Absorption of Water and Electrolytes

 About 90% of the water is  absorbed by the small intestine. The   colon reabsorbes the remaining water and other key nutrients from the indigestible material, solidifying it to form stool.Approximately 1,500-2000 mL of fluid enters the colon daily, but only 50 to 100 mL is excreted in feces.

Water absorption occurs via osmosis, driven by the osmotic gradient from electrolyte absorption. Sodium is actively absorbed through specific channels, while potassium is absorbed or secreted based on luminal concentration. Chloride ions are exchanged for bicarbonate ions due to this electrochemical gradient.

Colnic mucosa absorb nutrients after bacterial metabolism of carbohydrates that have not been absorbed in the small intestine 

Motility

In healthy adults, colonic transit normally requires from several hours to almost 3 days for completion of excretion.  The mean colonic transit time in healthy volunteers is 34 to 35 hours, with an upper limit of normal of 72 hours 

The colon exhibits two types of motility: haustral contraction and mass movement.

•  Low amplitude propagated contractions (LAPC) Haustral contractions, triggered by chyme, facilitate the slow movement and mixing of food to enhance water absorption. Those are slow, segmenting movements that further mix chyme, most comoon in proximal colon.  (5 to 40 mm Hg) and last about 3  seconds.They happen  about every 30 minutes. 

•  HAPC Increase upon awakening, are much more common during the day, and increase after meals
  Can transfer colonic contents over long distances, can be associated with internal anal sphincter relaxation, and can precede defecation.High amplitude propagated contractions (HAPC): Mass movements are stronger contractions that quickly push chyme toward the rectum.Some retrograde peristalsis delays the movement of chyme, while mass movements only push forward—occurring about four – six times daily—with a range of 2 to 24 daily. They have a pressure of 100  mm Hg, last 20 to 30 seconds, and can propragate ovver 30cm. They occur spontaneously, although also in response to pharmacological agents or colonic distention.; A majority originate in the proximal colon ,most do not propagate beyond the midcolon and fewer than 5% reach the rectum

 Both the low- and high-amplitude contractions are promoted by physical exercise as well as by emotional stress.  Both are reduced during sleep, although low amplitude are always present. It is known that HAPC reduce  over time in elderly patients, and also are notoriously reduced in frequency in those wih chronic constipation (some constipated patients can have as little as one per day or even none) .  Opposingly HAPC are increased in diarrhoea type IBS.  Low amplitude are worse understood, and despite increasing with age, they do not usually change the transit time.

The enteric nervous system typically inhibits colonic smooth muscle contractions. Hirschsprung’s disease is characterized by a congenital absence of enteric nerves, leading to obstruction due to tonic contractions.

Chyme entering the colon is mostly liquid, and progressive absorption along the large intestine results in solid feces. Increased motility reduces water absorption, potentially causing diarrhea, while decreased motility allows for excessive water reabsorption, leading to constipation.

Nervous factors

Parasympathetic nerves, mainly from the vagus for the cecum, ascending, and transverse colon, and from the sacral spinal cord for the descending colon, modulate colonic motility. Sympathetic activity inhibits contractions, coordinating colonic motility with the entire GI system. The colocolonic reflex causes relaxation in adjacent areas upon distention, partly mediated by sympathetic input, while the gastrocolic reflex increases colonic activity following stomach stretching.

Defecation

Defecation in healthy individuals starts with a predefecatory phase, marked by increased frequency and strength of intestinal contractions. This process can be triggered by stimuli like waking up or eating (gastroileal reflex also referred to as gastrocolic reflex). In people with slow-transit constipation, this phase is reduced or absent, and the reflex is weakened. Stool often reaches the rectum before the urge to defecate arises, which typically occurs when stool contacts receptors in the upper anal canal. Resisting this urge can cause stool to move backward, increasing colon transit time.

While sitting or squatting helps defecation by straightening the anorectal angle and relaxing the pelvic floor, studies on its benefits for constipation are limited. Defecation mainly involves striated muscles pushing out stool, with minimal help from colonic or rectal contractions. Relaxation of the puborectalis muscle and the anal sphincter during increased rectal pressure leads to stool expulsion. Normally, defecation involves minimal voluntary effort if intestinal muscle function is normal.

Vitamin Production and Absorption

The colon is vital for vitamin production, supporting a diverse bacterial environment. These bacteria, which are crucial for gut health, ferment substances to generate vitamins, particularly vitamin K and B vitamins like biotin or B12. These vitamins are absorbed into the bloodstream, helping to compensate for dietary deficiencies.

Microbiota and butyrate

Butyrate is a  four-carbon short-chain fatty acid,  produced through microbial fermentation of dietary fibers in the lower intestinal tract  and is a fundamental substrate for colonocyte and enterocyte oxidation pathways utilize (cellular fuel, a basic cellular need.)  Furthermore, intracellular accumulation of butyrate has evolved as a protective mechanism against carcinogenesis. 

The large intestine contains the largest bacterial ecosystem in the human body. About 99% of the large intestine and feces flora are made up of obligate anaerobes such as Bacteroides and Bifidobacterium.  

Bibliography

ASCRS Textbook of Colon and Rectal Surgery, Fourth Edition (2022)

Azzouz LL, Sharma S. Physiology, Large Intestine. [Updated 2023 Jul 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507857/

Thursby E, Juge N. Introduction to the human gut microbiota. Biochem J. 2017 May 16;474(11):1823-1836. doi: 10.1042/BCJ20160510. PMID: 28512250; PMCID: PMC5433529.

Sarna SK. Colonic Motility: From Bench Side to Bedside. San Rafael (CA): Morgan & Claypool Life Sciences; 2010. Colonic Motility Dysfunction. Available from: https://www.ncbi.nlm.nih.gov/books/NBK53473/