eli5: How did scientists meticulously unravel the chemical processes of food digestion within the closed system of live human beings?
In: Biology
Going through the entire digestive systems a bit much, but here’s the story of gastric acid specifically:
Gastric acid was discovered around the late 1700-early 1800s due to the work of various scientists, particularly one called Spallanzani who was *super* into making birds vomit up sponges and examining the fluids squeezed out. Heres a quote from John Young, an American scientist:
>”A frog was kept starving for two days; a piece of litmus was then forced into its empty stomach, and by means of a pair of forceps; upon being drawn out it was covered with gastric juice, and the litmus turned red… I took some meat on an empty stomach: in half an hour aftterwards by irritating my fauces, the meat was thrown up, and with it some gastric fluid; upon being tested, an acid was very evidently present. We have little hesitation therefore, in saying that the acid so constantly found in the stomach of man and almost (probably) all animals, is to be referred to their gastric fluid.”
Gastric acid was later confirmed to be hydrochloric acid via the works of William Prout, who experimented on rabbits, hares, horses, calves, and dogs. How the acid is made remains a mystery to him; he thinks it’s due to electricity.
A famous medic called William Beaumont to who we owe a lot of the understanding of the stomach had a patient, Alexis St. Martin, who had a hole blown into his stomach by a shotgun but survived, with a permanent open wound, through which Beaumont would put foods, examine the stomach lining, extract juices, etc. and generally just treat the man as a living experiment.
Pavlov (yes, that Pavlov) later proved that gastric acid secretion began as soon as food was *anticipated* by surgically making the same sort of holes in living dogs, and contributed a lot to us understanding at digestion has a nervous system component.
A man called Edkins then discovered Gastrin by observing how flushing the stomachs of anaethetised cats that had their vagus nerve severed with an extract from the pylorus of the stomach (which is where gastrin is made) caused gastric acid to be secreted. Another research team confirmed this further by removing the pyloric antrum and observing how gastric acid secretion was reduced.
The role of histamine then started to be uncovered by L. Popielski, who found injections of histamine caused increases in gastric acid secretion. Eventually a team at what would later become part of GlaxoSmithKline built on that knowledge to develop the first antihistamine for treating dyspepsia.
An then, in the 1970s, the ATP dependent proton pump responsible for generating gastric acid is discovered by studying the gastric mucosa extracts of pigs, and William Prouts ghost can finally rest. We put this knowledge almost immediately to use to develop proton pump inhibitors and after centuries of research we can finally treat heartburn.
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