Physiology, Large Intestine

Function

The large intestine has 3 primary functions: absorbing water and electrolytes, producing and absorbing vitamins, and forming and propelling feces toward the rectum for elimination. By the time indigestible materials have reached the colon, most nutrients and up to 90% of the water has been absorbed by the small intestine. The role of the ascending colon is to absorb the remaining water and other key nutrients from the indigestible material, solidifying it to form stool. The descending colon stores feces that will eventually be emptied into the rectum. The sigmoid colon contracts to increase the pressure inside the colon, causing the stool to move into the rectum. The rectum holds the feces awaiting elimination by defecation.

Mechanism

Motility

The intestinal wall is made up of multiple layers. The 4 layers of the large intestine from the lumen outward are the mucosa, submucosa, muscular layer, and serosa. The muscular layer is made up of 2 layers of smooth muscle, the inner, circular layer, and the outer, longitudinal layer. These layers contribute to the motility of the large intestine. There are 2 types of motility present in the colon, haustral contraction and mass movement. Haustra are saccules in the colon that give it its segmented appearance. Haustral contraction is activated by the presence of chyme and serves to move food slowly to the next haustra, along with mixing the chyme to help with water absorption. Mass movements are stronger and serve to move the chyme to the rectum quickly.

Absorption of Water and Electrolytes

Absorption of water occurs by osmosis. Water diffuses in response to an osmotic gradient established by the absorption of electrolytes. Sodium is actively absorbed in the colon by sodium channels. Potassium is either absorbed or secreted depending on the concentration in the lumen. The electrochemical gradient created by the active absorption of sodium allows for this. Chloride ions are exchanged for bicarbonate ions across an electrochemical gradient.

Production/Absorption of Vitamins

The colon also plays a role in providing required vitamins through an environment that is conducive for bacterial cultivation. The colon houses trillions of bacteria that protect our gut and produce vitamins. The bacteria in the colon produce substantial amounts of vitamins by fermentation. Vitamin K and B vitamins, including biotin, are produced by the colonic bacteria. These vitamins are then absorbed into the blood. When dietary intake of these vitamins is low in an individual, the colon plays a significant role in minimizing vitamin disparity.

Pathophysiology

Disorders of Large Intestinal Motility[5][6]

Irritable Bowel Syndrome

Irritable bowel syndrome is thought to be due to psychological factors influencing the motility of the large intestine via the extrinsic autonomic nervous system. During times of stress, segmentation contractions may be increased or decreased, resulting in constipation or diarrhea.

Hirschsprung Disease: Megacolon[7]

Hirschsprung disease is a disorder at birth that occurs when nerve cells are absent (Auerbach’s Plexus) in the muscles of the colon. This affects motility in the colon, making it difficult to pass stool.

Diverticulosis/Diverticulitis[8]

Diverticulosis is a disorder in which pockets develop in the colonic mucosa due to the weakness of the muscle layers in the colon wall. This usually occurs over time from chronic attrition of the aging process. Diverticulitis can develop if these pockets get infected or inflamed, causing abdominal pain and change in bowel movements. Diverticular disease is very common, especially in older adults.

Colitis[9]

Inflammatory Bowel Disease (Inflammatory)

Inflammatory bowel disease includes either Crohn's disease or ulcerative colitis. Both cause inflammation and scarring within the digestive tract, disrupting the normal function. The cause of inflammatory bowel disease is not known but is likely due to an abnormal response of the immune system. Ulcerative colitis is confined to the large intestine, whereas Crohn's disease can occur anywhere in the GI tract, from mouth to anus.

Ischemic

Ischemic colitis is more common in the elderly and occurs when there is decreased blood flow to the colon. Decreased blood flow can cause inflammation or injury to the colon. Some causes of ischemic colitis are atherosclerosis of arteries, low blood pressure, blood clots, and bowel obstruction.

Infectious 

Infectious colitis can occur from many different viruses, bacteria, or parasites. Infectious colitis most commonly occurs due to ingestion of contaminated food or water, introducing the infectious organism into the colon. The most common causes are Escherichia coli, Campylobacter, Shigella, and Salmonella. These infectious organisms invade the colon, cause inflammation, and affect the normal function, causing abdominal pain and diarrhea. Clostridium difficile is another organism that can cause colitis in association with antibiotic use. C. difficile is part of healthy, normal flora in the colon but can cause problems if it overgrows. Antibiotic use can destroy other susceptible normal flora in the colon, allowing overgrowth and invasion of C. difficile.

Clinical Significance

Disturbance or dysfunction of the large intestine’s normal physiology can result in poor quality of life and significant medical issues. Pathology of the large intestine is common. One out of every 10 Americans over the age of 40 have diverticular disease, and around 3 million people in the United States have inflammatory bowel disease.  It is important to incorporate a healthy diet and lifestyle to maintain a properly functioning colon. Eating a diet high in fiber and drinking plenty of water allows food to easily move through the colon, keeping the colon relatively clean, which can decrease the risk of diverticular disease. It is also important to maintain healthy colonic flora. Maintaining healthy colonic flora will decrease the risk of abdominal bloating, gas, diarrhea, constipation, and infectious colitis.[10]