respond by saving water: the brain's pituitary gland releases ADH (antidiuretic hormone), which tells the kidneys to put more water back into the blood. Urine becomes small and dark. How ADH works inside the kidney is in KidneyClear.

On a cold day you hardly sweat. ADH falls, and the spare water leaves as plenty of pale urine. Cold also shifts blood from the skin to the core, which makes the kidneys pass even more.

There is a limit. The day's waste needs at least about half a litre of urine to carry it, because kidneys can only concentrate urine so far. If sweat takes more than you drink, the body starts to dry out. Your urine colour is a handy guide: pale straw is good; dark yellow or amber means drink more. (Some vitamins and foods change the colour too.)

Most problems with the urinary tract are common and treatable, but they need the right help. This chapter explains them; it can't tell you what is happening in your own body. For that, see a doctor.

A urinary tract infection (UTI) happens when bacteria climb up the urethra into the bladder. It stings or burns when you pass urine, and you need to go often. Because the female urethra is short, about half of all women get one at some point. Drinking enough water and not holding on for hours help. A doctor can test the urine and treat it; fever or back pain means see one quickly.

Kidney stones are crystals that grow when urine is too concentrated. About 1 in 10 people get one, more in hot places. They hurt badly when they move down the ureter (KidneyClear has the full story).

Diarrhoea can drain water and salt fast, and it still kills about 440,000 young children a year worldwide. The fix is cheap: ORS, oral rehydration salts, a mix of salt and sugar that the gut absorbs together, pulling water in with it. Mix a packet exactly as it says, and see a doctor if a child is very sleepy, not passing urine or can't drink.

Bedwetting is normal in young children: about 1 in 6 five-year-olds, and most grow out of it. In older men the prostate often grows and squeezes the urethra, making the stream weak. And when kidneys fail, dialysis can clean the blood instead (see KidneyClear).