Digestion in the Large Intestine

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 fiber and pectin, enter here.

The organ's physiology is based on three main functions:

1. Reservoir function: the organ serves as a storage site for intestinal contents before their elimination.
2. Absorptive function: active reabsorption of water and electrolytes takes place. The volume dynamics are colossal: from 0.5 to 4 liters of liquid chyme enter the intestinal lumen per day, and after intensive fluid absorption, only 100–200 ml of dense contents remains.
3. Hydrolytic function: the 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 juice.

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 and has 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 in it.
• Solid part of the secretion: represented by mucous clumps. They consist of sloughed epithelial cells and abundant mucus produced by goblet cells. It is these mucous substances that form the basis of the secretion.

Secretion dynamics: at rest, in the absence of food stimuli, the juice is secreted in very small volumes. However, local mechanical stimulation of the intestinal wall can increase secretory activity by 8–10 times.

Role of the intestinal microflora

Fermentation processes, ensured by bacterial activity, take place in the large intestine. The microflora performs a number of critically important tasks to maintain homeostasis:

• Final decomposition: bacteria utilize undigested food residues.
• Fiber breakdown: bacterial enzymes successfully hydrolyze plant fibers that passed through the small intestine unchanged.
• Inactivation: microorganisms break down and inactivate digestive enzymes and biologically active substances.
• Protective function: normal flora suppresses the growth of pathogenic microorganisms, preventing infection of the body.
• Synthetic function: bacteria synthesize vital compounds, primarily vitamin K and B vitamins.

Completion of metabolism

In addition to content thickening and bacterial fermentation, the final stage of metabolism for several key compounds occurs in the large intestine. It is here that final metabolism takes place for:

• Proteins;
• Phospholipids;
• Bile acids and fatty acids;
• Bilirubin;
• Cholesterol.

The entire digestion process in an adult human takes from 1 to 3 days, with food residues spending the most time specifically in the lumen of the large intestine, where the aforementioned processes of compaction and fermentation take place.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of water and electrolyte absorption in the large intestine
• Processes of bacterial fermentation and their impact on the organism
• Regulation of motility and chyme transit time
• Bile acid and cholesterol metabolism in the distal GI tract
• Detailed cellular composition of crypts and function of goblet cells

Other topics in the "Digestion" section

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