Histological section. The<unk>  absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. The colon absorbs vitamins that are created by the colonic bacteria, such as thiamine, riboflavin, and vitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation). It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation. The<unk>  also secretes K+ and Cl-. Chloride secretion increases in cystic fibrosis. Recycling of various nutrients takes place in colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling. The appendix contains a small amount of mucosa-associated lymphoid tissue which gives the appendix an undetermined role in immunity. However, the appendix is known to be important in fetal<unk> fe as it contains endocrine cells that release biogenic amines and peptide hormones important for homeostasis during early growth and development. The appendix can be removed with no apparent damage or consequence to the patient. By the time the chyme has reached this tube, most nutrients and 90% of the water have been absorbed by the body. At this point some electrolytes<unk> ke sodium, magnesium, and chloride are left as well as indigestible parts of ingested food (e.g., a<unk>  part of ingested amylose, starch which has been shielded from digestion heretofore, and dietary fiber, which is<unk> ly indigestible carbohydrate in either soluble or insoluble form). As the chyme moves through the<unk> , most of the remaining water is removed, while the chyme is mixed with mucus and bacteria (known as gut flora), and becomes feces. The ascending colon receives fecal material as a<unk> quid. The muscles of the colon then move the watery waste material forward and slowly absorb all the excess water, causing the stools to gradually solidify as they move along into the descending colon. The bacteria break down some of the fiber for their own nourishment and create acetate, propionate, and butyrate as waste products, which in turn are used by the cell<unk> ning of the colon for nourishment. No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet; in other animals, including other apes and primates, who have proportionally<unk> r colons, more is made available, thus permitting a higher portion of plant material in the diet. The<unk>  produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the<unk> . The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral). Standing gradient osmosis Water absorption at the colon typically proceeds against a transmucosal osmotic pressure gradient. The standing gradient osmosis is the reabsorption of water against the osmotic gradient in the intestines. Cells occupying the intestinal<unk> ning pump sodium ions into the intercellular space, raising the osmolarity of the intercellular fluid. This hypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via tight junctions and adjacent cells, which then in turn moves across the basement membrane and into the capillaries, while more sodium ions are pumped again into the intercellular fluid. Although water travels down an osmotic gradient in each individual step, overall, water usually travels against the osmotic gradient due to the pumping of sodium ions into the intercellular fluid. This allows the<unk>  to absorb water despite the blood in capillaries being hypotonic compared to the fluid within the intestinal lumen. Gut flora The<unk>  houses over 700 species of bacteria that perform a variety of functions, as well as fungi, protozoa, and archaea. Species diversity varies by geography and diet. The microbes in a human distal gut often number in the vicinity of 100 trillion, and can weigh around 200 grams (0.44 pounds). This mass of mostly symbiotic microbes has recently been called the latest human organ to be "discovered" or in other words, the "forgotten organ". The<unk>  absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the<unk>  and absorbed by passive diffusion. The bicarbonate that the<unk>  secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids. These bacteria also produce<unk>  amounts of vitamins, especially vitamin K and biotin (a B vitamin), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who depends on absorption of vitamins formed by bacteria in the<unk>  may become vitamin-deficient if treated with antibiotics that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria. Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of the gases hydrogen, methane, and hydrogen sulfide. Bacterial fermentation of undigested polysaccharides produces these. Some of the fecal odor is due to indoles, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics. They are also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, thereby preventing infection or invasion. The two most prevalent phyla of the colon are Bacillota and Bacteroidota. The ratio between the two seems to vary widely as reported by the Human Microbiome Project. Bacteroides are implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'. A mucus layer protects the<unk>  from attacks from colonic commensal bacteria.
Large intestine
Small intestine
Stomach
Gastrointestinal tract
Liver
Gastric mucosa
Lung
Kidney