Gastrointestinal absorption

Absorption in different parts of the digestive tract

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

• Oral cavity. Absorption here is negligible. This is because food stays in the mouth for a very short time and does not yet have time to be broken down into monomers. Nevertheless, certain medications are able to cross the mucosa extremely quickly, which is actively used in medicine for their sublingual (under the tongue) administration.
• Stomach. The absorption process is minor. Only water with dissolved mineral salts, glucose, and some amino acids enter the bloodstream. At the same time, ethyl alcohol solutions (alcohol) are exceptionally well absorbed in the stomach.
• Small intestine. This is the main site of absorption for water, electrolytes, and nutrients. This process is inextricably linked with the hydrolysis of nutrients. Colossal efficiency is achieved due to the huge surface area of the mucous membrane. There are 30–40 villi per square millimeter of the mucosa, and the apical membrane of each enterocyte bears 1,700 to 4,000 microvilli. As a result, there are 50–100 million microvilli per 1 mm² of epithelium. Considering that the body has about $10^{10}$ absorptive cells and $10^{15}$ somatic cells, it turns out that a single intestinal cell provides nutrition for one hundred thousand other body cells.
• Large intestine. The main task of this section is the reabsorption of water and the formation of solid fecal masses. Glucose, amino acids, and other easily absorbed substances can also be assimilated here in small quantities. This is clinically important as it makes the use of nutrient enemas possible.

Cellular transport mechanisms

The transport of substances across enterocyte membranes is carried out through several fundamentally different pathways.

1. Passive transport. Occurs without the expenditure of cellular energy. It includes simple diffusion (the driving force is the concentration gradient of the solute), osmosis (movement of the solvent molecules themselves), and filtration (transfer of solution through pores under the influence of hydrostatic pressure).
2. Facilitated diffusion. Also proceeds without energy expenditure and strictly along the concentration gradient, but, unlike simple diffusion, it requires the participation of specific membrane carriers.
3. Active transport. Movement of substances against a concentration or electrochemical gradient. Requires mandatory expenditure of energy (ATP) and the participation of membrane pumps. A crucial example is the maintenance of the $Na^+$ and $K^+$ ion gradient between the cell and intercellular fluid. The key enzyme here is the $Na^+$, $K^+$-ATPase, which provides energy for all $Na^+$-dependent nutrient transport.
4. Endocytosis. Combines the processes of phagocytosis and pinocytosis, which are closely related to intracellular digestion.
5. Transcytosis. Transit passage of a substance through the cell: first, capture by endocytosis, then transport in a vesicle through the cytoplasm, and release outside by exocytosis. Transcytosis is of little importance for ordinary nutrients, but is critically important for the transfer of vitamins, enzymes, immune defense factors, and in newborns, for the transport of large proteins of breast milk.
6. Persorption. Transfer of substances not through the cells themselves, but through intercellular spaces. This is how part of the water, electrolytes, and to a lesser extent proteins (allergens, antibodies) and even bacteria pass.

Role of motility and intraluminal pressure

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

The significance of pressure is well illustrated by the following fact: if intraluminal pressure is increased to 8–10 mm Hg, the rate of $NaCl$ solution absorption from the small intestine increases by exactly two times. This clearly confirms the role of the filtration process.

In addition, the rate of absorption directly depends on the intensity of the small intestine's blood supply. The presence of digestive products in the intestinal lumen reflexively increases blood flow to this area.

Pumping function of villi

The mucous membrane is capable of active contractions because the villi and microvilli contain their own contractile elements. Villi work as a kind of micropump:

• During contraction, lymph saturated with absorbed substances is squeezed out of the central lymphatic vessel of the villus.
• During relaxation, special valves prevent the return of lymph, which provides the suction effect of the central lymphatic vessel.

Contractions of the microvilli themselves further enhance endocytosis processes.

Regulation of activity: At rest (on an empty stomach), contractions of the villi are rare and very weak. But as soon as chyme enters the intestine, the rhythm and force of contractions increase sharply. This is facilitated by mechanical irritation of the base of the villi and the chemical action of extractive substances of food, glucose, peptides, and amino acids. Neural regulation of these contractions is carried out with the participation of the enteric nervous system.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Physiological mechanisms and types of endocytosis
• The role of the enteric nervous system in the regulation of the GI tract
• Principles of operation of the enterocyte membrane $Na^+$, $K^+$-ATPase
• Reflex regulation of local intestinal blood supply
• Indications and mechanism of action of nutrient enemas

Other topics in the "Digestion" section

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