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Digestive System Gastrointestinal (GI) Tract • GI or alimentary canal – continuous, coiled, hollow tube that winds through ventral body cavity from mouth to anus – Entire GI is ~30 feet in cadaver, but shorter in living person due to muscle contractions – Includes mouth, pharynx, esophagus, stomach, small intestine, large intestine, and ends at the anus – Other accessory organs include salivary glands, pancreas, liver, gall bladder Anatomy of Digestive System Functions of Digestive System 1. Ingestion – putting food into mouth 2. Propulsion – moving food from one organ to the next 3. Mechanical digestion – breaks down food physically into smaller pieces 4. Chemical digestion – large molecules are broken down into their building blocks by enzymes 5. Absorption – transport of digested end products from the GI tract to the blood or lymph 6. Defecation – elimination of indigestible food that leaves through the anus in the form of feces Activities of Mouth, Pharynx, and Esophagus • Mouth – Chewing begins mechanical breakdown of food • Pharynx – Propels food into esophagus by peristalsis (involuntary smooth muscle contractions) • Epiglottis covers windpipe (trachea) to prevent food from moving down into lungs • Esophagus – Propels food into stomach via peristalsis Activities of Stomach • Stomach – a storage tank – C-shaped sac with sphincters at each end • Cardiac or esophageal sphincter prevents food from going back into esophagus • Pyloric sphincter opens to allow food to move into the small intestine – Physical digestion occurs as stomach grinds and churns food – Chyme - partially digested food that leaves the stomach – Chemical digestion occurs with enzymes • Gastric glands – in stomach lining; release gastric juice –Gastric juice – includes digestive enzymes, intrinsic factor, hydrochloric acid (HCl), and mucus »Intrinsic factor helps absorb vitamin B12 • Pepsin – enzyme that breaks down protein (activated by HCl) Activities of the Small Intestine • A tube that is 8 – 18 feet long and twisted into coils • The major digestive organ • Three subdivisions – Duodenum (5% of total length) – Jejunum (40% of total length) – Ileum (almost 60% of total length) • Nearly all food absorption occurs in the small intestine! • Duodenum – Pancreatic ducts dump pancreatic juice into duodenum – Bile duct (joins with pancreatic ducts) dumps bile from liver into duodenum • Jejunum and ileum - primary function is absorption • Intestinal villi tiny projections of inner wall of small intestine; greatly increase surface area for absorption – Each villus contains blood capillaries and a lacteal into which foodstuffs are absorbed • Ileocecal sphincter – opens to let chyme pass into large intestine Activities of the Large Intestine • Larger in diameter than the small intestine, but not nearly as long (~5 feet) • Major function is to dry out indigestible food residues by absorbing water and then eliminate them as feces • Subdivisions: cecum, appendix, colon, rectum, and anal canal • Cecum – first part of large intestine; sac-like with worm-like appendage called appendix • Colon – parts in order are: ascending (travels up), transverse (travels across), descending (travels down), and then sigmoid (S-shaped) • Rectum – stores feces and leads to anal canal • Anus – ending of GI; voluntary sphincter opens to release feces Activities of Accessory Organs • Teeth – breakdown, grind, and tear food into smaller pieces • Salivary glands release saliva; contain mucus to soften food and salivary amylase, which begins carbohydrate digestion – Parotid gland – largest, lie anterior and somewhat inferior to each ear – Sublingual gland – smallest; lie on floor of mouth inferior to tongue – Submanibular gland – lie in floor of mouth on inside surface of lower jaw • Pancreas – soft, pink, triangular gland; secretes pancreatic juice –Pancreatic duct – tube that carries pancreatic juice to duodenum – Pancreatic juice contains the following: • bicarbonate - neutralizes acidic chyme coming from stomach • amylase - enzyme that breaks down carbohydrates • lipase - enzyme that breaks down lipids • trypsin, chymotrypsin, and carboxypeptidase - enzymes that breakdown proteins • nuclease – enzyme that breaks down nucleic acids • Liver – largest gland in the body; digestive function is to produce bile – Bile – leaves liver through the hepatic duct and enters duodenum through bile duct • Yellowish-green liquid containing bile salts, bile pigments (bilirubin), cholesterol, electrolytes, and lipids (no enzymes) • Only the bile salts and lipids help with digestion • Bile salts emulsify fats by physically breaking large fat globules into smaller ones • Gall bladder – small, green sac under liver that stores bile Homeostatic Imbalances • Gall stones – if bile is stored in gall bladder too long or too much water is removed, the cholesterol in it crystallizes forming gall stones – Can cause blockage of hepatic or bile ducts and bile begins is released into bloodstream instead • Bile in blood and eventually tissues causes jaundice • Hepatitis – inflammation of liver; often cause by virus (also called hepatitis) • Cirrhosis – chronic inflammation of liver; can be from drinking too much alcohol • Heartburn – when cardiac sphincter does not close tightly and allows acidic stomach juices to enter esophagus – Hiatal hernia – superior part of stomach protrudes slightly above the diaphragm; which weakens cardiac sphincter • Peptic ulcers – erosion of stomach lining (open sore in membrane), which exposes the lining to acid; often caused by some bacteria (Helicobacteri pylori) • Pancreatitis – inflammation of pancreas caused by pancreatic enzymes breaking down the pancreatic tissue itself • Diverticulitis – diverticula are formed by the inner layer of the intestine protruding through the large intestine wall; causes inflammation and pain – Caused by not eating enough bulk (fiber); colon narrows and puts more pressure on walls • Diarrhea – large intestine does not absorb enough water from feces; loss of electrolytes and fluids can cause severe dehydration • Constipation – too much water is removed from feces; can be caused by low fiber diets • Impacted teeth – teeth that remain embedded in jawbone; can cause pressure and pain; often occurs with wisdom teeth • Appendicitis – infected appendix; bacteria can accumulate because its twisted • Hemorrhoids – enlarged and inflamed branches of the rectal vein in the anal canal that cause itching, pain, and sometimes bright red bleeding • Vomiting – complex reflex that empties stomach through esophagus, pharynx, and mouth (vomiting center in medulla oblongata)