Gastrointestinal Absorption

Absorption in Different Segments of the Digestive Tract

The rate of substance transfer varies significantly depending on the segment of the gastrointestinal tract.

• Oral Cavity. Absorption here is negligible. This is because food remains in the mouth for a very short time and is not yet broken down into monomers. Nevertheless, certain pharmacological agents can penetrate the mucosa extremely rapidly, which is widely utilized clinically for sublingual administration.
• Stomach. Absorption is minimal. Only water with dissolved mineral salts, glucose, and certain amino acids pass into the bloodstream. However, ethanol (alcohol) solutions are absorbed exceptionally well in the stomach.
• Small Intestine. This is the primary site for the absorption of water, electrolytes, and nutrients. This process is inextricably linked to the hydrolysis of nutrients. Massive efficiency is achieved through the enormous surface area of the mucous membrane. There are 30–40 villi per square millimeter of mucosa, and the apical membrane of each enterocyte bears 1,700 to 4,000 microvilli. Consequently, there are 50–100 million microvilli per mm² of epithelium. Given that the body contains approximately $10^{10}$ absorptive cells and $10^{15}$ somatic cells, a single intestinal cell provides nutrition for one hundred thousand other body cells.
• Large Intestine. The primary function of this segment is the reabsorption of water and the formation of solid fecal mass. Glucose, amino acids, and other readily absorbable substances can also be assimilated here in small quantities. This is clinically important as it makes the use of nutrient enemas feasible.

Cellular Transport Mechanisms

The transport of substances across enterocyte membranes occurs via several fundamentally distinct pathways.

1. Passive Transport. Occurs without the expenditure of cellular energy. It includes simple diffusion (driven by the concentration gradient of the solute), osmosis (movement of the solvent molecules themselves), and filtration (transfer of solution through pores driven by hydrostatic pressure).
2. Facilitated Diffusion. Also proceeds without energy expenditure and strictly down a concentration gradient, but, unlike simple diffusion, requires specific membrane carrier proteins.
3. Active Transport. The movement of substances against a concentration or electrochemical gradient. Requires the mandatory expenditure of energy (ATP) and the participation of membrane pumps. A prime example is the maintenance of the $Na^+$ and $K^+$ ion gradient between the intracellular and extracellular fluid. The key enzyme here is $Na^+$, $K^+$-ATPase, which provides energy for all $Na^+$-dependent nutrient transport.
4. Endocytosis. Encompasses phagocytosis and pinocytosis, which are closely linked to intracellular digestion.
5. Transcytosis. The transit of substances entirely across a cell: initial uptake via endocytosis, transport in a vesicle through the cytoplasm, and release on the opposite side via exocytosis. Transcytosis is of little significance for standard nutrients, but is critical for the transport of vitamins, enzymes, immune defense factors, and, in newborns, large maternal milk proteins.
6. Persorption. The transfer of substances not through the cells themselves, but via intercellular spaces. This accounts for the passage of a fraction of water, electrolytes, and to a lesser extent, proteins (allergens, antibodies) and even bacteria.

Role of Motility and Intraluminal Pressure

Intestinal motility plays a crucial role in nutrient assimilation. It not only generates the necessary intraluminal pressure, but also ensures the continuous renewal of the unstirred layer of chyme adjacent to the mucosa. This is critical for efficient hydrolysis and contact of breakdown products with the mucosa.

The importance of pressure is well illustrated by the following fact: if intraluminal pressure is increased to 8–10 mmHg, the absorption rate of an $NaCl$ solution from the small intestine increases exactly two-fold, clearly demonstrating the role of filtration.

Furthermore, the rate of absorption directly depends on the intensity of blood flow in the small intestine. The presence of digestive products in the intestinal lumen reflexively increases blood flow to that area.

The Pumping Function of Villi

The mucous membrane is capable of active contractions because villi and microvilli contain intrinsic contractile elements. Villi function as a specialized micropump:

• During contraction, lymph saturated with absorbed substances is squeezed out of the central lacteal (lymphatic vessel) of the villus.
• During relaxation, specialized valves prevent the return of lymph, producing a suction effect within the central lacteal.

Contractions of the microvilli themselves further enhance endocytosis.

Regulation of Activity: At rest (in the fasting state), villus contractions are rare and very weak. However, as soon as chyme enters the intestine, the rhythm and force of contractions increase sharply. This is driven by mechanical irritation of the base of the villi and the chemical action of food extractives, glucose, peptides, and amino acids. Neural regulation of these contractions is mediated by the enteric (metasympathetic) nervous system.