The colon, also called the colon, is the middle section of the large intestine. It is divided into four sections, beginning behind the appendix and ending at the junction with the rectum.
What is the colon?
The colon in humans is about one and a half meters long and has a lumen of about eight centimeters. In humans, its shape resembles a U that is upside down. This inverted U frames the small intestine. The main function of the colon is to recover water and electrolytes and to reabsorb food components that have not yet been absorbed by the small intestine.
Anatomy and structure
The colon begins after the appendix (caecum), that is, in the right lower abdomen. In the region of the appendix, the small intestine joins the large intestine. At the transition is the so-called Bauhin’s valve. It prevents the contents of the large intestine from flowing back into the small intestine. This ensures that the different bacterial populations of the large and small intestines do not mix. In addition, the flap ensures that the food is transported in portions. Almost immediately after the flap, the ascending part (colon ascendens) of the colon begins. This section is about 20-25cm long and merges seamlessly into a horizontal section at the level of the twelfth thoracic vertebra. This section is called the transverse colon. The left bend connecting the two parts is called the right flexure or flexura coli dextra. The transverse colon is joined by the left flexure (flexura coli sinistra), which merges into the descending colon, the descending large intestine. This is joined by the sigmoid (sigmoid colon), an s-shaped loop of colon. The colon ends with the beginning of the rectum, which in turn is bounded at its end by the anus. The colon shows a four-layered wall structure typical of the gastrointestinal tract. The innermost layer is the mucosa, a mucous membrane, which in turn is composed of three layers. The mucosa lies on a connective tissue layer (tunica submucosa). In this layer run the blood and lymph vessels that supply the colon, and absorb the absorbed nutrients, electrolytes and water. In addition, a nerve plexus, the submucosal plexus, is located in the connective tissue layer. Under the submucosa runs the tunica muscularis, a muscular layer formed by an inner annular muscle layer and an outer longitudinal muscle layer. These muscles serve to mix and transport the food pulp. Due to the peristaltic movements of the muscles and special constrictions, the muscle layer forms the taenia and haustra so typical of the colon. Between the two muscle layers runs another nerve plexus, the so-called Auerbach’s plexus. Depending on the section of the colon, either loose connective tissue or the peritoneum forms the fourth and final wall layer of the colon. The transverse colon is completely covered by peritoneum; the ascending and descending portions of the colon are covered by peritoneum only on their anterior surfaces.
Function and Tasks
The Bauhin’s valve releases the food pulp into the appendix in portions. The two nerve plexuses then provide the typical peristaltic, or undulating, contractions of the muscles of the colon. Mixed movements can be distinguished from transport movements. Mixing movements are caused by a contraction of the ring musculature and always run only over a short distance. They serve to vigorously mix the intestinal contents. This ensures adequate reabsorption of important nutrients. Mixing movements occur about 15 times per minute. Transport movements occur less frequently. These are long peristaltic waves that transport the food pulp into the rectum. The transport waves pass through the intestine about two to three times a day. This is often followed by defecation. The main function of the colon is to recover water and electrolytes. About one liter of fluid is recovered per day through the colon. In addition, an enzymatic conversion of certain food components takes place in the colon with the help of colon bacteria. The bacteria primarily break down vegetable fiber and produce vital nutrients such as vitamin K or vitamin B7 in the process.
Diseases
Inflammation of the colon is called colitis. It is usually accompanied by pain and diarrhea. A special form of colitis is ulcerative colitis.It belongs to the group of chronic inflammatory bowel diseases and causes severe bloody diarrhea and cramps in the colon due to mucosal damage with ulceration. Another chronic inflammatory bowel disease that can affect the small intestine as well as the colon is Crohn’s disease. Here, too, digestive disorders and diarrhea occur. Both diseases belong to the group of autoimmune diseases. Diverticula are the term used to describe bulges in the intestinal wall. If digested food residues collect in these diverticula, an inflammation, a so-called diverticulitis, can occur. Diverticulitis most frequently occurs in the area of the sigmoid. The symptoms are similar to those of appendicitis, except that the pain is more likely to be in the left lower abdomen. Diverticulitis is therefore also called left appendicitis. Colorectal cancer is the second most common cancer in Germany. Carcinomas of the colon and rectum account for more than 95% of all malignant colon tumors. The symptoms of a tumor in the colon are rather uncharacteristic. Early symptoms may include blood in the stool or sudden changes in bowel movements. Other symptoms include foul-smelling flatulence, pencil stool (narrow formation of the stool), and alternation between constipation and diarrhea. Blood loss can also cause anemia, with symptoms such as fatigue, chills, hair loss, and pallor.
Typical and common diseases
- Ulcerative colitis
- Crohn’s disease
- Diverticulitis
- Colorectal cancer