Human Digestive system

Digestive System Composed of Gastrointestinal (GI) Tract and accessory organs Breaks down ingested food for use by the body Digestion occurs by mechanical and chemical mechanisms Excretes waste products or feces through process of defecation

Gastrointestinal (GI) Tract / Alimentary Canal Continuous tube from mouth to anus Mouth Pharynx Esophagus Stomach Small intestine Large intestine

Accessory Digestive Organs Provide mechanical and chemical mechanisms to aid digestion Teeth Tongue Salivary glands Liver Gallbladder Pancreas

Digestion Phases Include Ingestion Movement Mechanical and Chemical Digestion Absorption Elimination

Mechanical and Chemical Digestion (1) Mechanical (physical) Chew Tear Grind Mash Mix Chemical Enzymatic reactions to improve digestion of Carbohydrates Proteins Lipids

Mechanical and Chemical Digestion (2) Mixing waves caused by peristaltic movement Chyme released in process of gastric emptying Proton pumps bring H+ into the lumen Carbonic anhydrase forms carbonic acid to provide H+ and bicarbonate ions (HCO3-)

Mechanical and Chemical Digestion (3) Chemical digestion stimulated by nervous system Parasympathetic neurons release acetylcholine Works with gastrin HCl released in presence of histamine Pepsin begins digestion of proteins Stomach protected by alkaline mucus secretion Gastric lipase digests triglycerides Few molecules absorbed by stomach Water, ions, short-chain fatty acids, alcohol

Mouth Teeth mechanically break down food into small pieces. Tongue mixes food with saliva Epiglottis is a flap-like structure at the back of the throat, preventing food from entering the trachea

Salivary Glands Release saliva to oral cavity 3 pairs of salivary glands Parotid Submandibular Sublingual

Composition of Saliva 99.5 % water 0.5% other solutes Ions Mucus Immunoglobulin A Enzymes Salivation controlled by autonomic nervous system Stimulated by various mechanisms

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Table 23.1

Pharynx Composed of skeletal muscle Lined by mucous membrane Nasopharynx Oropharynx Laryngopharynx

Esophagus Approximately 10” long Functions include: Secrete mucus Moves food from the throat to the stomach using muscle movement called peristalsis If acid from the stomach gets in here that’s heartburn. A good way to describe peristalsis is an ocean wave moving through the muscle. These diagrams don’t separate the esophagus from the mouth functions, you might want to talk about what happens in the mouth too.

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Stomach J-shaped muscular bag that stores the food, breaks it down into tiny pieces. Mixes food with digestive juices that contain enzymes to break down proteins and lipids. Acid in the stomach kills bacteria. Food found in the stomach is called chyme. The stomach takes around 4 hours to do it’s job on the food, depending on what kinds of food are digested.

Figure 23.10a

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Small Intestine Small intestines are roughly 7 meters long Lining of intestine walls has finger-like projections called villi, to increase surface area. The villi are covered in microvilli which further increases surface area for absorption. Duodenum Jejunum Ileum

Small Intestine Nutrients from the food pass into the bloodstream through the small intestine walls. Absorbs: 80% ingested water Vitamins Minerals Carbohydrates Proteins Lipids Secretes digestive enzymes

Large Intestine About 5 feet long Accepts what small intestines don’t absorb Rectum (short term storage which holds feces before it is expelled). Depending on the maturity of the group, you can talk about the feces leaving via the anus. Mention the appendix at the bottom of the ascending colon and that it might have been used long ago but is not today Mention the portions of the large intestine, ascending, transverse, descending, sigmoid, and rectum (last one if the audience is mature enough)

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Large Intestine It absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. Recycling of various nutrients takes place in colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling The colon absorbs vitamins that are created by the colonic bacteria. The standing gradient osmosis is the reabsorption of water against the osmotic gradient in the intestines.

Accessory Organs Not part of the path of food, but play a critical role. Include: Liver, gall bladder, and pancreas

Liver and Gallbladder Liver Largest gland at 1.4 kg (~3 lb) Closely associated with liver

Liver Directly affects digestion by producing bile Bile helps digest fat filters out toxins and waste including drugs and alcohol Livers can regenerate missing pieces if necessary. Is one of the largest organs in the body.

Gall Bladder Stores bile from the liver, releases it into the small intestine. Fatty diets can cause gallstones Explain to students that removing the stones typically means removing the gallbladder, but that the body eventually adjusts to not having the bile stored.

Pancreas Produces digestive enzymes to digest fats, carbohydrates and proteins Regulates blood sugar by producing insulin Explain the difference between the two types of diabetes. In type 1, the pancreas fails to produce enough insulin. In type 2, the body stops responding properly to the insulin it creates.

Pancreas Produces secretions to aid digestion Head Body Tail Pancreatic duct /duct of Wirsung Hepatopancreatic ampulla Sphincter of the heatopancreatic ampulla (sphincter of (Oddi) Regulates passage of pancreatic juice and bile Accessory duct (duct of Santorini)

Figure 23.13c

Histology of Pancreas Glandular epithelial cells 99% exocrine clusters Secrete pancreatic juice Fluid and enzymes Pancreatic islets (islets of Langerhans) 1% endocrine cells Hormones Glucagon Insulin Somatostatin Pancreatic polypeptide

Pancreatic Juice 1200-1500 mL/day pH 7.1-8.2 Water Salts Sodium bicarbonate Enzymes Pancreatic amylase Trypsin Entereokinase Chymotrypsin Carboxypeptidase Elastase Pancreatic lipase Ribonuclease and deoxyribonuclease

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