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10.2 The Human Digestive Tract Hierarchical Organization of the Body Atom molecules organelles cells tissues organs organ system organism * Four main types of tissues: connective, epithelial, nervous & muscle. Digestive System Overview Known as gastrointestinal (GI) tract or alimentary canal. Open at both ends to the outside world. Consists of long convoluted tube with accessory organs. Components: mouth, pharynx, esophagus, stomach, small intestine, large intestine & anus. Accessory organs include: salivary glands, pancreas, liver & gallbladder. Animation: Organs of Digestion Steps in Digestion 1. 2. 3. 4. Ingestion Digestion Absorption Elimination Types of Digestion Mechanical: physical breakdown of food Food mass is shredded, torn & churned. Occurs in mouth & stomach. Chemical Chemicals and enzymes break down food. Occurs in mouth (saliva amylase), stomach & small intestine. Structure of the Digestive Tract Lumen (hollow interior) surrounded by several layers of tissue: Mucosa Submucosa Circular smooth muscle Longitudinal smooth muscle Serosa Structure of the Digestive Tract Mucosa Highly folded; contains mucus/enzyme/hormone secreting cells & absorptive cells. Submucosa Connective tissue; contains blood vessels & nerves. Structure of the Digestive Tract Circular Smooth Muscle Responsible for peristaltic contractions; constricts lumen. Longitudinal Smooth Muscle Responsible for peristaltic contractions; shortens tract. Serosa Connective tissue; separates digestive tract from other abdominal organs. Organs of the Digestive System Oral Cavity Esophagus Stomach Small Intestine Large Intestine Oral Cavity Lips, tongue, teeth & jaw muscles break food down into smaller pieces. Food mixes with saliva and amylase, which begins the chemical digestion of carbohydrates. A bolus (moistened ball-like mass) forms and is swallowed. The bolus passes through the pharynx, past the epiglottis & through the esophagus. Structures of the Mouth Gums: part of the jaw that holds the teeth Frenulum: the layer of pink tissue that attaches the lip to the gum Hard Palate: front part of the roof of your mouth, made of bone and skin, also used in the breakdown of food Soft palate: back part of the roof of your mouth, made of muscle, expands for swallowing Uvula: a pinkish-red pointed structure hanging from the soft palate, direct food down the esophagus Lingual tonsil: a rough and bumpy covering near the back of the tongue Palatine Tonsil: an area found on each side of the throat, used to help prevent bacteria from getting into the body. Salivary Glands Parotid glands: lie just below the skin in front of each ear, secrete a watery fluid that contains salivary amylase. Sublingual glands: below the tongue in the floor of the mouth Submandibular glands: located below the roof of the tongue in the throat. N.B. Saliva contains an enzyme called amylase, which begins the chemical digestion of starch into sugar Esophagus Muscular tube that connects pharynx & stomach. The cardiac or lower esophageal sphincter connects the esophagus to the stomach. Constriction of this ring of smooth muscle prevents reflux (ensures one-way flow of food). Pressure exerted by food on the sphincter causes it to open, and food enters the stomach Peristalsis is a series of coordinated muscular contractions that propels food along the digestive tract into the stomach. Stomach J-shaped stretchable organ. Acts as a reservoir for food (1.5 L). Two sphincters control the movement of food coming into and out of the stomach. Cardiac sphincter: Between the stomach & the esophagus. Pyloric sphincter: Between the stomach & the small intestine (the duodenum). Specialized cells of the stomach 1. 2. 3. Mucous secreting cells: lubricate stomach’s walls and protects stomach lining Parietal cells: secrete hydrochloric acid Chief cells: secrete pepsinogen HCl + pepsinogen pepsin (enzyme) HCl breaks up connective tissue that holds food together The hormone gastrin regulates the amount of HCl produced Pepsin breaks protein molecules up into short polypeptides Animation: carbohydrate digestion - myDr.com.au Stomach Smooth muscle forms folds (rugae) that allow the stomach to expand. Mechanical digestion: Walls churn & squeeze bolus. Chemical digestion: Bolus mixes with gastric juices. Hydrochloric acid secreted by gastric glands. Pepsinogen released & converts to pepsin (enzyme that breaks down proteins). Bolus becomes a liquefied paste (chyme). http://www.mcgrawhill.ca/ school/applets/abbio/ch06 /gastric_three_phases_of. swf Stomach Mucus cells secrete mucus to line & protect stomach from HCl (aq). Ulcers: HCl (aq) burns a hole through the mucus, irritating the stomach cells below Usually the result of a particular type of bacteria (antibiotics required). Small Intestine Major site of digestion & absorption (80% occurs). About 6 m long & has a SMALLER diameter than the large intestine. Lined with tiny finger-like projections called villi, which project into the lumen. Microvilli line the villi. Villi & microvilli increase the surface area for absorption. Small Intestine Consists of three sections: Duodenum 2. Jejunum 3. Ileum. 1. Absorption Chemical digestion Chemical digestion & absorption Small Intestine Functions: http://www.mcgrawhill. ca/school/applets/abbi o/quiz/ch06/enzyme_a ction_and_the_h.swf Break down of carbs. By amylases, proteins by peptidases, and fats by lipases Absorption of monosaccharides, amino acids, fatty acids and glycerol (by microvilli) Mechanical digestion: Alternating contraction & relaxation of the smooth muscle mixes chyme with intestinal juices & secretions from the pancreas & liver. Nutrients absorbed into capillaries in the villi. Nutrients transported to the liver & then to all Tutorial 50.1 The Digestion and body cells. Absorption of Fats Products from fat digestion are absorbed into lacteals, which connect to the lymphatic system. Large Intestine Reabsorbs water, salt & some vitamins (no digestion). Holds & compacts unabsorbed material (cellulose/ bacterial fragments). About 1.5 m long & has a LARGER diameter than the small intestine. Consists of four sections: 1. 2. 3. 4. Caecum Colon Rectum Anus. Large Intestine Chyme passes from the small intestine into the caecum, through the ileocaecal valve. Waste products accumulate & are compacted into feces (3/4 water, 1/4 solid matter) Feces pass through the rectum & exit the body through the anus. Appendix attached to caecum, exact function is unknown. Large Intestine Defecation controlled by two sphincters: Rectal sphincter: Between the large intestine & the rectum. Anal sphincter: Between the rectum & the anus. The Digestive System Pancreas Finger-shaped organ approximately 15 cm in length. Located below the stomach. Contains a duct that empties into the small intestine Specialized for secreting hormones & enzymes necessary for proper digestion. Pancreas The acidity of chyme triggers cells in the duodenum to release a hormone called secretin. Secretin enters the bloodstream, signaling the release of bicarbonate ions (a base) from the pancreas. Bicarbonate ions neutralize the HCl in the duodenum, raising the pH so that pepsin becomes inactive and allows digestion of nutrients. Pancreas Other pancreatic enzymes: Lipases: digests lipids. Carbohydrases/pancreatic amylase: digests sugars & starches. Proteases: digests proteins. Trypsinogen: released & activated (trypsin) in the small intestine to digest proteins. Liver Functions: Regulates metabolism. Produces and secretes bile into the small intestine Removes any toxins (e.g., hydrogen peroxide, alcohol & drugs) via catalase. Excessive alcohol & drug use will lead to liver damage &/or death. Liver After skin, the liver is the second largest organ in the body (1.5 kg). The liver synthesizes & secretes 1 L of bile per day. Bile acts as detergent to breakdown or emulsify fat. Contains bile salts, bile acids, cholesterol, water, phospholipids, fatty acids water. & Liver & Gallbladder The liver has a left and right lobe. The gallbladder is under the right lobe & stores and concentrates bile. When fat enters the duodenum, CCK (a hormone) is released, signaling the gallbladder to release bile through the bile duct & into the duodenum. http://www. ahealthyme. com/Imageb ank/digestiv e.swf Liver & Gallbladder Tutorial 50.2 Insulin and Glucose Regulation Digestive system animation illustrating digestion Constipation Advice Digestive System / Drag & Drop Quiz (Organs of the Digestive System)