Intestinal Histophysiology
Intestinal histophysiology is the study of structure and function of tissues that form the small and large intestines. The intestines are tubular shaped organs that are part of the alimentary canal of the digestive system.
The intestines, like that of the rest of the digestive tract, consist of four layers of tissue: mucosa, submucosa, muscularis, and serosa. The innermost layer, the mucosa, opens into the interior of the intestines, the lumen. The mucosa is made up of several layers. The first, a layer of epithelial cells called the lamina epithelialis, lines the lumen. Different epithelial cell types enable digestion, absorption, secretion, and production of hormones within the intestines. Beneath the lamina epithelialis is a layer of loose connective tissue containing blood vessels and lymphatic vessels called the lamina propria. The last layer of the mucosa, the lamina muscularis mucosae, consists of smooth muscle and enables the mucosa to move.
The tissue layer of the intestines adjacent to the mucosa is the submucosa. Constituents of the submucosa include a meshwork of loose and dense connective tissue, submucous plexus, blood vessels, and lymphatic vessels. The submucous plexus is nervous tissue that is essential for sensation and regulation of the intestinal environment.
Between the submucosa and the outermost layer of the intestines is the muscularis. The muscularis consists of circular and longitudinal smooth muscle that enables the intestines to move food in the lumen through the digestive tract. Additionally, the muscularis contains the myenteric plexus, nervous tissue that controls motility of the intestines.
The outermost layer of the intestinal layers is the serosa. The serosa is composed of a layer of mesothelium protecting a layer of loose connective tissue.
The small intestine is an approximately 21-ft. long (6.4 m) continuation of the alimentary canal that receives gastric contents of the stomach called chyme. The small intestine has folds in its mucosa tissue layer that increase the surface area in the lumen and help to mix chyme as it passes through the small intestine. Protruding from the mucosal folds are projections called villi that extend into the lumen to increase the surface area for digestion and absorption of nutrients into the blood. Absorption is accomplished by an inner lining of epithelial cells called enterocytes. Not only do enterocytes secrete intestinal juices into the lumen, but they also secrete hormones into the blood that stimulate secretions from the liver and pancreas. The liver secretes bile salts that emulsify and absorb lipids and proteins in the small intestine. The pancreas secretes pancreatic juice containing sodium bicarbonate that creates optimal conditions for absorption to occur in the small intestine. Enterocytes further increase surface area by forming projections on the villi called microvilli. The enterocytes are replaced every few days by glands called the Crypts of Lieberkuhn.
The small intestine is divided into three anatomical regions: duodenum, jejunum, and ileum. The duodenum connects to the pyloric sphincter of the stomach, receiving gastric contents that have been liquefied by the time they reach the small intestine. The duodenum and the middle portion of the small intestine, the jejunum, complete digestion of any remaining macromolecules and begin absorption of water, electrolytes, and organic molecules. By the time the chyme reaches the ileum, digestion and absorption are complete. This final section of the small intestine is responsible for removing water and bile salts from the chyme as it continues into the large intestine.
The large intestine is responsible for recovering water and electrolytes, preparing the body for removal of waste products in the form of feces, and microbial fermentation. The large intestine can be classified into four anatomic regions; cecum, colon, rectum, and anal canal. The cecum is a closed pouch extending from the ileocecal valve that attaches to the vermiform appendix. Continuing from the cecum at the ileocecal valve is the colon.
The colon makes up the majority of the large intestine and is subdivided into the ascending, transverse, and descending colon. The colon is responsible for the absorption of excess water from the feces back into the bloodstream. Additionally, electrolytes, specifically sodium and chloride ions, can be absorbed in the ascending and transverse colon. The two main secretions into the lumen of the large intestine are bicarbonate ions and mucus. Bicarbonate ions are secreted to help neutralize the acid produced by microbial fermentation. Mucus helps to hold the feces together and acts as a lubricant as the feces passes through the large intestine. The descending colon is connected to the rectum where the feces are eliminated from the body through the anal canal.
The large intestine contains a normal flora of bacteria that enables the body to digest otherwise indigestible molecules by way of fermentation. For example, cellulose is a major component of the human diet. However, humans do not produce the enzyme cellulase to break down the carbohydrate and must depend of bacteria to digest it. One of the byproducts of microbial fermentation, intestinal gas or flatus, is eliminated through the anus. Bacteria also aid in the process of producing certain vitamins in the colon such as vitamin K and some of the B vitamins. The vitamin K produced in the large intestine is absorbed into the intestinal lymph and is the main source of vitamin K for the body. However, the B vitamins produced in the large intestine are not absorbed and are lost when the feces is eliminated.
Although one of the main functions of the intestines is water absorption, the feces actually consists of about 75% water. The remaining 25% of fecal material is composed of bacteria and undigested material such as fiber.
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