Large intestine

The large intestine is the region of the digestive tract from the ileocecal valve to the anus. Approximately 1.5 m in length, this organ has a larger diameter than the small intestine. The mucosa of the large intestine is composed of absorptive cells and mucus-secreting goblet cells. However, in contrast to the small intestine, the mucosa in this organ does not form villi. The large intestine consists of the following structures:

The cecum, which is the most proximal portion of the large intestine, receives chyme from the ileum of the small intestine through the ileocecal valve. The appendix, a small projection at the bottom of the cecum, is a lymphoid tissue. This tissue contains lymphocytes and assists in defense against bacteria that enter the body through the digestive system. The largest portion of the large intestine is the colon. It consists of four regions: ascending colon (travels upward toward the diaphragm on the right side of the abdomen), transverse colon (crosses the abdomen under the diaphragm), descending colon (travels downward through the abdomen on the left side), and sigmoid colon (S-shaped region found in the lower abdomen). The sigmoid colon is continuous with the rectum, which leads to the external surface of the body through the anus.

The large intestine typically receives 500 to 1500 ml of chyme per day from the small intestine. As discussed, most digestion and absorption have already taken place in the small intestine. In fact, the large intestine produces no digestive enzymes. At this point in the human digestive tract, chyme consists of indigestible food residues (e.g., cellulose), unabsorbed biliary components, and any remaining fluid. Therefore, the two major functions of the large intestine are:

The colon's absorption of most of the water and salt from the chyme results in this "drying" or concentrating process. As a result, only about 100 ml of water is lost through this route daily. The remaining contents, now referred to as feces, are "stored" in the large intestine until it can be eliminated by way of defecation.

The longitudinal layer of smooth muscle in the small intestine is continuous. In the large intestine, this layer of muscle is concentrated into three flat bands referred to as taniae coli. Furthermore, the large intestine appears to be subdivided into a chain of pouches or sacs referred to as haustra. The haustra are formed because the bands of taniae coli are shorter than the underlying circular layer of smooth muscle and cause the colon to bunch up, forming the haustra.

Motility of the large intestine. Movements through the large intestine are typically quite sluggish. It will often take 18 to 24 h for materials to pass through its entire length. The primary form of motility in the large intestine is haustral contractions, or haustrations. These contractions are produced by the inherent rhythmicity of smooth muscle cells in the colon. Haustrations, which result in the pronounced appearance of the haustra, are similar to segmentation contractions in the small intestine. Nonpropulsive, haustrations serve primarily to move the contents slowly back and forth, exposing them to the absorptive surface.

In contrast to segmentation contractions in the small intestine (9 to 12 per minute), haustral contractions occur much less frequently (up to 30 min between contractions). These very slow movements allow for the growth of bacteria in the large intestine. Normally, the bacterial flora in this region is harmless. In fact, some of the bacteria produce absorbable vitamins, especially vitamin K.

A second form of motility in the large intestine is mass movement. Three or four times per day, typically after a meal, a strong propulsive contraction occurs that moves a substantial bolus of chyme forward toward the distal portion of the colon. Mass movements may result in the sudden distension of the rectum that elicits the defecation reflex.

Secretion of the large intestine. The large intestine produces an alkaline mucus secretion, the function of which is to protect the mucosa from mechanical or chemical injury. Mucus provides lubrication to facilitate the movement of the contents of the lumen. Bicarbonate ion neutralizes the irritating acids produced by local bacterial fermentation; colonic secretion increases in response to mechanical or chemical stimuli. The mechanism of the enhanced secretion involves intrinsic and vagal nerve reflexes.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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