" first part
Gastric acid-peptic secretion: the gastric mucosa secretes a variable quantity between 500 and 3000 ml of gastric juice daily. It consists of mucus, water, electrolytes (among which hydrogen and chlorine prevail), pepsinogen and intrinsic factor (a molecule essential for the absorption of vitamin B12, essential for the synthesis of blood cells); gastric juice contributes in a way determinant to two important functions: the gastric acid barrier and the digestion. The secretory activity of the stomach is regulated by mechanisms of stimulation and inhibition. L "histamine exerts a powerful stimulating effect on acid secretion, mediated by receptors for the same present on acid-secreting cells. There are also other factors that can stimulate gastric acid secretory activity; these include hypoglycemia and the administration of insulin (mediated by the resulting hypoglycemia).alcohol and caffeine; the latter act directly on the mucosa.
Acid secretion is inhibited by Gastric Inhibitory Peptide(GIP) and other hormones produced by the duodenal and intestinal mucosa. The total quantity of hydrochloric acid secreted under stimulation conditions is directly proportional to the number of parietal cells present in the stomach; this explains the effect of net reduction of acid secretion obtained with gastric resection.
Stimulation of gastric acid secretion occurs in three distinct phases: cephalic, gastric and intestinal, combined with each other.
In cephalic phase Vagus nerve fibers are excited by visual or olfactory stimulation or by the simple thought of food. They, in turn, stimulate the parietal cells, the main cells and the cells of the pyloric antrum to secrete hydrochloric acid, pepsinogens and gastrin, respectively.
There gastric phase it is constituted by the secretion of acid determined by the entry of the alimentary bolus into the stomach and is probably regulated by gastrin.
There intestinal phase of acid secretion is always attributable to intestinal gastrin, but is considerably less important than the cephalic and gastric phase. L"inhibition of acid secretion it also includes three phases: one brain stage in which vagal stimulation, visual or olfactory, decrease; a antral phase in which the decrease of the pH in the antrum determines the inhibition of the liberation of gastrin; an intestinal phase, in which food-induced distension of the small intestine evokes an inhibitory reflex. Fats, carbohydrates and acidity in the duodenum inhibit acid secretion. The evaluation of gastric acid secretion is performed by aspiration of gastric juice in the fasting state in conditions of rest (basal) and after stimulation. It is an important test for diagnosing Zollinger-Ellison syndrome and acid hyper-secretion states.
Hormonal secretion: numerous hormones are secreted in the stomach and duodenum, with multiple effects on gastric, biliary and pancreatic secretion, gastro-intestinal motility, metabolism and the increase of other hormones.
Motility: when the food bolus enters the stomach, a relaxation of the gastric wall occurs, and this allows a considerable increase in the volume of the stomach, triggering peristaltic contractions (which allow the progression of the contents) which, in turn, determine the mixing and the fragmentation of solid foods until they are reduced to particles of a few millimeters in size. The pyloric sphincter opens periodically to allow the passage of fluids and small semisolid boluses. Immediately after these boluses pass, the pylorus closes and prevents reflux from the duodenum. The antrum, the pylorus and the first portion of the duodenum function in a coordinated way, so that their subsequent contraction determines the progression of the food bolus. Gastric emptying is regulated by many factors, such as vagal tone, distension of the duodenum, the amount of sugars and salts of the chyme that reaches the duodenum (the ex bolus) and its acidity, the protein and lipid content of the same chyme, the degree of continence of the pyloric sphincter.
The rate of gastric emptying largely depends on the nature of the food ingested. A food composition rich in carbohydrates determines a rapid gastric emptying, while the emptying is slower in the case of a diet rich in proteins and the emptying is even slower in the case of ingestion of fats.
Liquids are emptied more quickly (about 15 minutes), digestible solids more slowly (1/2 hour-2 hours depending on their composition), non-digestible foods, for example fibers, only after many hours. Fats, on the other hand, have a "braking" effect on gastric emptying. The surgical section of the vagus nerve causes a slowdown in the emptying of solids from the stomach due to a decrease in gastric peristalsis and decreased relaxation of the pyloric sphincter.
Digestion: an important phase of digestion takes place in the stomach, by the gastric juice containing hydrochloric acid and enzymes such as pepsinogens, gastric lipase and other important enzymes.
The hydrochloric acid secreted by the stomach is important for digestive purposes not only because it determines the acid pH essential for the activation of the pepsinogen, but also because it reduces the trivalent iron (ferric ion) into divalent iron (ferrous ion), so that this can be absorbed much more efficiently, in the reduced form, by the duodenal and jejunal mucosa.
The juice secreted by the duodenum, on the other hand, has a rather modest digestive function; its main action is to protect the duodenal mucosa from the acidity of the chyme and to prepare a suitable environment for the action of digestive enzymes. In the intermediate portion of the duodenum, pancreatic juice and bile also flow, which, in addition to carrying out digestive functions, help neutralize gastric acidity.
Other functions: at the gastric level occurs the "absorption of some substances such as water, ethyl alcohol and some drugs. In the duodenum the absorption of simple carbohydrates, numerous drugs and iron takes place. Finally, the stomach also has the function of control of bacterial growth present in food; these are in fact for the most part killed by the acidity of the gastric juice.
The increase in gastric pH, which can occur in the course of a decrease or absence of acid secretion, atrophic gastritis, therapy with anti-acid drugs and after surgical resection of the stomach, causes an increase in the gastric bacterial flora; this phenomenon can lead to to an increase in infections (gastro-intestinal and distant) and can determine the formation of nitrosamines, which are carcinogenic substances that derive from the ingestion of foods containing nitrites and nitrates by the bacteria themselves.
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