Why do we poop and pee? Pee is waste and extra water that your body does not need, while poop is the leftover parts of food that your body cannot use.

Food goes in at your mouth and poop comes out at your anus, at the other end of one long tube. Digestion begins in your mouth, where spit makes food easy to swallow, and food then moves along the gut by peristalsis, a wave of muscle squeezing in which the muscle behind the food contracts and pushes it forward while the muscle in front relaxes to let it move. Once you begin swallowing, the process becomes automatic, so you do not have to think about it. The stomach makes acid that helps break food down, and protein digestion begins there, where acid and an enzyme called pepsin break it down into smaller pieces. The stomach churns food with its juices into a thick mix called chyme. It is also a holding chamber, because you can eat much faster than the small intestine can digest and absorb, so the stomach passes food on only a little at a time. Stomach enzymes that digest protein could digest the stomach itself, so a thick coat of bicarbonate-rich mucus forms a barrier that neutralizes acid.

The liver makes bile, a digestive juice that helps break down fats, and the gallbladder stores the bile between meals and squeezes it through the bile ducts into the small intestine when you eat, so the fat-digesting juice arrives just as the food does. Bile acts like dish soap on grease: its salts break fats into tiny droplets (emulsify them) so the body can digest and absorb them. The liver also sends a yellow substance called bilirubin out in bile, and bilirubin helps give poop its brown color.

Digestion is chopping nutrients into pieces small enough to cross the gut wall, so proteins break into amino acids, fats into fatty acids and glycerol, and carbohydrates into simple sugars; only then can the body absorb them for energy, growth and cell repair. Tiny bumps called villi, about 20 to 40 in every square millimeter, give the small intestine a huge surface for taking in nutrients, and the pancreas adds more than a liter of digestive juice a day, with enzymes that break down carbohydrates, fats and proteins.

Digestion has helpers that are not part of your body. The bacteria living in the gut, called gut flora or the microbiome, help with digestion, and the bacteria in the large intestine can break down food that the body's own juices could not. The human gut holds roughly 10 to 100 trillion bacteria, and some of them ferment fiber that people cannot digest on their own into short-chain fatty acids such as acetic acid and butyric acid, which the body then absorbs. In one set of experiments, rodents raised in a sterile environment with no gut flora needed to eat 30 percent more calories just to stay the same weight as normal animals, which shows how much energy the microbes help extract from food. A single gram of poop, about the weight of a paper clip, can hold from about 100 billion to a trillion germs, but researchers have linked unbalanced gut flora to many inflammatory and autoimmune conditions, and a link is a correlation, so cause and effect is still being worked out.

The large intestine takes water out of leftover food and turns it into solid poop. Poop is more than leftover food, because the waste that reaches the large intestine also includes fluid and older cells from the lining of the gut. Digestion in the large intestine is done by its bacteria, which ferment leftover carbohydrates and make vitamins such as vitamin K that the body absorbs. The rectum, the last part of the large intestine, holds poop until it is time to go, and when it is full, nerves tell you that you need to poop. Two ring-shaped muscles called anal sphincters keep the anus closed, and you can choose when to open the outer one. Your body makes pee and poop on its own, but you get to choose when to go to the bathroom.

You have two kidneys, one on each side of your back, just under your ribs, and each kidney is about the size of a fist. Your kidneys clean your blood, taking out waste and extra water to make pee, and blood goes into each kidney through the renal artery and comes out cleaned through the renal vein. Pee is made of water, salts and waste that were filtered out of the blood, and two thin tubes called ureters carry it from the kidneys to the bladder. Urine holds high concentrations of urea, and when you are dehydrated, pee is darker and more concentrated and may smell of ammonia, which comes from urea. In people, a smaller share of urea also leaves in sweat, but the urinary system does most of the work.

Each kidney holds about a million tiny filters called nephrons. In each one, a knot of blood vessels called the glomerulus lets water and small wastes slip through its thin walls while proteins and blood cells stay in the blood, because the kidney filter sorts by particle size. A tube called the tubule then pulls most of the water and the minerals you need back into the blood, so nephrons make pee in three jobs: filtering the blood, taking useful things back into the blood (reabsorption), and adding extra wastes (secretion). Your kidneys filter about 150 quarts of blood in a single day, but only 1 to 2 quarts leave the body as urine, which means well over 98 percent of the fluid that is filtered is put back into the blood. Kidneys also do more than make urine: they remove acid made by the body's cells, keep the water, salts and minerals in the blood in balance so nerves and muscles work normally, and make hormones that help control blood pressure, build red blood cells and keep bones strong.

The bladder is a hollow, muscular, balloon-shaped organ that acts like a reservoir, and an adult bladder can hold about 1.5 to 2 cups of pee. Stretch receptors in the bladder wall send nerve signals to the brain as the bladder fills, and people first sense the urge to pee when the bladder holds about 150 milliliters, although the urge is easy to ignore for a while. Two sets of sphincter muscles hold the bladder closed: the inner one works automatically, and you can squeeze the outer one on purpose. To urinate, the brain tells the sphincters to relax and tells the muscular bladder wall to tighten, which squeezes urine out through the urethra. How much urine a person makes depends on many things, including how much liquid and food they take in and how much fluid they lose through sweating and breathing, and children make less urine than adults. Most children learn to control their bladder between ages 2 and 4, each in their own time.

Wombats are the only animals known to make poop shaped like cubes, as of November 2026. One idea is that the cube shape stacks well and does not roll, so wombats can pile them up to mark territory and attract mates. Researchers studied intestines from wombats that had died to learn how the cubes form, and the cube shape forms inside the intestine, not at the anus. A wombat's intestine is stiff in some parts and soft in others, and this uneven stretchiness helps mold the poop into cubes. To test the idea, scientists built mimic intestines: simply shoving beads through did not make cubes, but contractions of stiff and soft parts produced flat sides and corners. The study won the 2019 Ig Nobel Prize for Physics, an award for research that makes people laugh and then think.

In 1849 the London doctor John Snow published a paper, On the Mode of Communication of Cholera, in which he argued that cholera spread through contaminated water, not through bad air, which went against what most doctors of his day believed. Snow found the Broad Street pump partly by talking to people: with help from a local clergyman, Reverend Henry Whitehead, he interviewed residents of the neighborhood and identified the public pump on Broad Street as the source. Most people who died had drunk from that pump, and later it was found that a leaking sewer ran near the well. After the 1854 outbreak, he marked cholera deaths around Broad Street in Soho and showed that most were clustered near one public water pump. Officials removed the pump handle, but the worst of the outbreak had already passed, so the map mattered more for the proof than for the cure.

Snow also compared water companies. Houses supplied by the Southwark and Vauxhall company, which drew water from sewage-polluted parts of the Thames, had a cholera death rate 14 times that of houses supplied by the Lambeth company, which took cleaner water from further upriver. That comparison has been described as a natural experiment, because the water companies, not the researchers, decided who got which water.

Even so, Snow did not win the argument quickly: he failed to convince many in the medical establishment, including William Farr of the General Register Office, who accepted Snow's data but not his theory, and Farr changed his mind only after the last London cholera epidemic of 1866. Before germs were known, many doctors blamed miasma, a poisonous vapour of decaying matter that was marked by its foul smell; the idea was wrong, yet it still pushed cities to remove rotting waste and sewage, so a mistaken theory led to some useful cleanups.

London's sewage problem peaked in the hot summer of 1858, when the overwhelming smell of human waste from the River Thames became known as the Great Stink, and the old sewers emptied straight into the river. Engineer Joseph Bazalgette designed the new sewer system, and building began in 1859, with most of it done in less than a decade. It replaced or upgraded the old tangle of drains and added around 1,100 miles of new sewers, plus 82 miles of large intercepting sewers. The sewers carried waste downstream so that it could be pumped into the tidal Thames, and they helped public health a great deal. Britain had a final cholera outbreak in 1866, but the parts of London that were hit were not yet connected to the new system.

Waste is not wasted for long. Dung beetles turn poop into a nursery: many feed on the microbe-rich liquid part of mammals' dung and lay their eggs in balls made mostly of the leftover fibrous material, so the waste of one animal becomes food and a home for another. Bird and bat droppings called guano show the same idea on a global scale, because they became a worldwide fertilizer thanks to being rich in nitrogen, phosphate and potassium, the nutrients plants need.

Words to know

• peristalsis: gut squeezing waves
• microbiome: gut bacteria community
• secretion: adding wastes
• nephrons: tiny kidney filters
• miasma: bad air