The domestic cat’s digestive organs quite closely resemble those of humans. Working backwards they are the rectum, large intestine (colon), small intestine, duodenum, stomach, oesophagus and the mouth. One difference between humans and cats with respect to their digestive systems is that food stays in the mouth of a cat for a much shorter time. This is probably because cats (1) sheer (slice) meat i.e. the flesh of animals, with their scissor-like molars rather than masticate (grind and crush) it and (2) inherently they eat quickly in order to preserve their prey animal against scavengers. I’m referring of course to the inherited traits of the domestic cat’s wild cat ancestor. They are still there in abundance in the domestic cat. My cat can eat a mouse in 60 seconds!
The domestic cat needs a highly-acidic stomach in order to properly digest the flesh of animals and therefore digest their food properly. Eighty percent of the gastric juices secreted in the cat’s stomach are produced because of chemoreceptors in the stomach which detect meat-based proteins. The hydrochloric acid produced keeps the pH of the stomach contents very low at around 1 to 2. This leads to highly acidic stomach contents (chyme). This is important because digestive enzymes work more efficiently in an acidic environment. Also, the acidity of the environment kills pathogens such as bacteria and fungi. This is why cats have a high immunity to diseases passed on in food.
Commercially produced dry cat food is high in carbohydrates and plant protein and low in meat protein. Therefore when the food goes into the stomach chemoreceptors do not detect meat protein and therefore less than the normal amount of hydrochloric acid is produced leaving the pH of the stomach contents at around 4 to 5. The contents are less acidic and the pH is relatively high. The acidity of the chyme leaving the stomach triggers the next stage of digestion in the small intestine. It encourages the flow of bile and pancreatic enzymes which continue to digest the food. If the pH is a little high at 4-5, digestion is impaired throughout the rest of the digestive tract, so claims Lyn Thompson BVSc DipHom on her website Feline Nutrition.org.
It seems to mean that you cannot mix a raw diet with a commercially produced dry diet because a cat will struggle to digest the raw elements of their diet. This may lead to constipation, vomiting or possibly an obstruction. They can also suffer from a gastrointestinal infection or an upset gastrointestinal tract.
A reduced acidity in the stomach, on the face of it, makes cats more vulnerable to pathogens ingested in their food. The acidity is needed to sterilise ingested pathogens.