The Digestive System

Your digestive system is the long tube and the helper organs that turn the food you eat into the simple chemicals your cells can use. The whole digestive tract, from mouth to bottom, is about 9 metres long if stretched out, and food typically takes around 24 to 72 hours to make the full journey. Along the way, food is mashed, dissolved, attacked by acids and enzymes, and finally absorbed into the bloodstream. Anything your body cannot use is sent on its way out the other end.

  • Total lengthapprox. 9 metresFrom mouth to bottom
  • Food transit time24 to 72 hoursDepends on food and person
  • Stomach capacityapprox. 1 litre normallyCan stretch to approx. 4 litres after a big meal
  • Stomach acid pHapprox. 1.5 to 3.5Strong enough to dissolve metal
  • Small intestine surface areaapprox. 250 m²A whole tennis court, from the folds
  • Gut bacteriaapprox. 38 trillionAlmost as many as your own cells

The journey through the digestive system

Food goes on a long journey through your body.

  1. Mouth: teeth grind food; saliva starts breaking down starches.
  2. Oesophagus: a muscular tube that squeezes food down to the stomach.
  3. Stomach: stores food and mixes it with strong acid and enzymes for several hours.
  4. Small intestine: where most digestion happens and most nutrients are absorbed into the blood. About 6 metres long, but folded into your abdomen.
  5. Large intestine (colon): absorbs water and forms solid waste. About 1.5 metres long.
  6. Rectum and anus: store and finally pass out the leftover waste.

Two important helper organs work alongside the main tube:

  • Liver: produces bile (which breaks up fat) and processes nutrients from the blood.
  • Pancreas: produces strong digestive enzymes and the hormone insulin (which controls blood sugar).

The stomach: chemistry lab

Your stomach is a muscular sac that can hold about 1 litre normally and stretch to 4 litres after a really big meal. It does three jobs:

  1. Stores food while the rest of your digestive system catches up.
  2. Mixes food with strong stomach acid (mostly hydrochloric acid, pH 1.5 to 3.5) and digestive enzymes like pepsin, which start breaking down proteins.
  3. Kills most of the bacteria that come in with food.

Stomach acid is strong enough to dissolve metal. The stomach itself is protected by a thick layer of mucus that gets renewed every few days; without it, the acid would dissolve the stomach itself. When the protection breaks down (often due to bacteria called Helicobacter pylori), the result can be a painful stomach ulcer.

The small intestine: where the real digestion happens

The small intestine is the longest section of your digestive tract: about 6 metres long, folded into your abdomen. Despite the name, "small" refers only to its diameter (about 2.5 cm) compared to the large intestine.

The inside surface of the small intestine is not smooth: it is covered in millions of tiny finger-like projections called villi, which in turn are covered in even smaller projections called microvilli. Together they multiply the inner surface area to around 250 square metres, the size of a tennis court. This huge surface lets the small intestine quickly absorb almost all the nutrients from your food into your bloodstream.

The large intestine: water reclamation

By the time food reaches the large intestine, most of the useful nutrients have already been absorbed. The leftover material is mostly water, indigestible fibre and waste. The main job of the large intestine is to reclaim the water: it pulls water back into your bloodstream over several hours, leaving the rest as solid stool. The whole process takes around 12 to 36 hours and helps prevent dehydration.

The large intestine is also home to your gut bacteria: trillions of friendly microbes that live in your colon. They help digest fibres your own enzymes cannot break down, produce certain vitamins, and crowd out harmful bacteria.

Fact Your gut contains around 38 trillion bacteria, almost as many as your own human cells. Together they make up about 1 to 2 kg of your body weight, roughly the same as your liver. Different people have very different gut microbiomes, and the differences may affect everything from your weight and digestion to your immune system and even your mood. Modern medicine is just beginning to take seriously the idea that "you" are really a partnership between human cells and an enormous community of microbes.

The liver and pancreas

Two important helper organs work alongside the main digestive tube.

  • Liver: the largest internal organ. Produces bile (a green fluid stored in the gallbladder and used to break up fats in the small intestine), processes all the nutrients absorbed from food, stores energy as glycogen, breaks down old red blood cells, and detoxifies harmful chemicals like alcohol and many drugs.
  • Pancreas: sits behind the stomach. Produces strong digestive enzymes that finish the job of breaking down proteins, fats and carbohydrates in the small intestine. Also produces the hormone insulin, which controls how much sugar is in your blood.
Did you know? The liver is the only major human organ that can regrow itself. If you lose up to 75% of your liver to surgery or injury, the remaining tissue can grow back to roughly full size within just a few weeks. This regenerative power makes liver transplants from living donors possible: a portion of a healthy person's liver can be transplanted into a patient who needs one, and both halves will grow back into nearly full-sized livers within months.
Deeper dive: your microbiome and your gut-brain axis

One of the most exciting areas of modern biology is the study of the microbiome: the trillions of bacteria, fungi, viruses and other tiny organisms that live in and on your body. Most of them are in your gut. Together they outnumber your own human cells, and we are only just beginning to understand what they do.

What we know so far:

  • Different people have very different microbiomes, shaped by diet, illness, antibiotics, lifestyle and even how you were born (babies born by C-section have different gut bacteria from babies born vaginally).
  • The microbiome plays a major role in digestion: gut bacteria break down fibres your own enzymes cannot handle, and they produce vitamins B and K that your body absorbs.
  • It helps train your immune system: babies who grow up in too-clean environments tend to have more allergies and autoimmune conditions.
  • It influences your weight: some studies have shown that transferring gut bacteria from obese mice to thin mice can make the thin mice gain weight even on the same diet.
  • It has a surprisingly strong connection to your brain: signals travel back and forth between your gut and brain through the vagus nerve, and your gut bacteria produce many chemicals that can affect mood and behaviour. This is called the gut-brain axis. Some recent studies suggest gut bacteria may even play a role in conditions like anxiety and depression.

Modern medicine is now exploring how to deliberately reshape the microbiome to treat disease. Faecal transplants (yes, exactly what they sound like) are remarkably effective for some types of stubborn gut infection. Probiotic foods and supplements may help maintain a healthy microbiome, though the science is still developing. The "second brain" in your belly is one of the most exciting frontiers in modern biology.

For the bacteria that live in your gut, see single-celled organisms. For how nutrients get to your cells, see the circulatory system.