Intestine, Large Health Article

Definition

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.