Digestive Dive: Gut Expedition

A nod to Scuba Dive (Durell Software, 1983). Three of the six stages are caves to explore as in the original: dive from the surface, fetch something from near the bottom, bring it back up and come up for air. The other three are fast rides down the gut. At each stage you’re someone new: a nano-diver, a bolus of food, a drop of chyme, a gut bacterium and, at the end, a toy brick that a child swallowed.

The science

• Mouth: saliva’s amylase starts cutting starch into maltose. Streptococcus mutans turns sugar into acid that dissolves tooth enamel, and fluoride helps enamel repair itself and resist acid. Near the throat, Streptococcus pyogenes (strep throat) meets the tonsils. When you swallow, the epiglottis covers the windpipe.
• Oesophagus: the bolus is carried down by peristalsis, and the lower oesophageal sphincter keeps stomach acid out (when it doesn’t, that’s heartburn).
• Stomach: parietal cells make hydrochloric acid, which switches on pepsin, the enzyme that cuts proteins. Helicobacter pylori survives the acid; Barry Marshall and Robin Warren, in Perth, showed that it causes most stomach ulcers. You help an endoscope take out two magnets and a button battery: a swallowed button battery, or more than one magnet, is an emergency. Then the pylorus lets chyme out in short squirts.
• Small intestine: in the duodenum, bicarbonate from the pancreas neutralises the acid, bile from the liver (stored in the gallbladder) breaks fat into droplets for lipase, and trypsin carries on with proteins. In the jejunum, lactase, on the microvilli of the villi, splits lactose into glucose and galactose, but only so fast: this is why many people with lactose intolerance are fine with a small portion at a time. Lactose you don’t digest goes on to the colon, where bacteria turn it into gas (and in the game, into extra gas bubbles). The ileum takes up vitamin B12, and the ileocaecal valve leads on.
• Caecum and appendix: you’re Roseburia, one of the gut bacteria that ferment fibre into short-chain fatty acids such as butyrate, the main fuel of the cells lining the colon. Clostridioides difficile can take over after antibiotics, and Salmonella causes food poisoning. The appendix may keep a reserve of good bacteria, which gives you an extra life.
• Colon: it takes back water, so things get firmer and the way narrower. Goblet cells make mucus, the pouches (haustra) squeeze, mass movements push everything along, and some bacteria make vitamin K. At the rectum, the internal sphincter opens by itself, and the external one waits until the child has found a toilet.

Most small, smooth objects that children swallow pass through on their own in a few days. A button battery, or more than one magnet, is the exception: go to hospital straight away.

Where the game cuts corners

• There are no nano-divers (yet). Doctors do take swallowed objects out with an endoscope, but they steer it themselves, from outside, while the child is asleep under anaesthetic.
• The trip is squeezed into a few minutes. In life, food spends a few hours in the stomach and one to three days on the whole journey.
• Chyme doesn’t go looking for bicarbonate, bile or enzymes: they pour into the duodenum and the gut’s squeezing mixes them in. Lactase isn’t used up either; it can only split lactose so fast.
• Pepsin doesn’t chase anyone. It just floats about cutting proteins, which is scary enough.