The colon, or large intestine, is an essential part of the human digestive system, often overlooked compared to the small intestine or stomach, but essential for proper digestion and overall health. In addition to its primary function of absorbing water and salts, the colon plays a critical role in the formation and elimination of stool, regulation of the gut microbiota, and interaction with the immune system. Its complex anatomy and multiple biological functions make the colon a key organ for digestive health and overall well-being. In this article, we will explore the anatomy of the colon, its main functions, and the mechanisms by which it maintains digestive balance. We will also review some of the most common disorders that can compromise the functioning of the colon, as well as current therapeutic strategies to manage and prevent colonic diseases.
Colon anatomy: structure and divisions
Colon is a portion of the large intestine and represents the last part of the gastrointestinal tract before the rectum. It has a total length of about 1.5 meters and is divided into several anatomical sections:
Cecum: This is the first part of the colon, located in the lower right abdomen, where the contents from the ileum of the small intestine enter the colon through the ileocecal valve. The cecum is also the attachment site of the appendix, a small structure that, although historically thought to be nonessential, has immune functions and may play a role in maintaining the intestinal flora.
Ascending colon: The part of the colon that extends vertically upward from the cecum along the right side of the abdomen. This is where the absorption of water and mineral salts begins.
Transverse colon: It extends horizontally from the hepatic angle, located below the liver, to the splenic angle, near the spleen. It is the central part of the colon and plays an important role in the absorption of fluids and the fermentation of undigested fiber by intestinal bacteria.
Descending colon: Extends along the left side of the abdomen and carries fecal contents to the lower part of the large intestine.
Sigmoid colon: The final, S-shaped part of the colon that connects to the rectum. The sigmoid colon stores feces until the bowel reflex is activated.
Rectum: This is the last section of the intestinal tract and serves as a temporary reservoir for feces before they are eliminated through the anus during defecation.
Functions of the Colon
The colon performs many essential functions for digestion and maintaining health. Among these, the most important are the absorption of water and mineral salts, the fermentation of dietary fiber by the intestinal microbiota, and the elimination of feces.
Absorption of water and electrolytes
One of the main functions of the colon is the recovery of water from the intestinal contents. When chyme, the semi-digested material from the small intestine, enters the colon, it is still relatively liquid. Through a process of active and passive absorption, the epithelial cells lining the colon reabsorb approximately 90% of the water and electrolytes (such as sodium and chloride) present in the fecal contents, helping to form solid stool. This absorption mechanism is essential for preventing dehydration and maintaining the body’s water balance.
Fiber Fermentation by the gut microbiota
The colon is the main home of the gut microbiota, a complex community of trillions of bacteria that play critical roles in digestion, vitamin synthesis, and immune regulation. Many dietary fibers, which cannot be digested by the small intestine, are fermented by colonic bacteria, producing short-chain fatty acids (SCFAs) such as butyric, propionic, and acetic acid. These SCFAs not only provide energy to the colonic epithelial cells, but also have anti-inflammatory properties and contribute to metabolic health. SCFA production is closely linked to overall gut health and can also affect systemic metabolism. An imbalance in the gut microbiota, known as dysbiosis, can impair these processes and contribute to a variety of disorders, such as irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), and even metabolic conditions such as obesity and diabetes.
Elimination of Stool
The colon plays a key role in the formation and elimination of stool. After water and electrolytes are reabsorbed, the stool contents are compacted in the descending and sigmoid colon. Stool is composed of water, bacteria, indigestible fiber, and undigested food debris, as well as dead skin cells and mucus. When the rectum fills, nerve receptors send signals to the brain that stimulate the sensation of wanting to defecate. The process of defecation is regulated both by the autonomic nervous system, which controls the smooth muscles of the colon and rectum, and by voluntary control of the external anal sphincter muscles.
The Role of the colon in immune health
The colon is not only a digestive organ, but also an important site of immune regulation. Much of the immune system of the gut is located in the colon, where immune cells constantly monitor the intestinal contents for pathogens and foreign agents. Immune cells, such as macrophages and dendritic cells, are present in Peyer’s patches and other lymphoid structures associated with the gut, known as GALT (Gut-Associated Lymphoid Tissue). These cells play a crucial role in immune surveillance, ensuring that the body can defend itself from pathogens while tolerating the beneficial bacteria of the microbiota. The microbiota itself interacts closely with the immune system, influencing its response and helping to keep inflammation in check.
Gut bacteria can stimulate immune cells to produce anti-inflammatory cytokines and strengthen the intestinal barrier, preventing pathogens and toxins from entering the bloodstream. Aside from species such as Prevotella, Escherichia, Aerobacter, Akkermansia and Proteus, the colon is home to a large portion of lactic acid bacteria (Lactobacillus) and the majority (85%) of bifidus bacteria (Bifidobacterium). According to the latest research in the field, lactobacilli maintain immune homeostasis, while bifidobacteria cooperate for physiological functions, including defecation. Chronic and obstinate constipation seems to be due to a lack of bifidobacteria.
Common colon disorders
The colon can be subject to a number of disorders that compromise its function and general well-being. Among the most common are:
Irritable Bowel Syndrome (IBS): A functional disorder that causes abdominal pain, bloating, and changes in bowel habits (diarrhea, constipation, or both). The causes of IBS are not fully understood, but factors such as altered colon motility, visceral hypersensitivity, and changes in the gut microbiota are thought to be involved.
Inflammatory Bowel Disease (IBD): These include Crohn’s disease and ulcerative colitis, two conditions characterized by chronic inflammation of the intestines. Crohn’s disease can affect any part of the gastrointestinal tract, while ulcerative colitis is limited to the colon and rectum. Both diseases can cause debilitating symptoms, including abdominal pain, chronic diarrhea, and weight loss.
Diverticulitis: A condition in which small pouches, called diverticula, form in the walls of the colon. When these diverticula become inflamed or infected, they can cause severe abdominal pain, fever, and changes in bowel habits.
Colon Cancer: Colorectal cancer is one of the most common types of cancer in the world. Early detection through screening, such as colonoscopy, is crucial to improving survival rates. Diet, lifestyle and genetics are important risk factors for developing this type of cancer.
Disease prevention and treatment
Maintaining a healthy diet rich in dietary fiber, fruits, vegetables, and fermented foods is essential for a healthy colon and gut microbiota. Dietary fiber promotes regularity of the intestines and the production of SCFAs, improving digestive function and reducing the risk of intestinal disorders. When fiber intake from vegetables is limited by factors such as “palatal” fiber, fiber from fruits or whole grains can be used instead. Alternatively, probiotic introduction is always recommended for colon “housekeeping”: yogurt, kefir and other fermented beverages are always welcome to balance gut health. Also commercially available probiotic formulas are widely available as free supplements.
Regular physical activity, adequate hydration, and moderate stress are other factors that can help maintain a healthy colon. For more serious conditions, such as IBD or colorectal cancer, treatment may include anti-inflammatory drugs, immunosuppressants, corticosteroids, surgery, or targeted cancer therapies. As you can see from this basic information, the colon is a critical organ for digestion, nutrient absorption, and immune health regulation. A balanced diet, a healthy lifestyle and early diagnosis of colon disorders are crucial to maintaining well-being and preventing serious and invalidating diseases.
- edited by Dr. Gianfrancesco Cormaci, PhD, specialist in CLinical Biochemistry.
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