As part of our ongoing series on the human body, we turn our attention now to the large intestine, also called the colon. How much do you know about it?
In previous Anatomy & Physiology posts, we discussed chewing and swallowing as the beginning point of digestion. Following this, food travels down the esophagus to the stomach, where some additional digestion takes place. The majority of digestion takes place after the food leaves the stomach, and travels to the small intestine; here is the main site of digestion and absorption. The food then leaves the small intestines, and travels to the large intestine, as Dr. Halpern explains below:
THE ROLE OF THE LARGE INTESTINE
The large intestine is a 1.5 m-1.8 m tube that runs from the terminal end of the ileum (ileocecal valve) to the anus. It is divided into thee major portions: the cecum, the colon and the rectum. The portion that exits the small intestine called the cecum is a very short 6 cm and consists primarily of a pouch or wrapping around the end of the small intestine and the start of the large intestine. It also contains the appendix. The cecum prevents regurgitation of food back into the small intestine. The majority of the large intestine is the colon. The final portion, the rectum is a short section that is continuous with the anus through which elimination of feces takes place. The colon is divided into four sections: the ascending colon, transverse colon, descending colon and sigmoid colon.
The functions of the large intestine can be divided into three main areas: absorption of water and electrolytes, formation of feces and microbial fermentation. Water absorption occurs in response to an osmotic gradient. Fecal material is the end product after remaining water and electrolytes have been absorbed. Microbial fermentation is due to a plethora of bacteria living in the large intestine. These bacteria secrete substances that assist in the digestion of some otherwise difficult-to-digest food constituents such as cellulose. However, digestion of these substances is minimal. The bacteria also help to digest remaining carbohydrate and protein. When poorly digested food reaches the large intestine, the microbes begin to act on the food particles in an exaggerated manner producing excessive amounts of gas. The gasses produced include: methane, nitrogen, hydrogen, carbon dioxide and oxygen. Bacteria in the large intestine are also responsible for breaking down bilirubin (bile secretions) into its constituent pigments. This gives stool its color. As a by-product of their activity, bacteria produce a small amount of B vitamins and vitamin K (used for clotting).
The appendix is a small structure the size of a worm near the junction of the small and large intestines. It is very similar in structure to the rest of the large intestine but contains more lymphoid material. Its opening generally closes during middle age. As a lymphoid structure, it is prone to inflammation.
The process of chyme (partially digested food) moving into the large intestine begins in the stomach with the secretion of the hormone gastrin. Gastrin signals the ileocecal valve to allow chyme to enter into the large intestine. Once in the large intestine, chyme is moved through via the process of peristalsis (rhythmic muscular contractions in the wall of the large intestine). Chyme moves very slowly at an average rate of 1cm per hour. Variations in the amount of time food stays in the large intestines occur: the slower the eliminative process, the drier and harder the stools. A healthy large intestine also produces some mucous (kapha) that allows feces to easily move through rectum and anus. Feces accumulate in the rectum until it begins to stretch the muscular wall. This triggers a reflex that loosens the internal sphincter of the anus allowing feces to be eliminated. There is an external sphincter as well that is under voluntary control and this allows conscious control over elimination.
Common and important diseases of the large intestine include Irritable Bowel Syndrome, Crohn’s disease, Ulcerative Colitis, Diverticulitis and Appendicitis.
Irritable Bowel Syndrome is a non-inflammatory condition of the large intestine characterized by chronic gas, cramping and alterations of elimination. Some cases present primarily with loose stools often with urgency. Crohn’s disease and Ulcerative Colitis are widespread inflammatory diseases. Whereas Crohn’s disease affects both the small and large intestine, Ulcerative Colitis is generally restricted to the large intestine. Both produce bleeding and mucous and excessive elimination along with gas and discomfort. Diverticulitis is a condition where pockets form in the wall of the large intestine due to a localized weakness in the wall of the muscle. Food particles become trapped in the pockets leading to a localized infection. Symptoms include cramping pain, gas, nausea, fever and loose stools. Appendicitis means inflammation of the appendix. It occurs when fecal material or chyme get trapped in the appendix. This causes pressure to build up on the inside. This restricts blood flow and the area become gangrenous. The area becomes inflamed and can rupture into the abdominal cavity if it is not treated. Symptoms can include pain around the umbilicus later locating to the right lower quadrant, nausea, loss of appetite, vomiting, diarrhea or constipation and abdominal swelling with a possible low fever.
Colon and colo-rectal cancer is the leading cause of cancer deaths in the United States. Colo-rectal cancer produces blood in the stools and, as the condition becomes more advanced, there is weight loss. Often, there are no other early symptoms though pain is possible.
Signs and symptoms of large intestine disease vary considerably with the underlying cause and can overlap with small intestine disease. In general, the following signs and symptoms point in the direction of the large intestine:
- * Diarrhea
- * Bleeding (red blood in the stools)
- * Mucous in the stools
- Ayurvedic theory teaches that the large intestine (Sanskrit: pakvasaya; intestines or receptacle of digestion or digested food) is the home site of the vata dosha and specifically apana vayu. Apana vayu is the force that regulates downward motion and thus it is the force that controls peristalsis. Two important factors controlling the pace of elimination are temperature and moisture. As the body or, specifically, the colon becomes colder, vata dosha becomes vitiated and apana vayu works to retain stool as an effort to hold onto body heat. As the body and, specifically, the colon becomes warmer, apana vayu works to eliminate stool more quickly in an effort to release the heat. Thus, apana vayu plays an important role in regulating body temperature, agni and pitta dosha. Moisture is also an important factor. When, through the intake of foods that are too dry or when the body becomes dehydrated, the colon becomes too dry, a person’s stools become hard and difficult to eliminate. Thus, due to either dryness or coldness, vata dosha is vitiated resulting in constipation. Likewise, heat and moisture lead to an easier passage of stools and, when in excess, results in diarrhea.
In the event that too much heat (pitta) builds up in the large intestine, ulcerations and bleeding will occur. It is the natural effect of kapha dosha to try to protect against the heat of pitta. Thus, as heat builds up and ulcerations occur, mucous is also produced. This occurs in Ulcerative Colitis and Crohn’s disease.
Excerpt from “Principles of Ayurvedic Medicine,” by Dr. Marc Halpern, Founder of the California College of Ayurveda