Large Intestine

Human anatomy > Digestive System > Large Intestine

The body’s large intestine is in charge of three major duties. First, it receives the undigested food from the small intestine. Next, it is the receptacle of excess water and electrolytes that have been unused by the chimes. And, it facilitates the expulsion of solid waste materials out of the gastrointestinal tract. Measuring more or less 5 feet in length, and about 2 ½ inches in width, the large intestine is twice as big as the diameter of the small intestine. It got its name from its length. The large intestine begins where the small intestine ends. It goes all the way, on the right side of the body, until the liver. It then traverses through the abdomen, and goes down to the anus where it ends. The large intestine’s traverse portion is backed up by mesocolon, a specialized part of the mesentery structure.

The large intestine’s role in human digestion is actually only minor; it just absorbs the leftover food, water and electrolytes. It is more concerned with storing fecal matter, and expelling it from the body.

The Sections Of The Large Intestine

Structural and functional differentiation of the large intestine into smaller parts yield the following categories: the cecum, the rectum, the anal canal and the colon. The cecum is the dilated pouch under the ileocecal valve, the boundary that separates the large intestine from the small intestine, and the barrier that prevents the entry of backwash from the large intestine into the small intestine. The cecum is also what holds the appendix (which appears like a finger-like projection). Though a lot of scientific debates have been waged over the true purpose of the appendix, emerging research say that the organ helps reduce infection risks. That’s because researchers have found quite a presence of lymphatic tissues in the appendix. But, there are still those who believe that the existence of the appendix inside the human body is just an ancestral reminder of what our forefathers had before.

Large intestine

The superior part of the cecum continues to form part of the colon. The colon can transverse, ascend and sigmoid sections of the other parts of the large intestine. The colon drops from the cecum along the right wall of the stomach, against the inferior part of the liver. Once reaching the liver, the colon goes left following the hepatic flexure and goes upward the abdominal cavity, becoming the transverse colon. When it has reached the abdominal cavity, it then follows the spleenic flexure. This forms a right angle that forms the descent of the colon down to the pelvis. At this point, the colon passes through the abdominal cavity again in an S manner, marking the start of its sigmoid part. The rectum makes up the remaining 7 ½ inches. The last three centimeters is what’s known as the anal canal.

The rectum is positioned before the sacrum, and it’s secured in that position by the peritoneum. The anus is the opening of the anal canal that serves as the exit to the body. The wastes deposited in the anal canal are being regulated by two sphincter muscles, both voluntary and involuntary muscles. The internal sphincter is made up of smooth muscle tissues. The external sphincter is formed by skeletal muscles.

Anal columns are the long partitions of vascular mucous membranes along the anal canal. The large intestine and the small intestine both have the same basic structural tunics, although they do have quite a few differences. The large intestine is marked by the absence of intestinal villi, and the presence of ample goblet cells that produce protective mucous in the mucosal layer. Along the large intestine, the muscularis layer has three bands referred to as taeniae coli. A lot of bulges called haustra or sacculations are found, too. Epiploic appendages - pouches that are stuffed with fats - are attached to the superficial portion of the epiploic appendages.

The large intestine is supplied with nerves by the superior and inferior mesenteric plexus. The celiac plexus also does a little innervation. Parasympahetic innervation of the large intestines are done by pelvic splanchnic nerves and the vagus nerves.

A person knows that defecation is needed when bowel pressures trigger the sensory fibers that send the message that the colon is full. Blood supply to the large intestine is done by the superior and inferior mesenteric branches. The two’s mesenteric veins are also in charge of venous drainage. They connect with the hepatic portal vein and from there, the venous drainage is deposited in the liver.

Structural Modifications Of The Large Intestine

Every time the ileocecal valve opens and closes, the lare intestine receives the 15 milliliters or so of pasty chyme that has not been digested. When a person eats and food enters the body, the rythmical opening and closing of the valve as well as the peristaltic action is intensified. This is what is called gastroileal reflex. The large intestine acts as the receptor of the material and allows buildup in the large intestine and in the colon.

The large intestine deals with three functional motions. These are: haustral churning, mass movement and peristalsis. The peristalsis that the large intestine does is fairly similar to what the small intestine does, the only difference is that the large intestine goes about the process slower.

A relaxed haustrum is the process by which waste materials get into the large intestine and the ascending colon. When the buildup increases, the muscularis contracts and distention starts to happen. This is what is called haustrum churning. This process achieves two purposes. The first is that the waste materials travel down to the next haustrum. And, the amount of water and electrolytes that are absorbed increase as the materials encounter mucosa. When water from the materials are removed, feces is formed.

When feces is formed, the body needs to expel it. The feces move closer to the rectum as peristaltic action triggers it. This is what’s called mass movement, and it happens two to three times a day, whenever a meal or a snack is eaten. That is, a healthy adult does. Even when the food is still being eaten, mass movement can already happen. Science terms it as gastrocolic reflex. It usually happens in infants, because babies are known to defecate even while they are still eating.

While the feces is traveling towards the rectum, nutrients like water, Na+ and K+ are absorbed by the body. The colon’s mucosa soaks up about 850 milliliters of water in a day.

When the fecal matter has reached the rectum, the body feels the need to expel it, and it gets ready for the expulsion. But, if the impulse is willfully negated, the internal anal sphincter allows the material to move back up into the colon, and it closes. If it has to come to it, the fecal material will get stored in the sigmoid colon. The desire to defecate rises when rectal pressure elevates. When this happens, the sphincter prepares for release of the fecal material. Once the fecal material reaches the anal canal, most people would not be able to repress the urge anymore.

Defecation takes place as the rectum muscles contract, and rectal pressure increases. The pelvic skeletal muscles and the sphincter muscles contract and relax (respectively) to help defecation. This would put up the fecal matter for easier release. So, the waste material moves from the rectum to the anal canal, and the anus excretes the fecal matter.

© Copyright 2010-2011