Secretory Apparatus and Mucosal Cells
The secretory function of the small intestine is carried out by Lieberkühn glands, or intestinal crypts. Several specialized cell types are involved in this complex process, each performing a specific task to ensure normal food digestion.
- Goblet cells are responsible for producing complex protective and digestive secretions. Their granular endoplasmic reticulum actively synthesizes protein components, while the Golgi apparatus produces mucopolysaccharides. Their secretion exhibits pronounced enzymatic, including proteolytic, activity.
- Enterocytes secrete fluid rich in hydrolytic enzymes, which are critical for the final stages of nutrient breakdown.
- Argentaffin cells, located predominantly at the base of the crypts, perform endocrine functions, locally regulating motility and secretory processes.
Fulfilling a dual role, the intestinal epithelium acts as a selective filter. It secretes substances into the intestinal lumen (some of which are derived from the bloodstream) while actively and passively transferring breakdown products from the lumen and mucosal surface into the circulatory and lymphatic systems.
Physicochemical Properties and Composition of Intestinal Juice
Intestinal juice is a complex multicomponent biological fluid. The baseline pH is 7.2–7.5, but at the peak of secretory activity, when digestion is most intense, alkalinity can rise to 8.6.
The chemical composition of the juice can be divided into two main groups:
- Inorganic components: maintained at a concentration of about 10 g/L. This group includes vital electrolytes — chlorides, bicarbonates, and sodium, potassium, and calcium phosphates.
- Organic substances: the fluid fraction contains large amounts of mucus, proteins, amino acids, urea, and various cellular metabolites.
Mucus is a critical organic component of the juice. It serves an essential protective function, reliably preventing mechanical damage and aggressive chemical action of acidic chyme on the delicate mucosa. Additionally, mucus possesses enzymatic properties, containing high enzymatic activity that hydrolyzes nutrients right at the wall surface.
Morphokinetic Secretion
A unique physiological feature of the small intestine is morphokinetic (or morphonecrotic) secretion. Essentially, this mechanism represents a continuous cellular conveyor belt that operates throughout a person's life.
The life cycle of an intestinal epithelial cell is strictly programmed and includes three consecutive stages:
- Continuous generation of new cells at the base of the intestinal crypts.
- Gradual migration of maturing enterocytes along the length of the villi.
- Shedding (desquamation) of senescent cells from the villus tips into the lumen of the organ.
The rate of this turnover is colossal. Complete replacement of the entire epithelial layer in an adult occurs on average every 3–6 days (in some cases, 1.4 days). Shedding rates remain high, with about 2% of cells sloughed off per hour. As a result, approximately 250 grams of epithelial cells are discarded daily into the intestinal lumen. Upon disintegration in the lumen, these cells release their contents, serving as a powerful source of enzymes.
Enzymatic Profile
The bulk of intestinal juice enzymes is synthesized directly within the small intestinal mucosa, with only a small fraction excreted in transit from the blood. In total, intestinal juice contains over 20 different hydrolytic enzymes.
Key groups of intestinal juice enzymes:
- Proteolytic: enteropeptidase (the key activator of pancreatic trypsinogen), specific peptidases (dipeptidase, aminopolypeptidase), and duodenase.
- Lipolytic: intestinal lipase and phospholipase.
- Amylolytic and disaccharidases: amylase, lactase, maltase, sucrase, invertase.
- Others: alkaline phosphatase and nucleases.
An important physiological pattern is the proximal-distal gradient. This means that the concentration and functional activity of digestive enzymes are maximal in the proximal segments of the small intestine and predictably decrease toward the distal segments and the large intestine.