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I read in my book that during digestion our stomach secretes hydrochloric acid ($HCl$). Why we do not get hurt from it? It was also said that mucous provides protection, but from where is it coming?

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    $\begingroup$ Great answers, but it also is important to note that our HCl in the stomach acid has a low concentration as well, but high enough to kill many pathogens, in particular, bacteria. Also, some of our epithelial cells are destroyed, but the epithelial cells have the highest cell turnover rate in our body, only about one day... $\endgroup$ – TanMath Aug 22 '15 at 22:32
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Goblet cells.

These are specialized epithelial cells whose job it is to produce mucins. Mucous role is not only important in protecting the epithelium of the digestive tract from HCl, it is also a very important substance in the innate immune system, acting as a barrier defense to trap pathogens and/or prevent them from breaching the epithelium and gaining access to sterile parts of the body.

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Our stomachs are lined with a special mucal layer which protects the organ from its own secretions. People with stomach ulcers have a hole in this layer, allowing the acid to get to the skin underneath. HCl is present in our stomach to aid digestion - especially with dismantaling the bolus (food) coming from the oesophagus. The HCl separates the larger bolus to more smaller pieces. The stomach has a mucus which protects the tissues from being damaged. People with gastric problems may not have a sufficient layer of mucus to protect their tissue and the pain is from the acid eating through the tissue. Thus, a common solution for people who have Gastric problems is to use tablets which neutralizes the acid so that it is weaker.

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    $\begingroup$ The idea of acid causing ulcers was disproven by Dr. Barry Marshall, for which he won a Nobel Price in Medicine and Physiology, and is now known to be cause by inflammation due to an H.pylori infection. $\endgroup$ – AMR Aug 22 '15 at 18:57
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    $\begingroup$ @AMR, afaik Marshall disproved the idea that ulcers are caused by stress, spicy food, smoking, etc. Actually, H.pilory disrupts the mucus at some regions of stomach, thereby enabling the acid to get through and damage the inner layer of stomach. $\endgroup$ – Ivan Aug 22 '15 at 19:48
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    $\begingroup$ @Ivan if you read my comment, I say "causing ulcers." The causative agent for an ulcer in 90% of the cases is an H.pylori infection and the damage done to the endothelial cells by the enzymes produced by these pathogens. Acids will aggravate the ulcer, but does not cause it. When you read the list of substances that Marshall disproved, acid is one of those substances. In fact much of the gastritis that occurred when he infected himself with H.pylori was in the absence of acid. $\endgroup$ – AMR Aug 22 '15 at 21:29
  • $\begingroup$ actually the ulcers are caused.. when say.. we didn't have food for long time, then the acid present in the stomach needs to dissolve some materials as, food is not there it starts to effect the layering of the stomach and thus, some kind of blisters and boils.. appears on the layering of stomach causes ulcer. $\endgroup$ – Ashwin.N Aug 23 '15 at 7:35
  • $\begingroup$ @Ashwin.N no, that is incorrect. If you read the previous comments, you'd have seen that this has been disproved. Acid on its own does not cause ulcers - H. pylori does. $\endgroup$ – MattDMo Aug 23 '15 at 13:33
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Simply, it's because of the mucous layer around the stomach, that prevents the strong acid, Hcl to hurt our stomach. Not only this, if there were no mucous, the Hcl would digest the protein present in the epithelial cells, so the stomach itself would get digested. So the main protagonist here is the mucous layer.

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