The journey of a sandwich
Follow a bite of lunch from your teeth to the end of your gut, and meet the organs that turn it into you.
6 minutes · 8 stops
- It starts with your teeth
Your front teeth slice, your canines grip and your back teeth grind. Each bite is crushed with a force strong enough to crack a nut. Chewing does more than break food up: it mixes it with saliva so it is soft and slippery enough to swallow, and it releases flavours that tell your stomach food is on its way.
- Saliva gets to work
Three pairs of salivary glands squirt saliva into your mouth. It moistens food, starts digesting starch straight away and washes acid off your teeth. You make somewhere around a litre a day. Just smelling a sandwich can start the flow, which is why your mouth waters before you take a bite.
- A carefully timed swallow
Your tongue pushes the chewed food back into the throat, and dozens of muscles fire in a precise order. The soft palate seals off your nose and a flap called the epiglottis tips over your airway. For a split second you stop breathing so the food heads down the right pipe.
- Down the food pipe
The oesophagus doesn't rely on gravity. Waves of muscle squeeze the food down behind your windpipe and heart and through your diaphragm in a few seconds. A ring of muscle at the bottom opens to let it into the stomach, then closes to keep stomach acid from coming back up.
- The stomach churns
Your stomach is a stretchy, muscular bag. It bathes the food in strong acid and enzymes and churns it into a thick soup. A layer of mucus stops it digesting itself. Over a few hours it lets the soup out in small squirts into the small intestine.
- Bile and juices join in
In the first part of the small intestine, the duodenum, digestive juice from the pancreas and bile from the liver and gallbladder join the food. The bile breaks fat into tiny droplets, and the pancreas adds enzymes that split proteins, fats and starches into pieces small enough to absorb.
- Where your lunch becomes you
The small intestine is several metres long and lined with millions of tiny finger-like folds. This is where most nutrients pass into your blood. Sugars, amino acids and fats travel first to the liver, which processes them before they reach the rest of your body.
- The colon finishes the job
What is left passes into the colon, home to trillions of bacteria. They feed on fibre your body cannot digest, making useful chemicals as they go. The colon soaks up water, slowly turning the leftovers into stool. The whole journey usually takes between one and three days.