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The Digestive System Tri-State Business Institute Micheal H. McCabe Introduction: The digestive system is also called the alimentary canal and the gastrointestinal tract. The organs of the digestive system have four primary functions: – – – – Ingestion Digestion Absorbtion Elimination Ingestion: Ingestion involves all behaviors associated with the acquisition and consumption of food and beverage. Ingestion involves not only the organs and structures of the alimentary tract, but the entire organism – body and mind. Ingestion is often governed as much by social convention as by hunger. Digestion: Food is broken down mechanically in the mouth by chewing – this increases the surface area of the food and speeds dissolution. Chemical action serves to break down food into its component parts. Complex materials are broken down into simpler compounds. Solvents, including water and strong acids, dissolve nutrients and allow for absorbtion. Chemical Digestion: Enzymes – are chemicals that speed-up chemical reactions and help in the breakdown of complex nutrients. Stomach Acids – primarily hydrochloric acid – dissolve minerals, and break down complex materials (like cellulose). Bile – emulsifies fat to allow absorbtion. Absorbtion: Digested food must by absorbed into the bloodstream by passing through the walls of the small intestine. Carbohydrates (sugars) and amino acids are distributed by the bloodstream throughout the body where they provide energy and raw materials to the individual cells. Absorbtion, Continued: Amino acids are used as raw materials to build new protein structures within the cells. Excess carbohydrates are stored within the liver as starch (glycogen.) Fats are broken down into fatty acids and glycerol. Fatty acids are then stored in adipose tissue as an energy reserve. Elimination: Many materials that are ingested cannot be absorbed. These materials are considered solid waste. The large intestine (colon) collects and concentrates this waste (called feces). Wastes ultimately pass from the body via the anus (defecation.) Anatomy of the Digestive System The Oral Cavity: The oral cavity (mouth) is the first part of the digestive system. Food is ingested (eaten) and the digestive process begins within the mouth. Mastication (chewing) is the first step in the mechanical breakdown of nutrients. Anatomy of the Oral Cavity: Cheeks – form the lateral walls of the mouth. Lips – surround the opening of the mouth. Hard Palate – forms the anterior portion of the roof of the mouth. Soft Palate – muscular structure forming the posterior portion of the roof of the mouth. Rugae – are irregular ridges in the mucous membrane covering the anterior portion of the hard palate. Uvula – small appendage of the soft palate. Serves as an accessory organ for speech and acts as a sense organ in swallowing. Anatomy of the Oral Cavity, Continued: Tongue – large muscular organ located on the floor of the oral cavity and attached to the mandible by muscles. Moves food around during mastication and swallowing. Mastication – is the act of chewing. Deglutition – is the act of swallowing. Papilla – are small raised areas on the tongue that contain taste buds; specialized sense organs that respond to the chemical composition of food. Tonsils – are masses of lymphatic tissue located on both sides of the oropharynx. Anatomy of the Oral Cavity, Continued: Gums – are made of fleshy tissue and surround the sockets of the teeth. Teeth – are specialized structures of several types that are used to cut, pierce, and grind food during mastication. A complete set of adult dentition includes 32 permanent teeth. “Milk Teeth” – sometimes present in newborns are not true teeth – these are specialized structures of cartilage. “Baby Teeth” are called deciduous teeth and are replaced by larger adult teeth between the ages of 5-10 years. Diagram of the Oral Cavity: The Dental Arch (Upper): Tooth Classification: Central Incisors – are designed to cut food – the have a sharp chisel-shaped edge that allows you to sever a portion of food from a larger mass. Lateral Incisors – also serve to cut food, the arch-shaped arrangement of the central and lateral incisors allows a discrete “bite” of food to be taken from a larger mass. Canines – are pointed and provide the ability to pierce through tough membranes present in food. Sometimes called “Fangs” or “cuspids”– they serve as a killing instrument in carnivorous animals. Bicuspids – Also called pre-molars – these teeth serve to crush food and break fibers up. Tooth Classification: Molars – serve to pulverize and grind the brokenup food particles into a fine mash. Dentists use special terms to describe the surfaces of teeth: Labial – is used to describe the surface of incisors and canine teeth adjacent to the lips. Buccal – describes the surface of bicuspid and molar teeth nearest the cheek. Some dentists use the term facial to describe both the labial and buccal surfaces. Opposite the facial side, all teeth have a lingual surface near the tongue. Tooth Classification: The mesial surface of a tooth is the face nearest the midline of the body. The distal surface of a tooth is the face farthest from the medial line. Bicuspids and Molars have an additional surface called the occlusal surface. This is where the teeth come together when chewing food. Incisors and Cuspids both have a sharp incisal edge. That serves as the cutting edge. Inner Anatomy of a Tooth: Inner Anatomy of a Tooth: Crown – the portion of the tooth visible above the gum line. Root – the portion of the tooth below the gum line. Enamel – is the dense, hard, white substance that forms the outermost protective layer of the crown. Enamel is the hardest substance in the human body. Inner Anatomy of a Tooth: Dentin – forms the main substance of the tooth. Dentin is a yellow substance, similar to bone, that lies beneath the enamel and extends throughout the crown. Pulp – lies beneath the dentin. Pulp is a soft and delicate tissue that forms the center of the tooth. Blood vessels, nerve endings, connective tissue, and lymph tissue are all found in the pulp canal (also called the root canal.) Salivary Glands: Three pairs of salivary glands surround the oral cavity. These glands produce Saliva that contains important digestive enzymes such as salivary amylase that begin chemical digestion of food while still in the mouth. Salivary Glands: The Gastrointestinal Tract: Pharynx: The pharynx (throat) is a muscular tube appx. 5 inches long, lined with mucous membrane. The pharynx serves as a common passageway for both air traveling from the nose to the trachea, and food traveling from the oral cavity to the esophagus. When swallowing (deglutition) occurs, a flap of tissue, the epiglottis, covers the trachea so that food can’t enter. Esophagus: The esophagus is a muscular tube extending from the pharynx to the stomach. Rhythmic contractions of muscles in the wall of the esophagus propel food towards the stomach. Peristalsis is the name given to this progressive, involuntary, rhythmic contraction of smooth muscle observed in most of the organs in the digestive system. Stomach: Food passes from the esophagus into the stomach through the cardiac (esophageal) sphincter. The cardiac sphincter normally closes after passing a bolus of food into the stomach – this prevents gastric reflux (heartburn.) Abnormalities with the cardiac sphincter may result in “GERD” (gastroesophageal reflux disease.) Stomach, continued: Within the stomach, gastric acid (primary hydrochloric acid) and enzymes breakdown food particles into simpler substances that can later be absorbed by the intestines. The stomach “churns” – rhythmically contracts to thoroughly mix food with stomach acids and enzymes. Anatomy of the Stomach: Yet More About the Stomach: The stomach is a hollow, muscular organ that serves as a “staging area” where digesting food is held prior to passage into the small intestine. Rugae is a specialized tissue present in the stomach, urinary bladder, and similar hollow organs that allows the stomach to expand in size without causing injury. Specialized cells in the stomach wall produce hydrochloric acid and digestive enzymes. The Duodenum: After food has been thoroughly mixed with stomach acid, the mixture (now called chyme) passes through the pyloric sphincter into the duodenum. Within the duodenum, bile and pancreatic juice are added to the chyme to emulsify fats and break proteins up into the 29 basic amino acids. The duodenum is the shortest segment of the small intestine; normally about a foot in length. Accessory organs of Digestion: The liver is a multi-purpose organ that produces bile as one of its principal functions. Bile is a “salt” made from acid and alkali compounds that serves to emulsify fats to allow absorbtion. The pancreas is also a multi-function organ that produces digestive enzymes needed to break down complex proteins into component amino acids. The Gallbladder and Bile Ducts: Bile is manufactured by specialized cells in the liver. It moves through the hepetic duct to the cystic duct and is stored in the gallbladder until needed. When fatty foods are ingested, the gallbladder contracts, forcing bile into the duodenum via the cystic duct and common bile duct. The Pancreas and Pancreatic Duct: Digestive enzymes are made by specialized cells in the pancreas and secreted via the pancreatic duct into the duodenum. The pancreatic duct communicates with the common bile duct and duodenum – infection or inflammation within any of these organs is readily transmitted to the other two. Example: pancreatitis resulting from infection can present clinically with jaundice as the gallbladder and liver become involved. Likewise, gallstones or inflammation of the gallbladder can result in inflammation of the pancreas. Absorbtion of Nutrients in the Small Intestine. Absorbtion of nutrients begins in the duodenum as chyme moves past millions of microscopic villi. The villi resemble small fingers extending from the intestinal wall. Each villi contains a rich bed of capillaries that serve to absorb nutrients and carry it into the bloodstream. Each villi also contains a specialized lymph vessel called a lacteal that serves to absorb emulsified fat and conduct it into the lymphatic circulation. The Jenunum: The second segment of the small intestine is called the jejunum. The jejunum is appx. 8 feet long and continues the process of digestion and absorbtion. Chyme is passed from the jenunum into the third section of the small intestine. The Ileum: The ileum is the third section of the small intestine and is the last section of the gastrointestinal tract to be concerned with the absorbtion of nutrients. The ileum is also the longest section of the small intestine and is approximately 11 feet long. By the time that chyme leaves the small intestine via the ileocecal valve, most water-soluble and emulsified nutrients including carbohydrates, protein, and fat have been absorbed. The remaining matter consists largely of waste and is called feces, or stool, once it enters the large intestine. The Large Intestine: The large intestine is divided anatomically into seven parts: – – – – – – – The The The The The The The cecum appendix ascending colon transverse colon descending colon sigmoid colon rectum The Cecum: The cecum is a pouch on the lower end of the ascending colon. Fecal matter enters the cecum through the ileocecal valve where the cecum communicates with the ileum. The principal function of the large intestine is recovery of water, minerals, bile, and enzymes from the waste stream. “Recover, Recycle, and Reuse.” The Appendix: The appendix has no clear function in humans. If clogged or blocked by fecal matter, the appendix can become infected and inflamed – “appendicitis.” Rupture of an inflamed appendix can result in peritonitis, sepsis, and death. The Ascending Colon: The ascending colon extends upward from the cecum (lower right side) to the undersurface of the liver. Below the liver, a 90 degree bend in the colon (called the hepatic fixture) occurs and fecal matter passes into the transverse colon. Transverse Colon: The transverse colon passes across the abdominal cavity towards the spleen. A second 90 degree bend in the colon occurs near the spleen (splenic fixture) and fecal matter progresses into the descending colon. Descending Colon: From the splenic fixture, fecal matter moves downward through the descending colon toward the pelvic crest. At the pelvic crest, the descending colon makes an “S” turn and becomes known as the sigmoid colon. Sigmoid Colon: The “S” shaped section of the colon serves to carry fecal matter from the abdominal cavity into the pelvic cavity. The lower (distal) end of the sigmoid colon connects with the rectum. The Rectum: The rectum is an expansive, muscular organ that serves to store fecal matter until it can be expelled from the body. The opening of the rectum is called the Anus and is equipped with two sphincters – the inner sphincter is involuntary and controlled by the autonomic nervous system. The outer sphincter is under voluntary control – to a point. Elimination: The elimination reflex is triggered by a sense of pressure in the rectum. When the sensation of pressure in the rectum becomes apparent, the inner (involuntary) sphincter relaxes and the need to pass stool becomes “urgent.” Voluntary control of the external sphincter allows reasonable selectivity regarding the time and place of elimination. Elimination, Continued: The extent of voluntary control is limited in scope and duration. If voluntary defecation isn’t undertaken, stool continues to collect in the rectum and pressure increases. Beyond a certain point, voluntary control of the external sphincter is lost and defecation occurs automatically. Diagram of the Gastrointestinal Tract: