Across the whole planet, humans eat on average between 1 and 2.7 kilograms of food a day. That’s over 365 kilograms a year per person and more than 28,800 kilograms over the course of a lifetime. And every last scrap makes its way through the digestive system.
Comprised of ten organs covering nine meters, and containing over 20 specialized cell types, this is one of the most diverse and complicated systems in the human body. Its parts continuously work in unison to fulfil a singular task: transforming the raw materials of your food into the nutrients and energy that keep you alive.
Spanning the entire length of your torso, the digestive system has four main components. First, there’s the gastrointestinal tract, a twisting channel that transports your food and has an internal surface area of between 30 and 40 square meters, enough to cover half a badminton court.
Second, there’s the pancreas, gallbladder, and liver, a trio of organs that break down food using an array of special juices. Third, the body’s enzymes, hormones, nerves and blood all work together to break down food, modulate the digestive process and deliver its final products. Finally, there’s the mesentery, a large stretch of tissue that supports and positions all your digestive organs in the abdomen, enabling them to do their jobs.
The digestive process begins before food even hits your tongue. Anticipating a tasty morsel, glands in your mouth start to pump out saliva. We produce about 1.5 litres of this liquid each day. Once inside your mouth, chewing combines with the sloshing saliva to turn food into a moist lump called the bolus.
Enzymes present in the saliva break down any starch. Then, your food finds itself at the rim of a 25-centimetre-long tube called the oesophagus, down which it must plunge to reach the stomach. Nerves in the surrounding esophagal tissue sense the bolus’s presence and trigger peristalsis, a series of defined muscular contractions. That propels the food into the stomach, where it’s left at the mercy of the muscular stomach walls, which bound the bolus, breaking it into chunks.
Hormones, secreted by cells in the lining, trigger the release of acids and enzyme-rich juices from the stomach wall that starts to dissolve the food and break down its proteins. These hormones also alert the pancreas, liver, and gallbladder to produce digestive juices and transfer bile, a yellowish-green liquid that digests fat, in preparation for the next stage.
After three hours inside the stomach, the once shapely bolus is now a frothy liquid called chyme, and it’s ready to move into the small intestine. The liver sends bile to the gallbladder, which secretes it into the first portion of the small intestine called the duodenum.
Here, it dissolves the fats floating in the slurry of chyme so they can be easily digested by the pancreatic and intestinal juices that have leached onto the scene. These enzyme-rich juices break the fat molecules down into fatty acids and glycerol for easier absorption into the body.
The enzymes also carry out the final deconstruction of proteins into amino acids and carbohydrates into glucose. This happens in the small intestine’s lower regions, the jejunum and ileum, which are coated in millions of tiny projections called villi. These create a huge surface area to maximize molecule absorption and transference into the bloodstream.
The blood takes them on the final leg of their journey to feed the body’s organs and tissues. But it’s not over quite yet. Leftover fibre, water and dead cells sloughed off during digestion make it into the large intestine, also known as the colon. The body drains out most of the remaining fluid through the intestinal wall. What’s left is a soft mass called stool.
The colon squeezes this byproduct into a pouch called the rectum, where nerves sense it expanding and tell the body when it’s time to expel the waste. The byproducts of digestion exit through the anus and the food’s long journey, typically lasting between 30 and 40 hours, is finally complete.