Note in the middle panel, a light micrograph, that the microvilli are visible and look something like a brush. For this reason, the microvillus border of intestinal epithelial cells is referred to as the "brush border". Most of the discussion on following pages focuses on enterocytes, the epithelial cells which mature into absorptive epithelial cells that cover the villi. These are the cells that take up and deliver into blood virtually all nutrients from the diet.
However, several other important cell types populate the small intestinal epithelium:. If the small intestine is unable to absorb enough water, a person may get diarrhea. The small intestine consists of three distinct parts :. It is difficult to measure the length of the small intestine in a healthy living person, but researchers estimate that it ranges from about 9. The three sections of the small intestine differ significantly in length:. Older research found that the total length of the intestines correlates with weight, such that people who weigh more have longer intestines.
Younger people and males also have, on average, longer intestines. The small intestine is long enough that a person can undergo small bowel resection surgery to remove part of it if an underlying disease or condition causes it to stop functioning. The large intestine , which doctors also call the colon, has a larger diameter than the small intestine.
The primary function of the large intestine is to reabsorb fluids, electrolytes, and vitamins and then form and propel feces toward the rectum for elimination. It contains four distinct parts:. As with the small intestine, the total length of the large intestine varies from person to person. Heavier people, younger people, and males generally have longer intestines.
In most people, it is about 5 ft long. The colon is the longest part of the large intestine, while the cecum measures about 2. MOCA Domain:. Anatomy and Biomechanics. Possible Appearance:. Background Information:. Topic Certainty Cholera Speculative. References Comparative physiology of the vertebrate digestive system , Stevens, Charles E.
Attached to the teniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages. The purpose of these is unknown. Although the rectum and anal canal have neither teniae coli nor haustra, they do have well-developed layers of muscularis that create the strong contractions needed for defecation.
The stratified squamous epithelial mucosa of the anal canal connects to the skin on the outside of the anus. This mucosa varies considerably from that of the rest of the colon to accommodate the high level of abrasion as feces pass through. Two superficial venous plexuses are found in the anal canal: one within the anal columns and one at the anus. Depressions between the anal columns, each called an anal sinus, secrete mucus that facilitates defecation. The pectinate line or dentate line is a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses, and represents the junction between the hindgut and external skin.
The mucosa above this line is fairly insensitive, whereas the area below is very sensitive. The resulting difference in pain threshold is due to the fact that the upper region is innervated by visceral sensory fibers, and the lower region is innervated by somatic sensory fibers. Most bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes.
However, trillions of bacteria live within the large intestine and are referred to as the bacterial flora. Most of the more than species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K. Some are linked to increased immune response.
A refined system prevents these bacteria from crossing the mucosal barrier. Dendritic cells open the tight junctions between epithelial cells and extend probes into the lumen to evaluate the microbial antigens.
The dendritic cells with antigens then travel to neighboring lymphoid follicles in the mucosa where T cells inspect for antigens. This process triggers an IgA-mediated response, if warranted, in the lumen that blocks the commensal organisms from infiltrating the mucosa and setting off a far greater, widespread systematic reaction.
The residue of chyme that enters the large intestine contains few nutrients except water, which is reabsorbed as the residue lingers in the large intestine, typically for 12 to 24 hours. Thus, it may not surprise you that the large intestine can be completely removed without significantly affecting digestive functioning. For example, in severe cases of inflammatory bowel disease, the large intestine can be removed by a procedure known as a colectomy. Often, a new fecal pouch can be crafted from the small intestine and sutured to the anus, but if not, an ileostomy can be created by bringing the distal ileum through the abdominal wall, allowing the watery chyme to be collected in a bag-like adhesive appliance.
In the large intestine, mechanical digestion begins when chyme moves from the ileum into the cecum, an activity regulated by the ileocecal sphincter. Right after you eat, peristalsis in the ileum forces chyme into the cecum. When the cecum is distended with chyme, contractions of the ileocecal sphincter strengthen.
Once chyme enters the cecum, colon movements begin. Mechanical digestion in the large intestine includes a combination of three types of movements.
The presence of food residues in the colon stimulates a slow-moving haustral contraction. This type of movement involves sluggish segmentation, primarily in the transverse and descending colons. When a haustrum is distended with chyme, its muscle contracts, pushing the residue into the next haustrum. These contractions occur about every 30 minutes, and each last about 1 minute.
These movements also mix the food residue, which helps the large intestine absorb water. The second type of movement is peristalsis, which, in the large intestine, is slower than in the more proximal portions of the alimentary canal. The third type is a mass movement.
These strong waves start midway through the transverse colon and quickly force the contents toward the rectum. Mass movements usually occur three or four times per day, either while you eat or immediately afterward.
Distension in the stomach and the breakdown products of digestion in the small intestine provoke the gastrocolic reflex, which increases motility, including mass movements, in the colon. Fiber in the diet both softens the stool and increases the power of colonic contractions, optimizing the activities of the colon.
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