Physiology of digestion

Basic concepts: from process to system

The work of the gastrointestinal tract is based on a physiological process—a strictly verified sequence of actions that is always aimed at a specific result. The specific activity of any structure is called a function. Four main functions are characteristic of the digestive tract:

• Secretory
• Motor
• Absorptive
• Excretory

Organs and tissues that are hereditarily fixed to perform a common task form a physiological system. For example, the digestive system anatomically unites the structures responsible for food processing, just as the respiratory system is responsible for gas exchange.

Functional systems according to Anokhin

Soviet physiologist P.K. Anokhin introduced a broader concept—the functional system. This is a temporary alliance of organs and tissues from completely different anatomical groups created to maintain vital constants.

Let us consider this using the functional system of nutrition as an example. Its main task is to maintain a stable level of glucose, fatty acids, and amino acids in the blood. As soon as these parameters deviate from the norm, the receptor apparatus catches the changes and transmits a signal to the nerve center. Next, through mechanisms of higher nervous activity, as well as neural and humoral regulation, a cascade of reactions is triggered. The digestive, circulatory, and excretory systems are simultaneously involved in this process.

Key properties of such systems:

1. Dynamism—the same organ can participate in the work of different systems.
2. Self-regulation—as soon as the nutrient level returns to normal, the system disintegrates.
3. Feedback—the nerve center constantly receives information on whether the result has been achieved or not.

Lipid digestion and pharmacological support

The digestion, absorption, and transport of exogenous fats occur predominantly in the small intestine and represent a multi-stage process. In various pathologies (chronic pancreatitis, gastritis, hepatitis) or dietary errors, this mechanism malfunctions.

To compensate for disorders, drugs that mimic natural physiology are used, for example, double-layer dragees. Their structure replicates the stages of the GI tract:

• Outer layer dissolves in the acidic environment of the stomach. It contains extracts of the gastric mucosa and bile, amino acids, and pancreatin.
• Inner layer is covered with an acid-resistant coating. It reaches the intestine, where it releases enzymes (lipase, amylase, proteases) and bile extract, ensuring full breakdown of nutrients.

Age-related features and GI dysfunctions

The work of the digestive system heavily depends on age. In infants, functional disorders are most often associated with feeding errors and anatomical and physiological immaturity. They exhibit physiological hypochlorhydria (decreased gastric barrier function) and a deficiency of secretory IgA on the mucous membranes. This makes young children particularly susceptible to intestinal infections and secondary enzymopathies.

In older children, dietary routine disturbances come to the fore, along with increasing physical and psycho-emotional overloads. Gastrointestinal dysfunctions often occur against the background of comorbid central nervous system pathologies.

The multifactorial nature of functional dyspepsia includes:

• Gastric motility disorders.
• Visceral hypersensitivity.
• Genetic predisposition.
• Low-grade inflammation of the upper GI mucosa (after infections or due to food allergy).

Interestingly, the presence of Helicobacter pylori without duodenal ulcer disease does not provoke dyspepsia by itself. However, biliary tract dysfunctions, in which bile enters the intestine asynchronously, inevitably disrupt digestion and cause pathological refluxes (duodenogastric and gastroesophageal).

Class and exam questions

What else to study on this topic

• Phases of digestion, absorption, and transport of exogenous fats in the small intestine.
• Neural and humoral regulation in the functional system of nutrition.
• Pathogenesis of duodenogastric and gastroesophageal refluxes.
• The role of visceral hypersensitivity in the development of dyspepsia.
• The impact of comorbid CNS pathology on GI motor function.

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.