Most people do not understand the functions and purpose of the large intestine. If you think the large intestine or colon is limited to defecation, please consider these other very important perspectives.
Sustenance is taken through the oral cavity where the cephalic phase (smell, sight and taste) of gastric secretion and mechanical digestion begins. Taste signals are transmitted to the brain which are paramount for both the digestion and the elimination processes.
Additional gastric and intestinal phases further digestion through the stomach and duodenum. Salivary amylase, lingual lipase, mucin, hydrochloric acid, pepsin, sodium bicarbonate, pancreatic lipase, bile, gastrin, secretin, and cholecystokinin (to name only a few) assist the body in making foods available to be changed into energy, and for absorption in the small and large intestines.
From the stomach these combined are now chyme which enter the small intestine for absorption, then will pass through the Ileo-cecal sphincter and Ileo-cecal valve. Although described as a valve, technically the ICV is not a true valve. Valves in the body are in the heart, veins, and the lymphatics.
Sodium ions are transported from the lumen across the epithelium by virtue of the epithelial cells having very active sodium pumps. The colonic epithelium is actually more efficient at absorbing water than the small intestine. Water in the chyme is absorbed by the large intestine and returns that material to a more solid consistency for defecation.
Goblet cells secrete mucin and with water form a beneficial mucus which protects the intestinal epithelium, and lubricates the feces to not adhere to the bowel wall, allowing it to easily pass and aids in the formation of stool, and is essential for proper bowel function and complete elimination.
In the gastrointestinal tract, bacteria may be free-living or attached to mucus, mucosa surface, food particles or digestive residues. The attached bacteria produce microcolonies, leading to the development of biofilms.
The large intestine serves in the last stages of digestion with a slightly acidic pH of 5.6 ~ 6.9. Several vitamins needed for normal metabolism are bacterial products that are absorbed in the large intestine; some B vitamins, vitamin K, and electrolytes.
Microbial bacteria ferment carbohydrates and convert proteins to amino acids. Any remaining carbohydrates and proteins in the large intestine may release ammonia, carbon dioxide, indole, phenol and skatole, all of which are toxins and considered normal without threat. If the pH of the large intestine moves towards alkaline, this may be associated with an imbalance of intestinal floras.
The large intestine eliminates these by defecation or via the hepatic portal system of the liver where they are denatured into less toxic compounds which can be excreted primarily through the kidneys.