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The Digestive System
Chapter 18
Structures of the Digestive System
Gastrointestinal tract (GI) the digestive system is an irregular tube, open at both ends (mouth and anus), also called the alimentary canal or digestive tract and is about 9 m (29 feet) in length (in adult). Main organs of Digestive system : mouth; pharynx; esophagus; stomach; small intestine (Duodenum, Jejunum, lleum) ; large intestine (Cecum , Ascending colon, Transverse colon, Descending colon, Sigmoid colon); rectum; anal canal Accessory organs : teeth and tongue, salivary glands; liver; gallbladder; pancreas; vermiform appendix Ingestion : Process of taking food into the mouth, starting it on its journey through the digestive tract Digestion includes o Physical breakdown of food. o Chemical conversion of food into nutrients and its absorption and o Elimination of waste The breakdown or digestion of food material is both mechanical and chemical in nature. o Mechanical digestion—chewing, swallowing, and peristalsis break food into tiny particles, mix them well with digestive juices, and move them along the digestive tract o Chemical digestion—breaks up large food molecules into compounds having smaller molecules; brought about by digestive enzymes Foods undergo three forms of processing in the body: digestion, absorption, and metabolism. Digestion and absorption are performed by the organs of the digestive system. Metabolism, on the other hand, is performed by all body cells.
Wall of the Digestive Tract
The digestive tract has been described as a tube with a lumen that extends from the mouth to the anus. Tissue layers of the wall of the digestive tube from inside to outside o Mucosa— The mucosa of the esophagus, for example, is composed of tough and abrasion-resistant stratified epithelium. The mucosa of the remainder of the tract is a delicate layer of simple columnar epithelium structured for absorption and secretion. The mucus produced by either type of epithelium coats the lining of the alimentary canal. o Submucosa—connective tissue contains blood vessels, glands and nerves. o Muscularis—two layers of smooth muscle that move food through the tube by rhythmic muscular waves known as peristalsis (Peristalsis is a rhythmic, wavelike contraction of the gut wall caused by repeating waves of ringlike contraction along the circular muscle layer in the muscularis. This type of sequenced contraction squeezes and pushes ingested food material forward through the digestive tube’s internal pathway) o Serosa—serous membrane that covers the outside of abdominal organs also known as visceral peritoneum. The loops of the digestive tract are anchored to the posterior wall of the abdominal cavity by a large double fold of peritoneal tissue called the mesentery.
o Dental caries is the most common diseases of the mouth and teeth in the developed world.
Pharynx
o Muscular tube (throat) lined with mucous membrane o Functions as part of both respiratory and digestive systems as it acts as a common passage for food and air. o Subdivided into three anatomical segments -Nasopharynx, oropharynx and laryngopharynx. o The swallowing of a mass of chewed food is called deglutition. o First, mastication (chewing) involves voluntary movements that result in formation of a ball or bolus of food in the mouth that is then moved involuntarily through the oropharynx and into the esophagus and, finally, into the stomach.
loss), nausea (unpleasant feeling that often leads to vomiting), and emesis (vomiting) Gastric Ulcer : o An ulcer is a craterlike wound or sore in a membrane due to tissue destruction caused by acid in gastric juice. Often occurs in duodenum or stomach. o This can cause burning pain and may ultimately result in hemorrhage, perforation, widespread inflammation, scarring, and other very serious medical complications. o Excessive acid secretion was thought for many years to be the primary cause of ulcers. It is now known that most gastric and duodenal ulcers result from infection with the Helicobacter pylori (H. pylori) bacterium. o Long-term use of certain pain medications such as aspirin and ibuprofen, called nonsteroidal antiin- flammatory drugs (NSAIDs) can also cause ulcers. o Currently, the standard antibiotic-based treatment used most often to both heal ulcers and prevent recurrences is called triple therapy Stomach cancer- Stomach cancer has been linked to excessive alcohol consumption, use of chewing tobacco, eating smoked or heavily preserved food, and to H. pylori infection.
Small Intestine
Size —about 7 meters (20 feet) long but only 2 cm or so in diameter Divisions: Small Intestine has three segments: Duodenum, Jejunum and Ileum o Most of the chemical digestion occurs in the first subdivision of the small intestine, known as the duo- denum. The duodenum is C shaped and curves around the head of the pancreas. The middle third of the duodenum contains the openings of ducts that empty pancreatic digestive juice and bile from the liver. Bile - greenish yellow fluid produced in the live and released from the gallbladder helps to breakdown the large fat molecules in to smaller one (emulsification). Pancreatic Juice: Proteases (e.g., trypsin)- converts proteins (intact or partially digested) into peptides and amino acids Lipases- converts emulsified fat (by bile) into fatty acids, monoglycerides, and glycerol
Pancreatic amylase- converts starch into Maltose sugar. o The intestinal glands present in the mucous linings secrete the intestinal digestive juice that is rich in a variety of enzymes as well as water and ions. Intestinal Enzymes: Peptidases- converts peptides into amino acids Sucrase- converts sucrose (cane sugar) into glucose and fructose (simple sugars, or monosaccharides) Lactase- converts lactose (milk sugar) into glucose and galactose (simple sugars) Maltase-converts maltose (malt sugar) into glucose o The wall of small intestine has multiple circular folds called plica that makes it well suited to absorption of food and water. These folds are themselves covered with thousands of tiny fingers like projection called villi which increase the absorptive surface. Each villus is itself covered by epithelial cells, which each have a brushlike border composed of microvilli. The microvilli further increase the surface area of each villus for absorption of nutrients o Inside each villus lies a rich network of blood capillaries that absorb the products of carbohydrate and protein digestion (sugars and amino acids) and lymph capillaries called lacteals that absorb fats. o The main functions of the small intestine are digestion and absorption. Nearly all the chemical digestion and absorption of the digestive system occurs in the small intestine.
Liver
Liver is largest gland fills upper right section of abdominal cavity and extends over into left side’. It secretes a yellowish or greenish fluid called bile. The function of bile is similar to how soap helps break up fat when washing dishes because it mechanically breaks up or emulsifies fats. Liver has a variety of metabolic functions. Ducts o Hepatic duct drains bile outside the liver. o Cystic— duct by which bile enters and leaves gallbladder. o Common bile—formed by union of hepatic and cystic ducts and drains bile from hepatic or cystic ducts into duodenum
Gallbladder
Location—undersurface of the liver Function—concentrates and stores bile produced in the liver. Bile-Chemically, bile contains significant quantities of cholesterol and substances (bile salts) that act as detergents to mechanically break up, or emulsify, fats. In addition to emulsification of fats, bile that is eliminated from the body in the feces serves as a mechanism for excreting cholesterol from
Pancreatic disorders
Pancreatitis —inflammation of pancreas. Acute pancreatitis results from blocked ducts that force pancreatic juice to backflow. Diabetes mellitus - the inability of the islet cells to make insulin as a result blood glucose increase.
Large Intestine
The large intestine is only about 1.5 meters (5 feet) in length. Undigested and unabsorbed food material enters the cecum of large intestine after passing through a sphincter-like structure called the ileocecal valve. Divisions
- Cecum
- Colon—ascending, transverse, descending, and sigmoid (S shaped bend)
- Rectum
- Anal canal and anus (Opening to exterior) Function : o The remains of the food material after digestion in the small intestine is acted on by beneficial intestinal microbiome or flora causing release of additional nutrients from cellulose and other fibers and are absorbed in the colon. o These normal florae are also responsible for the synthesis of vitamin K needed for normal blood clotting and for the production of some of the B-complex vitamins. After they are formed, these vitamins are absorbed from the large intestine and enter the blood. The intestinal microbiome also plays a role in supporting immune functions that protect us from many intestinal diseases. o Some bacteria also produce gases that escape from the colon through the anus—a phenomenon called flatulence or flatus. o No villi are present in the mucosa of the large intestine. As a result, much less surface area is available for absorption. Salts, especially sodium, are absorbed by active transport, and water is moved into the blood by osmosis. o Two sphincter muscles stay contracted to keep the anus closed except during defecation —the elimination of feces. Smooth or involuntary muscle composes the internal anal sphincter, but voluntary skeletal muscle forms the external anal sphincter.
Disorders of the large intestine
Often relate to abnormal motility (rate of movement of contents). Diarrhea— Diarrhea is characterized by frequent passing of watery feces. In inflammatory conditions such as dysentery, the watery feces also may contain mucus, blood, or pus. It results from abnormally in creased intestinal motility; may result in dehydration or convulsions Constipation—results from decreased intestinal motility produce hard stool. Diverticulitis is an inflammation of abnormal saclike pouches of the intestinal wall called diverticula. Diverticula often develop in adults older than 50 years of age who eat a low-fiber diet. Diverticulitis can be a cause of constipation. Colitis refers to any inflammatory condition of the large intestine. If present for prolonged periods of time, inflammatory bowel disease becomes a significant risk factor for colorectal cancer. Symptoms of colitis include diarrhea and abdominal cramps or constipation. Some forms of colitis also may produce bleeding and intestinal ulcers. Colitis may be a result of emotional stress, as in irritable bowel syndrome. It also may result from an autoimmune disease, as in ulcerative colitis. Another type of autoimmune colitis is Crohn disease , which often also affects the small intestine.
Appendix and Appendicitis
Vermiform appendix is blind tube attached directly to cecum, no important digestive function in humans Appendicitis —inflammation or infection of appendix. If appendix ruptures, infectious material may spread to other organs.
Peritoneum
The peritoneum is a large, moist, slippery sheet of serous membrane that lines the abdominal cavity and covers the organs located in it, including most of the digestive organs. o Parietal layer of peritoneum lines abdominal cavity o Visceral layer of peritoneum covers abdominal organs also known as serous layer (serosa). o Peritoneal space is the space between parietal and visceral layers. It contains peritoneal fluid to keep both layers of the peritoneum moist and able to slide freely against each other during breathing, digestive movements, and twisting or bending of the torso. Peritonitis —inflammation of peritoneum resulting from infection or other irritant; often a complication of ruptured appendix Ascites —abnormal accumulation of fluid in peritoneal space; often causes bloating of abdomen.
Digestion
Definition —a complex process that transforms food into a form that can be absorbed and used by cells. Enzymes — are functional protein molecules that act as catalysts. That is, they speed up specific chemical reactions without themselves being changed or consumed during the reaction process. E.g protease. Lipase etc Carbohydrate digestion —mainly in small intestine (salivary amylase partially in the mouth cavity breaks starch into double sugar.) Protein digestion —starts in stomach; completed in small intestine Fat digestion- Bile contains no enzymes but emulsifies fats (breaks fat droplets into very small droplets). Fat digestion begins in the small intestine
