The large intestine is located in the abdominal cavity. It is the site of the last phases of digestion and consists of three segments: the cecum, the colon, and the rectum. The colon divides into ascending colon, transverse colon, descending colon, and sigmoid colon. The large intestine is called large because its diameter is considerably greater than the diameter of the small intestine.
Description
The large intestine is the terminal part of the digestive system. This important system is responsible for the ingestion and digestion of foodstuffs. Along the digestive tract, food is broken down into nutrient molecules small enough to pass into the bloodstream. Nutrient molecules are mostly absorbed in the small intestine, with the remainder being absorbed in the large intestine, which also prepares waste for elimination from the body through the anus.
The large intestine is called the ascending colon as it starts from the cecum, which marks the end of the small intestine. The caecum contains the worm-shaped appendix. The ascending colon then passes along the right abdominal wall to the inferior surface of the liver and bends sharply at a right angle to the left at a curve called the hepatic flexure. At this point, it crosses the abdominal cavity, passing to the left abdominal wall and is known as the transverse colon. Under the spleen, it bends again at the splenic flexure, and is known as the descending colon, passing along the left abdominal wall to the pelvic region. The colon then forms an S-shaped curve and is called the sigmoid colon. The rectum marks the end of the colon. It is a storage site for solid waste which can then exit the body through an external opening called the anus, controlled by muscles called sphincters.
The large intestine is about 6 ft (1.8 m) long and about 2 in (5 cm) wide in the average, normal adult.
Function
The large intestine has three major functions:
Recovery of water and essential ions. When the partially digested foodstuffs reach the end of the small intestine (ileum), roughly 80% of their water contents has been absorbed, but considerable water and small ions, such as sodium and chloride, still remain and must be recovered by further absorption. The colon then absorbs most of the remaining water and ions and additionally secretes bicarbonate ions as well as mucus, an important lubricant that protects the intestinal lining.
Formation and storage of feces. As matter moves through the colon, it is dehydrated, mixed with bacteria and mucus, and formed into feces for subsequent storage and elimination. The composition of normal feces is approximately 75% water and 25% solid waste, mostly consisting of bacteria and roughage, that is, undigested protein, fat, fibers, dried digestive juices, and dead cells. Its typical brown color is due to pigments resulting from the bacterial degradation of bilirubin and fecal odor results from gases released by bacteria.
Bacterial fermentation. Fermentation refers to the enzymatic decomposition and utilization of foodstuffs by bacteria. The large intestine has a rich bacterial life that produces a wide variety of enzymes capable of fermenting many of the nutrient molecules that would oth erwise not be absorbed. A normal adult harbors some 450 different species of bacteria in the colon and most of these are anaerobes, meaning bacteria that survive only in oxygen-free environments. Bacterial populations in the large intestine digest carbohydrates, proteins, and lipids that escape digestion and absorption in the small intestine and they also manufacture vitamin K and certain B vitamins.
The movement of bulk matter in the colon is referred to as large intestinal motility and it consists of four different types of muscle-assisted contractions:
Segmentation contractions. These contractions mash and mix the partially digested foodstuff, exposing it to the mucous membrane where nutrient absorption occurs.
Peristaltic contractions. These contractions are wavelike contractions that allow material to advance from the small intestine through the colon.
Antiperistaltic contractions. These contractions occur in a backwards direction toward the ileum, so as to slow down the forward movement of matter through the colon. This provides more residence time for the absorption of nutrients.
Giant migrating contractions. These contractions represent a type of motility only seen in the colon. Giant migrating contractions are a type of very intense and prolonged peristaltic contraction that can strip a large segment of colon free of contents.
Following the ingestion of food, large intestinal motility increases significantly, triggered by the duodenocolic reflex, which is stimulated by the presence of fat in the small intestine. Additionally, giant migrating contractions push feces into the empty rectum. Stretching of the rectum in turn stimulates the defecation reflex. This is a reflex controlled by the pelvic nerves, and it results in relaxation of the ring-like internal anal sphincter, the muscle that constricts or closes the anus. This is followed by voluntary relaxation of the external anal sphincter and defecation.
Role in human health
The importance of the large intestine in human health is mostly derived from its role in removing water from food residues and transporting it into the bloodstream. Along with this water, dissolved minerals and ions, notably sodium, potassium, and chlorine, are also transported to the bloodstream. Without these chemicals, the blood is chemically unbalanced, a condition that can lead to serious illness and even death.
Common diseases and disorders
Appendicitis. Appendicitis is an inflammation of the appendix. It occurs due to accumulated waste material that cannot move out of the appendix easily because it only has one opening. The symptoms of appendicitis are muscular rigidity, sharp pain in the right lower abdomen, and vomiting.
Colitis. Colitis is commonly known as irritable bowel or spastic colon. It refers to the inflammation of the inner lining of the colon. Colitis is related to stress and can lead to ulcerative colitis, in which open sores appear in the mucous membrane of the colon.
KEY TERMS
Abdomen—A part of the body that lies between the thorax and the pelvis. It contains a cavity (abdominal cavity) that holds organs such as the pancreas, stomach, intestines, liver, and gallbladder. It is enclosed by the abdominal muscles and the vertebral column (spine).
Abdominal cavity—The hollow part of the body located between the diaphragm, which is the thin muscle below the lungs and heart, and the pelvis, the basin-shaped cavity that contains the reproductive organs, bladder, and rectum. The abdominal cavity contains the abdominal organs.
Anaerobes—Bacteria that live only in environments that are virtually oxygen-free.
Anal sphincter—The ring-like band of muscle that constricts or closes the anus.
Anus—The terminal opening of the digestive tract.
Appendix—A worm-shaped structure projecting from the cecum.
Bilirubin—A waste product from the breakdown of heme, the active group of hemoglobin, the molecule that carries oxygen in the blood. Bilirubin is produced in the liver and excreted in bile.
Cecum—The pouch-like start of the large intestine that links it to the small intestine.
Colon—Part of the large intestine, located in the abdominal cavity. It consists of the ascending colon, the transverse colon, the descending colon, and the sigmoid colon.
Digestion—The conversion of food in the stomach and in the intestines into substances capable of being absorbed by the blood.
Digestive system—Organs and paths responsible for processing food in the body. These are the mouth, the esophagus, the stomach, the liver, the gallbladder, the pancreas, the small intestine, the large intestine, and the rectum.
Feces—Waste product of digestion formed in the large intestine. About 75% of its mass is water, the remainder is protein, fat, undigested roughage, dried digestive juices, dead cells, and bacteria.
Fermentation—Enzymatic breakdown and utilization of foodstuffs by bacteria, as occurs in the large intestine.
Goblet cells—Mucus-secreting cells found scattered among other cells in the epithelium of many organs, especially in the intestinal and respiratory tracts. They are most abundant in the colon.
Hepatic flexure—Sharp right-angle bend of the ascending colon under the liver as it becomes the transverse colon.
Ion—Elements consist of positively charged nuclei surrounded by negatively charged electrons. These charges are balanced and the overall charge of an element is zero. An element becomes an ion, that is a charged species, if it gains or losses electrons. Many small ions are essential for the functioning of the body. The major ones include: the potassium ion (K+), the sodium ion (Na+), the chlorine ion (Cl), and the HCO3- ion.
Large intestinal motility—Muscle-assisted contractions occurring in the colon to facilitate movement of intestinal contents. There are four type of motility: segmentation contractions, peristaltic contractions, antiperistaltic contractions and giant migrating contractions.
Large intestine—The terminal part of the digestive system, site of water recycling, nutrient absorption, and waste processing located in the abdominal cavity. It consists of the caecum, the colon, and the rectum.
Mucous membrane—The lubricated lining of several body organs that contains mucus-secreting glands.
Mucus—The slimy secretion of glands found on mucous membranes composed of various proteins, salts, and white blood cells.
Pelvis—The basin-shaped cavity located below the abdomen that contains the reproductive organs, the bladder, and rectum.
Peristalsis—A pattern of wave-like muscle contractions that allows material to advance through the digestive tube.
Peritoneum—The thin membrane that lines the abdominal and pelvic cavities, and covers most abdominal organs.
Rectum—The rectum is a short, muscular tube that forms the lowest portion of the large intestine and connects it to the anus.
Small intestine—The part of the digestive tract located between the stomach and the large intestine. It consists of the duodenum, the jejunum, and the ileum.
Splenic flexure—Sharp right-angle bend of the transverse colon under the spleen as it becomes the descending colon.
Colorectal cancers. Colorectal cancers start in the innermost layer of the tissues of the large intestine and can grow through some or all of the layers. They can develop in any of the four sections of the colon or in the rectum.
Constipation. Constipation is caused primarily by insufficient fiber in the diet, lack of exercise, or not drinking enough fluids. As a result, fecal matter hard ens and large intestinal motility is impaired.
Diarrhea. Diarrhea is a condition characterized by frequent, loose, watery stools that range from yellowish to light brown to green in color. If enough water is lost, dehydration occurs.
Diverticulosis. Diverticulosis is characterized by out ward ballooning (diverticula) of the large intestine wall caused by chronic constipation.
Dysentery. Dysentery is a general term for various dis orders characterized by severe diarrhea, inflamed intes tines, and intestinal bleeding. Some forms of dysentery may clear up by themselves, while other forms may continue for years without treatment.
Hemorrhoids. Hemorrhoids are commonly known as piles. They are dilated veins in the anus and rectum.
Peritonitis. Peritonitis refers to inflammation of the peritoneum, the membrane that lines the abdominal and pelvic cavities. It can occur as a result of a ruptured appendix, which empties its contents of fecal matter and waste into the abdominal cavity. This condition is extremely serious.
Ptosis of the colon. Also known as prolapsed colon, this is a common condition that occurs when the colon falls from its normal position to a lower position.
BOOKS
Ballard, Carol. The Stomach & Digestive System. Toronto: Britnell Book Wholesalers, 1997.
Janowitz, Henry D. Your Gut Feelings: A Complete Guide to Living Better With Intestinal Problems. New York: Taylor & Francis, 1995.
Johnson, L.R., and T.A. Gerwin, eds. Gastrointestinal Physiology. St. Louis: Mosby, 2001.
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