This layer is formed of surface epithelium, underlying connective tissue lamina propria, and a small amount of smooth muscle muscularis mucosae. In some regions, the mucosa is folded with tiny projections that extend into the passageway, or lumen, of the digestive tube; the folds increase the absorptive surface area. The mucosa also has glands that are tubular invaginations into which the lining cells secrete mucus and digestive enzymes. The mucosa protects the tissues beneath it and carries on secretion and absorption.
The submucosa contains considerable loose connective tissue as well as glands, blood vessels, lymphatic vessels, and nerves. Its vessels nourish the surrounding tissues and carry away absorbed materials.
This layer, which provides movements of the tube, consists of two coats of smooth muscle tissue. The fibers of the inner coat encircle the tube. When these circular fibers (they are closed spirals) contract, the diameter of the tube decreases. The fibers of the outer muscular coat run lengthwise. When these longitudinal fibers (open spirals) contract, the tube shortens.
Serosa or serous layer:
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The serous layer, or outer covering of the tube, is composed of the visceral peritoneum, formed of epithelium on the outside and connective tissue beneath. The cells of the serosa protect underlying tissues and secrete serous fluid, which moistens and lubricates the tube's outer surface so that the organs in the abdominal cavity slide freely against one another.
Explain what is peristalsis and how it is carried out
Peristalsis is the process through which foodstuff is propelled distally through the esophagus and intestines. It is a wavelike muscle contraction in the esophagus, stomach and intestines. There are two types of waves: the short reflex and the long reflex. It is a manifestation of two major reflexes within the enteric NS. They are stimulated by a bolus of foodstuff present in the lumen. Peristaltic movement of the interneurons is stimulated in two ways:
One group of interneurons activates excitatory motor neurons above the bolus - these neurons, which contain acetylcholine and substance P, stimulate contraction of smooth muscle above the bolus.
Another group of interneurons activates inhibitory motor neurons that stimulate relaxation of smooth muscle below the bolus. These inhibitor neurons appear to use nitric oxide, vasoactive intestinal peptide and ATP as neurotransmitters.
It begins in the esophagus when a bolus of food is swallowed. The strong wave-like motions of the smooth muscle in the esophagus carry the food to the stomach. Next, peristalsis continues in the small intestine where it mixes and shifts the chyme back and forth, allowing nutrients to be absorbed into the bloodstream through the small intestine walls. Peristalsis concludes in the large intestine where water from the undigested food material is absorbed into the bloodstream. Finally, the remaining waste products are excreted from the body through the rectum and anus.
Identify three ways the small intestine is modified to increase the surface area for digestion and absorption
The plicae circulares, villi, and microvilli are modifications of the small intestine for digestion and absorption.
The inner wall of the small intestine has a velvety appearance throughout its length, due to many tiny projections of mucous membrane called intestinal villi. These structures are most numerous in the duodenum and the proximal jejunum. They project into the lumen of the alimentary canal, contacting the intestinal contents. Villi greatly increase the surface area of the intestinal lining, aiding absorption of digestive products.
Each villus consists of a layer of simple columnar epithelium and a core of connective tissue containing blood capillaries, a lymphatic capillary called a lacteal, and nerve fibers. At their free surfaces, the epithelial cells have many fine extensions called microvilli that form a brushlike border and greatly increase the surface area of the intestinal cells, further enhancing absorption. The blood capillaries and lacteals carry away absorbed nutrients, and impulses transmitted by the nerve fibers can stimulate or inhibit activities of the villus.
Between the bases of adjacent villi are tubular intestinal glands (crypts of Lieberkühn) that dip into the mucous membrane. The deeper layers of the small intestinal wall are much like those of other parts of the alimentary canal in that they include a submucosa, a muscular layer, and a serosa. The lining of the small intestine has many circular folds of mucosa, called plicae circulares that are especially well developed in the lower duodenum and upper jejunum. With the villi and microvilli, these folds help increase the surface area of the intestinal lining.
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How is digestive activity provoked after eating? What activates the secretion of digestive juices into the lumen or hormones into the blood?
The digestive activity is provoked after eating when the bolus reaches the back part of the tongue. Sensors, which are the mechanoreceptors and chemoreceptors, located in the walls of the gastrointestinal tract, respond to stretching by the introduction of food into the lumen. These sensors are able to respond to changes in solute concentration and pH as well as the presence of substrates and end products of digestion. When chime enters the small intestine cholecystokinin is secreted by the i-cells to initiate secretion of bile.
What is bile and where is it produced? What is its digestive function? Where is it stored and concentrated?
Bile is a yellowish-green liquid that hepatic cells continuously secrete. In addition to water, it contains bile salts, bile pigments, cholesterol, and electrolytes. Of these, bile salts are the most abundant. They are the only bile substances that have a digestive function. Hepatic cells use cholesterol to produce bile salts, and in secreting these salts, they release some cholesterol into the bile.
Cholesterol by itself has no special function in bile or in the alimentary canal. Bile pigments (bilirubin and biliverdin) are breakdown products of haemoglobin from red blood cells. These pigments are normally excreted in the bile. The yellowish skin, sclerae, and mucous membranes of jaundice result from excess deposition of bile pigments.
Bile salts aid digestive enzymes. Molecules of fats clump into fat globules. Bile salts reduce surface tension and break fat globules into droplets called emulsification. Monoglycerides that form from the action of pancreatic lipase on triglyceride molecules aid emulsification. Lipases can then digest the fat molecules more effectively. Bile salts enhance absorption of fatty acids and cholesterol by forming complexes (micelles) that are very soluble in chyme and that epithelial cells can more easily absorb. Lack of bile salts results in poor lipid absorption and vitamin deficiencies. The mucous membrane of the small intestine reabsorbs nearly all of the bile salts, along with fatty acids. The blood carries bile salts to the liver, where hepatic cells resecrete them into the bile ducts. Liver cells synthesize bile salts, which replace the small amounts lost in the feces.
(2013,05). Arbl.cvmbs.colostate.edu. Physiology of Peristalsis. Retrieved February 02, 2013, from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html