Why do we poop and pee? Pee is waste and extra water that your body does not need. 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.
The trip starts with chewing. Digestion begins in your mouth, and spit makes food easy to swallow. Once you begin swallowing, the process becomes automatic, so you do not have to think about it. After that, food moves along the gut by peristalsis, a wave of muscle squeezing. The muscle behind the food contracts and pushes it forward while the muscle in front relaxes to let it move.
The stomach makes acid that helps break food down. Protein digestion begins here, where acid and an enzyme called pepsin break it down. The stomach churns food with its juices into a thick mix called chyme. It is also a holding chamber: 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. After a meal the stomach slowly empties its contents into the small intestine, and it is usually empty within 2 to 4 hours. 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. 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 makes roughly half a liter to a liter of bile a day. Bile also helps color poop. The liver sends a yellow substance called bilirubin out in bile, and bilirubin helps give poop its brown color.
The small intestine takes nutrients from digested food into the blood, and it takes in water as well. Digestion is chopping nutrients into pieces small enough to cross the gut wall. 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. 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. 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. The large intestine's bacteria also make vitamins such as vitamin K that the body absorbs. A single gram of poop, about the weight of a paper clip, can hold from about 100 billion to a trillion germs.
A baby's gut community starts at birth and slowly changes. By about age two, the gut flora has gradually become a state that looks like an adult's, in step with the growth of the gut lining and its protective barrier. Some baby animals get their gut bacteria in a more surprising way. The young of elephants, giant pandas, koalas and hippos eat feces from their mothers or others in the herd to get the bacteria they need to digest the tough plants in their habitat, like borrowing a starter kit for the gut. Rabbits eat a special kind of dropping called a cecotrope: small particles of fiber move into the cecum, where microbes ferment them, and the nutrient-rich packages that result are eaten so the rabbit can absorb them. Cecotropes are different from regular feces, which are mostly indigestible parts.
The large intestine takes water out of leftover food and turns it into solid poop. Poop is more than leftover food: the waste that reaches the large intestine also includes fluid and older cells from the lining of the gut. The rectum, the last part of the large intestine, holds poop until it is time to go. 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.
You have two kidneys, one on each side of your back, just under your ribs. Each kidney is about the size of a fist. Your kidneys clean your blood, taking out waste and extra water to make pee. Blood goes into each kidney through the renal artery and comes out cleaned through the renal vein. Urine is a mix of water and wastes, and it holds high concentrations of urea. A smaller share of urea also leaves in sweat, but the urinary system does most of the work. When you are dehydrated, pee is darker and more concentrated and may smell of ammonia, which comes from urea. People's urine also carries plant nutrients. Typical design values for sewage planning are about 4 kg of nitrogen, 0.36 kg of phosphorus and 1.0 kg of potassium excreted in urine per person per year.
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. 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). Desert kangaroo rats save water with their kidneys, because they make very concentrated urine, and Merriam's kangaroo rats can get enough water from seeds without drinking.
Your kidneys filter about 150 quarts of blood in a single day, but only 1 to 2 quarts leave the body as urine. That means well over 98 percent of the fluid that is filtered is put back into the blood, so the kidneys work mostly by sorting what to keep. Kidneys 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. 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. People first sense the urge to pee when the bladder holds about 150 milliliters, but 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. 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. Researchers studied intestines from wombats that had died to learn how the cubes form. 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 wombat poop study won the 2019 Ig Nobel Prize for Physics, an award for research that makes people laugh and then think. 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.
Whale poop floats, so researchers who spot it can scoop it up with a net. Earlier research had found major nutrients, such as nitrogen and carbon, in whale poop samples. Scientists then analyzed five whale poop samples and found iron in every one, plus copper (as of February 2025). Researchers do not yet know where the whale poop ligands that make iron usable come from, but suspect bacteria in the whales' stomachs (as of February 2025).
In 1849 the London doctor John Snow published a paper arguing that cholera spread through contaminated water, not through bad air, which went against what most doctors of his day believed. After the 1854 outbreak, Snow 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.
Words to know
- peristalsis: gut squeezing waves
- microbiome: gut bacteria community
- reabsorption: taking back in
- glomerulus: kidney filter
- ligands: binding molecules
