Functions of the Large Intestine
Anatomically, the large intestine is about 1.3 m long. By the time the food bolus reaches this section, the main digestion and absorption of nutrients are already completed in the small intestine. Only digestive juices and a small amount of indigestible substances, such as cellulose and pectin, arrive here.
The physiology of the organ is based on three main functions:
- Reservoir function: the organ serves as a storage site for intestinal contents before elimination.
- Absorptive function: active reabsorption of water and electrolytes occurs. The volume dynamics are colossal: from 0.5 to 4 liters of liquid chyme enter the lumen per day, and after intensive fluid absorption, only 100–200 ml of solid content remains.
- Hydrolytic function: hydrolysis of food residues is carried out. This process involves enzymes delivered with the chyme, microbial enzymes, and a small amount of enzymes from the large intestine's own secretion.
Characteristics of Intestinal Juice
The secretion of the large intestine differs significantly from the juices of the upper gastrointestinal tract. It consists of liquid and solid parts, possessing a strongly alkaline reaction (pH 8.5–9.0).
- Enzymatic activity: extremely low. The juice is poor in enzymes, although trace amounts of cathepsin, peptidases, lipase, amylase, and nucleases can be detected.
- Solid part of the secretion: represented by mucous clumps consisting of desquamated epithelial cells and abundant mucus produced by goblet cells. Mucous substances constitute the basis of the secretion.
Secretion dynamics: at rest, in the absence of digestive stimulation, juice is secreted in very small volumes. However, local mechanical irritation of the intestinal wall can increase secretory activity by 8–10 times.
Role of the Intestinal Microbiota
Fermentation processes take place in the large intestine, driven by bacterial activity. The microflora performs several critically important tasks to maintain homeostasis:
- Final decomposition: bacteria utilize remnants of undigested food.
- Cellulose breakdown: bacterial enzymes successfully hydrolyze plant fibers that bypassed the small intestine unchanged.
- Inactivation: microorganisms split and inactivate digestive enzymes and biologically active substances.
- Protective function: normal flora suppresses the growth of pathogenic microorganisms, preventing systemic infection.
- Synthetic function: bacteria synthesize vital compounds, primarily vitamin K and B-complex vitamins.
Completion of Metabolism
In addition to content compaction and bacterial fermentation, the final stage of metabolism for several key compounds occurs in the large intestine. It is here that final processing takes place for:
- Proteins;
- Phospholipids;
- Bile acids and fatty acids;
- Bilirubin;
- Cholesterol.
The entire digestive process in adults takes from 1 to 3 days, with food residues spending the most time in the large intestine cavity, where the described compaction and fermentation processes occur.