How does your stomach work?

Michigan’s Mackinac Island in the early nineteenth century didn’t seem like a very likely place for someone to discover how digestion worked, but that’s exactly where Army surgeon Dr. William Beaumont found himself when he made such a discovery in 1822.

Beaumont was caring for young French-Canadian fur trader Alexis St. Martin who had a gaping hole in his side after suffering an accidental gunshot. Beaumont patched him up as best as he could, but he didn’t think St. Martin would live.

St. Martin surprised everyone though; not only did he live, but his stomach, which had also been hit, fused with his skin to create a permanent opening (or fistula) from outside of his body directly into his stomach.

Beaumont realized that he had something unprecedented on his hands. Up to that point, no one really knew how digestion happened; with this patient, the doctor had a human guinea pig where he could literally see digestion happening before his eyes. He tricked St. Martin into signing an agreement to be his servant for many years, during which time he also experimented on him. He put different types of food into St. Martin’s stomach for different lengths of time to see what happened. He collected St. Martin’s gastric juices to study them.

The results of Beaumont’s studies clarified exactly how a stomach works, but it came at a great cost for St. Martin. When the patient died in 1880, his family even let his body partially decompose before they buried it, lest any more scientists come to poke and prod their loved one.

How Stomach Digestion Works

The first thing that a bite of swallowed food (called a bolus) encounters when it reaches the stomach is the cardiac sphincter. This tight, circular muscle closes off the top of the stomach like a purse string and keeps in gastric acid.

The cardiac sphincter relaxes to allow the bolus to pass in and then closes up tightly behind it. If you’ve ever suffered from heartburn after eating, you know exactly how painful it can be if the cardiac sphincter doesn’t close all the way and lets acid into your esophagus!

The stomach itself is basically a big muscle bag. There are three separate layers of muscles, each of which contract in a different direction, smashing and mixing your food with gastric juice into a soft, pulpy mess called chyme. By turning food into chyme, the surface area of your food is drastically increased, allowing digestive enzymes to get into all the nooks and crannies of your food and digest it more fully.

After the chyme is fully mixed together, your stomach slowly releases it through another purse-string muscle—the pyloric sphincter, at the end of your stomach. Don’t fear, though; even though the mixture is very acidic and could normally burn your tissues, it’s neutralized by a squirt of bicarbonate ions (produced by your pancreas) as it enters the small intestine.

It’s the Pits: Gastric Pits in the Stomach

But where does the acid come from, and how does it not digest your stomach itself?

The answer is the pits—the gastric pits. These deep, microscopic tunnels go all the way into the lining of your stomach. At the end of each tunnel is a cluster of three types of cells that produce gastric juice.

Parietal cells are responsible for producing the hydrochloric acid for which gastric juice is so well-known. Chief cells produce pepsinogen—an inactive form of pepsin that only works once it comes into contact with hydrochloric acid. Mucus neck cells (a horrible name for anything, by the way) produce a lot of mucus that coats and protects the lining of your stomach. Even with this protection, though, the life of a cell lining the stomach is rough: they generally live for about 5 days only. To compensate, your stomach is constantly growing new cells.

What does gastric juice do?

Gastric juice seems like an odd thing for the human digestive system to tolerate. Why bother with a dangerous acidic cocktail that often backfires on unsuspecting victims?

Gastric juice (and its potency) is important for three main reasons. First, it starts the process of digesting protein, which is one of the main types of macromolecules needed by your body. The acid itself starts to unravel some of the tightly-wound protein molecules (a process called denaturation) in your food so that digestive enzymes can get in and start breaking it down. Acid also activates the pepsinogen into pepsin; once the long chains of proteins are unwound, the pepsin can get in and start slicing and dicing the bonds holding the protein together.

Second, the gastric juice also kills any pathogens (bacteria, viruses, and fungi) that may be in your food. It’s not a perfect system, though, which is why sometimes we still get food sickness. But it would probably be a lot worse without the acid bath.

Finally, gastric juice is important because it lubricates your food and makes it easier to flow down the rest of your digestive tract where digestive enzymes can get in and digest your food more fully. You don’t want to get plugged up!

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Written by Lindsay VanSomeren

Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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