How do the intestines work?

After your stomach (see StomachClear) has churned a meal into a soupy paste called chyme, it lets it out a squirt at a time through a ring of muscle, the pylorus. Everything after that is the intestines. We are looking at the belly from the front, so the patient's right is on your left.

First comes the small intestine, "small" because it is narrow (about 2.5 cm across), not short. Its first 25 cm is the C-shaped duodenum, where bile from the liver and juices from the pancreas pour in. Then comes the jejunum (about two-fifths of the rest, wider and redder) and the ileum (the last three-fifths). In a living person the small intestine is about 3 to 5 metres long. You may read "6 to 7 metres": that figure comes from bodies after death, when the muscle in the wall has relaxed and the tube stretches.

The coils are not loose. A fan-shaped, fatty sheet called the mesentery ties them to the back wall and carries their blood vessels. Its root is only about 15 cm long, but its free edge is metres long, so it is pleated like a frilly skirt.

At the lower right, the ileum joins the large intestine through the ileocaecal valve, a one-way door. Below it is a pouch, the caecum, with the finger-like appendix. The colon climbs up (ascending), crosses under the liver and stomach (transverse), comes down the left side (descending), makes an S (sigmoid) and ends in the rectum. It is about 1.5 m long and about 6 cm wide, with pouches called haustra.

Food can only get into your body by crossing the lining of the small intestine. The more lining there is, the faster that happens. So the gut folds its lining three times over.

First, the inside is ridged with circular folds (plicae circulares), which about 1.6× the area. The folds are covered in millions of tiny fingers, villi, each 0.5 to 1 mm tall, which add about 6.5×. Between them are little pits, the crypts, where new lining cells are made. Each lining cell is topped with a fuzz of microvilli, the brush border, which adds about 13× more.

Together that is about 30 m², roughly half a badminton court. Many books used to say "a tennis court" (about 260 m²). In 2014 Swedish researchers Herbert Helander and Lars Fändriks measured it carefully and found the old figure was far too big. Still, 30 m² packed into a tube you could hold in two hands is impressive.

Inside each villus are tiny blood vessels, capillaries, which take sugars and amino acids to the liver (see LiverClear), and a central lymph vessel, the lacteal, which takes fats. How the molecules actually get across depends on osmosis and diffusion (see OsmosisClear).

The wall of the intestine has two layers of muscle: an inner ring of circular muscle that squeezes the tube narrower, and an outer layer of longitudinal muscle that shortens it. You never have to think about them. A network of nerves in the gut wall, the enteric nervous system, runs them by itself. It talks to your brain in both directions, which is why nerves and stress can upset your stomach: the gut–brain axis (see BrainClear).

Peristalsis is a travelling squeeze. Muscle contracts just behind the food and relaxes just in front, so the food is pushed forwards, like toothpaste squeezed along its tube. Each wave moves at about 0.5 to 2 cm a second, but only for a few centimetres, so chyme creeps along at about 1 cm a minute. That is why it takes roughly 2 to 6 hours to get through the small intestine. The colon is slower still: often 12 to 48 hours or more.

Segmentation is mixing. Rings contract, then relax while the rings between them contract, about 12 times a minute near the start and 8 or 9 times near the end. The chyme sloshes back and forth, mixing with juices and touching the lining again and again.

Between meals, the gut sweeps itself clean. Every 90 to 120 minutes a strong wave, the migrating motor complex, travels all the way down the small intestine and pushes out leftovers and bacteria. Pushing gas and fluid makes noise: the rumble you hear when you are hungry.

Food is made of molecules far too big to get through the gut lining. Digestion cuts them into small pieces, mostly in the first part of the small intestine, the duodenum.

Two juices arrive there through one small opening. Bile, made by the liver and stored in the gallbladder (see LiverClear), works like washing-up liquid: it breaks big blobs of fat into tiny droplets so enzymes can reach them. Pancreatic juice from the pancreas (see PancreasClear) carries enzymes, the molecular scissors, plus bicarbonate, which neutralises the stomach acid. Each makes about a litre a day.

The scissors work in two steps. Pancreatic enzymes do the rough cutting: amylase chops starch, trypsin and others chop proteins, and lipase chops fats. Then enzymes fixed on the villi, in the brush border, do the fine cutting. In the end, carbohydrates become sugars like glucose, proteins become amino acids, and fats become fatty acids and monoglycerides.

Most of this is absorbed in the duodenum and jejunum. A few things wait for the end: vitamin B12 and bile salts are taken up in the last part of the ileum, and fibre is not digested at all, but goes on to the colon.

By the time chyme reaches the colon, most of the nutrients are gone. What is left is water, salts, fibre and a few other leftovers. The colon has two big jobs: save water and house bacteria.

Water. About 8 to 9 litres of water pass into your gut each day: what you drink plus litres of juices. The small intestine takes most of it back. About 1.5 litres reaches the colon, and the colon absorbs nearly all of that, leaving only about 100 mL in the stool. That is why the contents go from runny on the right side to firm on the left.

Bacteria. Your colon is home to about 38 trillion bacteria, from hundreds of species. You may have read that bacteria outnumber your own cells ten to one. A careful count in 2016 by Ron Sender, Shai Fuchs and Ron Milo found the real ratio is closer to 1.3 to 1: about as many bacteria as human cells.

You can't digest fibre, but many of these bacteria can. They ferment it into short-chain fatty acids such as butyrate, the favourite fuel of your colon's lining cells. Gut bacteria also make vitamin K and some B vitamins, crowd out harmful germs, and help train your immune system. Scientists are still working out how they talk to the rest of the body, including the brain (see BrainClear).

This chapter shares general facts, not medical advice. If you are worried about your own gut, talk to a doctor.

Fibre and water. Fibre from whole grains, dal, vegetables and fruit keeps stool soft and bulky and feeds your gut bacteria. The WHO advises adults to eat at least 25 g a day. Enough water helps too.

Diarrhoea is when the gut pours out more water than it takes back, often because of an infection. The danger is dehydration: losing water and salts. It still kills about 440,000 children under five each year. The fix is surprisingly simple: oral rehydration solution (ORS), a measured mix of clean water, salt and sugar. Sugar and sodium are carried into the lining cells together, and water follows them back in, even during cholera. ORS was proven in the 1960s and 70s in Dhaka and Kolkata, and in 1971 the Indian doctor Dilip Mahalanabis used it to save thousands of refugees in camps in West Bengal. The Lancet later called it potentially the most important medical advance of the 20th century. Zinc helps too. A doctor should see anyone very drowsy, not drinking, with blood in their stool, or a baby with diarrhoea.

Lactose intolerance. Milk sugar, lactose, needs the enzyme lactase. Most people worldwide make less of it after childhood, which is normal. Undigested lactose goes to the colon, where bacteria ferment it, and that can cause gas, bloating and loose stools.

Coeliac disease (about 1 in 100 people) is an immune reaction to gluten, a protein in wheat, barley and rye, that flattens the villi. Appendicitis is an inflamed appendix: pain often starts near the navel and moves to the lower right. It needs a doctor urgently. Probiotics (live "good" bacteria) are widely sold; the evidence varies a lot by product and condition, and many claims are not proven.