THE LARGE INTESTINE

The large intestine is wider than the small intestine and begins beyond the ileocecal sphincter and ends at the anus. The colon is lined with transporting epithelial cells called colonocytes, which absorb fluid and transport electrolytes but do not express digestive enzymes. 

Nutrient absorption should be completed by the time the luminal contents reach the large intestine. The main functions of the large intestine are completion of fluid absorption and the storage and elimination of fecal waste. Most water absorption occurs in the right colon. Prior to defecation, fecal waste is stored in the left colon . By the time fecal material reaches the rectum, it consists of a small volume of K+-rich fluid containing undigested plant fibers, bacteria, and inorganic material. 

Carbohydrates are the major source of calories. Approximately 60% is starch, about 30% is sucrose, and about 10% is lactose. Digestion reduces carbohydrates to their component monosaccharides: glucose, galactose, and fructose.

Areas and functions of the large intestine. The right colon includes the cecum, the ascending colon, and the proximal half of the transverse colon; the left colon includes the distal half of the transverse colon, the descending colon, the sigmoid colon, and the rectum.

Gut flora consists of a complex of microorganism species that live in the digestive tracts of animals and is the largest reservoir of human flora or microbiota. The human body carries about 100 trillion microorganisms in its intestines, a number ten times greater than the total number of human cells in the body. Bacteria make up most of the flora in the colon and up to 60% of the dry mass of feces. 99% of the bacteria come from about 30 or 40 species.

Bacteria in the gut fulfill a host of useful functions for humans, including digestion of unutilized energy substrates, stimulating cell growth, repressing the growth of harmful microorganisms, training the immune system to respond only to pathogens, and defending against some diseases.

Gut Bacteria’s functions 

( 1 ) Carbohydrate fermentation and absorption. 

• Without gut flora, the human body would be unable to utilize some of the undigested carbohydrates it consumes, because some types of gut flora have enzymes that human cells lack for breaking down certain polysaccharides. Carbohydrates that humans cannot digest without bacterial help include certain starches, fiber, oligosaccharides and sugars that the body failed to digest and absorb.

( 2 ) Trophic effects. 

• The gut flora increase growth of intestinal epithelial cells and control their proliferation and differentiation.

( 3 ) Suppression of pathogenic microbial growth. 

• Another important role of helpful gut flora is that they prevent species that would harm the host from colonizing the gut through competitive exclusion, an activity termed the "barrier effect".

( 4 ) Immunity.

• The bacteria stimulate the lymphoid tissue associated with the gut mucosa to produce antibodies to pathogens. The immune system recognizes and fights harmful bacteria, but leaves the helpful species alone, a tolerance developed in infancy.

( 5 ) Metabolic function. 

• There are the syntheses of vitamins like biotin, vitamins B (e.g. folate) and vitamin K , as well as absorption of ions including magnesium, calcium and iron.