Gastric absorption and the protective barrier
Under normal physiological conditions, the stomach rapidly absorbs water and low-molecular-weight electrolytes. It is also characterized by a high absorption rate for alcohol and various toxic agents. However, when destructive changes occur in the gastric wall, the barrier function declines. The main consequence of this is facilitated penetration of macromolecules, particularly antigenically foreign proteins, into the body's internal environment. This inevitably triggers immunopathological processes, ranging from classic allergic reactions to severe states of immune autoaggression.
The gastric barrier consists of mucus, bicarbonates (which neutralize acid), and a strict pH gradient (from 2.0 in the lumen to 7.0 at the epithelial membranes). The barrier can be disrupted by Helicobacter pylori, ethanol, NSAIDs, bile salts, or an excess of lipid peroxidation products. Aggressive factors cause a sharp spike in hydrogen ion concentration within cells, death of epithelial cells, destruction of tight junctions, edema, and hemorrhage.
Disorders of the digestive function of the intestine
Digestive pathology occurs when there are combined or isolated disorders of the secretion of digestive glands. As a result, two main types of digestion are impaired: luminal (in the intestinal lumen) and membrane (parietal).
Key causes of disorders:
- Pancreatic achylia: loss of the exocrine function of the pancreas. It occurs due to a decrease in organ mass (pancreonecrosis, tumor, resection), duct obstruction, or their dyskinesia (smooth muscle cell spasm).
- Impaired bile secretion: deficiency of bile in the small intestine against the background of liver and biliary tract diseases.
- Decreased secretion of Brunner's glands: lack of protective mucus and bicarbonates in the duodenum. Causes include mucosal hypotrophy, wall ischemia, acute enteritis, and intoxication.
- Goblet cell dysfunction: impaired mucus secretion in the crypts and intestinal villi.
Absorption disorders and malabsorption syndrome
Disorders of luminal and membrane digestion systematically lead to the development of malabsorption syndrome. Its main threats to the body are a total deficiency of energy and substrate supply, as well as the risk of intestinal autoinfection and intoxication.
Absorption of hydrolysis products is impaired due to the following reasons:
- Digestive insufficiency: defects in nutrient breakdown.
- Accelerated transit: reduction of the contact time between chyme and the mucosa (observed in diarrhea, irritable bowel syndrome).
- Mucosal hypotrophy: physical reduction of the absorptive surface area.
- Mucosal shielding: excess inflammatory exudate or mucus prevents nutrients from contacting the villi (typical for acute intestinal infections).
- Reduction of intestinal length: consequences of surgical resection due to necrosis or tumors.
- Vascular disorders: microcirculation disturbances in the intestinal wall, including ischemia, venous hyperemia, and stasis.
Briefly: answers to topical questions
What is the mechanism of digestive impairment in bile deficiency?
The mechanism is associated with impaired delivery of bile and bile acids to the intestine. In bile deficiency, the following failures occur:
- Impaired fat emulsification — bile acids lower surface tension, break large fat droplets into smaller ones, and increase the surface area for enzyme action.
- Impaired pancreatic lipase activation — bile acids participate in the activation of pancreatic lipase together with colipase.
- Impaired mixed micelle formation — micelles ensure the transport of fatty acids, 2-monoacylglycerol, and cholesterol to the brush border of enterocytes for absorption.
As a result, the rate of fat digestion and absorption decreases, and steatorrhea develops—the excretion of undigested fats in the feces.
What coprogram parameters are characteristic of pancreatic achylia?
In pancreatic achylia associated with impaired exocrine pancreatic function, the coprogram may reveal signs of digestive enzyme insufficiency:
- Creatorrhea — the presence of undigested striated muscle fibers; indicates impaired gastric digestion or insufficiency of pancreatic enzymes involved in proteolysis.
- Steatorrhea — a high content of neutral fat and/or fatty acids; possible in pancreatic insufficiency due to impaired lipase action.
- Amylorrhea — excess starch; possible in pancreatic insufficiency due to impaired amylase action.
What mechanisms underlie vitamin B12 malabsorption?
The mechanisms and causes underlying vitamin B12 malabsorption may include:
- Endogenous hypovitaminosis — impaired intestinal absorption of cobalamins.
- Deficiency or absence of intrinsic factor (Castle factor) synthesis.
- Gastrointestinal pathology — enterocolitis, neoplasms, gastrointestinal diseases with malabsorption syndrome.
- Autoimmune mechanisms — formation of antibodies against the "vitamin B12 / intrinsic factor" complex, against Castle's intrinsic factor, and against gastric parietal cell proton pump proteins; a consequence may be hypoacid gastritis.
- Fish tapeworm infestation — competitive consumption of the vitamin.
- Severe dysbiosis.
The causes of malabsorption are easy to remember by the acronym SUPER: Sascular disorders, Ucceleration of transit, Pancreatic/digestive insufficiency, Encapsulation/shielding of the mucosa, Resection (reduction in length).
Class and exam questions
How does gastric pH change under normal and pathological conditions?
Normally, a gradient exists: the pH on the surface of the mucus layer is about 2, while in the submembranous part of the epithelium, it is above 7.0. During mechanical or chemical damage to the mucous membrane, this protective gradient is destroyed.
What does the absorption of macromolecules in the stomach lead to?
The facilitated penetration of antigenically foreign proteins through the damaged gastric wall triggers immunopathological processes. This leads to the development of allergic reactions and states of immune autoaggression.
What is pancreatic achylia and why does it occur?
This is a disorder of the exocrine function of the pancreas. It develops due to a decrease in pancreatic mass (pancreonecrosis), duct obstruction, their dyskinesia, or neurohumoral regulation disorders.
What are the risks of intestinal mucosal shielding?
In acute intestinal infections or chronic enteritis, an excess of exudate and mucus covers the intestinal surface. This physically prevents the contact of chyme with the epithelium and blocks absorption.
What else to study on this topic
- Mechanisms of development of intestinal autoinfection in malabsorption syndrome.
- The role of lipid peroxidation products in the destruction of the gastric barrier.
- Pathogenesis of biliary tract and pancreatic duct dyskinesia.
- The effect of non-steroidal anti-inflammatory drugs (NSAIDs) on epithelial tight junctions.
- Differences in pathogenesis between luminal and membrane digestion disorders.
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