Introduction to the Digestive System and Nutrient Absorption

1. Chapter 3 Anatomy for Nutrition’s Sake

2. ASK YOURSELF: True or False? 1. Essential nutrients are those nutrients that can be made by the body. 2. The body’s cells need nutrients 24 hours a day. 3. Each part of the digestive tract has specific functions. 4. Digestion begins in the mouth. 5. It takes approximately two days for a meal to empty from the stomach.

3. 6. Nutrients are absorbed in the small intestine. 7. The small intestine is longer than the large intestine. 8. The small intestine is lined with finger- like projections (called villi) that increase the absorptive area so that if the lining of the small intestine could be spread out flat, it would cover an area the size of a tennis court. 9. The liver determines the metabolic fate of every nutrient absorbed. 10. The pancreas plays no role in the digestive process.

4. Introduction to the Human Body • The body is composed of millions of cells. • Each cell is a self-contained living entity. • The body cells need: • Energy (or fuel) • Oxygen • Water • Nutrients Cells: The smallest units in which independent life can exist. All living things are single cells or organisms made of cells.

5. Introduction to the Human Body Cells are organized into tissuesdesigned to perform specialized tasks. Some of these tasks include the formation of muscles and organs. Several organs work cooperatively to form a body system. The digestive system consists of organs and tissues working together to supply energy, water, and essential nutrients to every body cell.

6. Digestive System Hypothalamus: Detects fuel deprivation Generates nerve impulses that signal hunger to the conscious part of the brain The hypothalamus monitors the body’s conditions & sends signals to the brain’s thinking portion, the cortex, which decides on actions.

7. Cortex Hypothalamus Spinal cord

8. Digestive System • The gastrointestinal (GI) tract supplies the body with a constant supply of water and nutrients by controlling: • Passage of food through GI tract • Secretion of digestive juices and enzymes • Digestion of food • Absorption of water and nutrients • Circulating blood through digestive system to distribute absorbed substances

9. Digestive System Each part of the digestive tract has specific functions such as: Basic passage of food Temporary storage unit Digest food Absorb food The digestive system

11. Salivary glands Mouth Esophagus Tongue Airway to lungs Stomach Liver Gallbladder Pancreas Pancreatic duct Pyloric sphincter Bile duct Small intestine Colon (large intestine) Appendix Rectum Anus

13. Layers of the Digestive Tract Mesentary (outermost): Attaches the intestines to the abdominal wall. Muscularis externa: Moves food along the GI tract. Mixes food with digestive juices and enzymes. Mucosa (innermost): Secretes gastric juices and digestive enzymes to aid digestion. Secretes mucus to help protect the lining of the GI tract. Layers of the digestive tract Peristalsis

14. The Mouth • Digestive process begins in the mouth through mechanical digestion (physically breaking down foods). • Breaks down indigestible membranes and uncovers nutrient-rich portion of foods. • Exposes surface area of food to digestive enzymes. • Contains salivary glands which secrete digestive enzymes and help to moisten food and make it easier to swallow.

15. The Mouth • When food is chewed the mixture of food particles and saliva is called a bolus. • The bolus moves from the mouth to the esophagus through swallowing. • The first part of swallowing which moves food toward the pharynx is voluntary. • After food reaches the pharynx, swallowing is considered to be involuntary and cannot be stopped. • The trachea (windpipe) closes and the pharynx propels the bolus toward the esophagus.

16. The Esophagus • No digestion takes place in the esophagus. • Food passes from the mouth to the stomach and travels through the esophagus. • Sphincters open and close to allow the bolus to pass. • Sphincter: a circular band of muscle fibers that constrict a passage or close a natural opening in the body. • Peristalsis:Longitudinal and circular muscle layers of the esophagus rhythmically push the bolus down the esophagus.

17. The Stomach • Three jobs of the stomach: • Stores food until it can be processed. • Forms chyme: the semi-liquid blend of food and gastric secretions that forms in the stomach during digestion. • Controls movement of chyme into the small intestine at a rate suitable for digestion and absorption by the small intestine. • An empty stomach holds about 3 tablespoons but can stretch to about one liter for holding food and drinks.

18. The Stomach Food passes through the gastroesophageal sphincter to enter the stomach. Peristalsis causes the stomach to churn, mixing the food with HCL and gastric secretions to form chyme. The pyloric sphincter controls the emptying of the stomach. Anatomy of the stomach

19. Esophagus Smooth muscle Gastroesophageal sphincter Stomach folds Pyloric sphincter Duodenum

20. The Stomach Mixing of stomach contents and emptying of the stomach

21. Stomach Esophagus 1 Gastroesophageal sphincter Pyloric sphincter Duodenum Direction of movement of peristaltic contraction 6 3 4 2 5 Movement of chyme Peristaltic contraction Peristaltic contraction Gastric mixing Gastric emptying

22. The Stomach • The stomach empties in about four hours. • Liquids pass the quickest. Solids stay until mixed with stomach secretions. • Carbohydrate passes more quickly than protein. Fat takes the longest to digest. • The rate of emptying is controlled by the central nervous system (CNS) and hormonal mechanisms.

23. The Small Intestine The small intestine is about 20 feet in length. It is smaller in diameter than the large intestine, thus the name “small” intestine. It finishes the job of digestion and absorption of nutrients.

24. The Small Intestine • Chyme must touch the walls to make contact with secretions and be absorbed at the proper places. • Peristalsis moves chyme through the small intestine. • Stimulated by fiber • Slowed by fat • The ileocecal valve controls the flow of contents going into the large intestine.

25. The Small Intestine • The small intestine works to break down food and nutrients to their smallest absorbable components. • This breakdown is complex, requiring many chemical reactions: • Hormonal messages tell the gallbladder to send its emulsifier, bile. • Hormones send messages to the pancreas to release the neutralizer, bicarbonate. • Pancreatic and intestinal enzymes act to break bonds of larger nutrients.

26. The Small Intestine Bile A compound made from cholesterol by the liver, stored in the gallbladder, and secreted into the small intestine. It emulsifies lipids to ready them for enzymatic digestion. Emulsifier A compound with both water-soluble and fat-soluble portions that can attract lipids into water to form an emulsion.

27. The Small Intestine After breakdown, these nutrient components are able to cross through the absorptive cells lining the small intestine and enter the blood and lymph supply. These cells are selective and efficient as they absorb enough nutrients to nourish all the body’s other cells.

28. The Small Intestine • The intestinal tract lining contains an enormous absorbing surface. • Villi Fingerlike projections of the sheet of cells that line the GI tract; the villi make the surface area much greater than it would otherwise be. • Microvilli Tiny hairlike projections on each cell of the intestinal tract lining that can trap nutrient particles and translocate them into the cells.

29. DIGESTIVE TRACT SECRETIONS

30. Details of the lining of the small intestine

31. Between the villi tubular glands secrete enzyme-containing intestinal juice. Stomach Small intestine Folds with villi on them A villus Capillaries The wall of the small intestine Is wrinkled into thousands of folds and is carpeted with villi. Lymphatic vessel Muscle layers beneath folds Artery Vein This photograph shows part of a human intestinal cell with microvilli. Lymphatic vessel Microvilli Three cells of a villus. Each cell is covered with microvilli.

32. The Small Intestine Simple sugars and water-soluble vitamins are absorbed high in the small intestine because they require no special handling. Nutrients released more slowly are absorbed further down. When the mixture reaches the large intestine, water, fiber, and dissolved minerals are all that is left. The lymphatic system and circulatory systems take over the job of transporting nutrients.

33. The Large Intestine The large intestine is about 1.5 meters long and is larger in diameter than the small intestine. No digestive enzymes are secreted because chemical digestion is complete. Anatomy of the large intestine

34. Transverse colon Descending colon Ascending colon Ileocecal valve Appendix Sigmoid colon Cecum Rectum Internal anal sphincter (smooth muscle) External anal sphincter (skeletal muscle) Anal canal

35. The Large Intestine • Naturally occurring bacteria reside in the large intestine and are responsible for: • Digestion of some fiber and leftover carbohydrate. • Synthesis of absorbable vitamin K. • Primary functions of the large intestine: • Absorb dissolved minerals and water. • Eliminate waste products.

36. The Large Intestine Final waste products are mixed with mucus to form fecal matter. Fecal material normally consists of: water, undigested fiber, bilirubin, bacteria, small amounts of salt. Peristalsis is slower and occurs less frequently. When the rectum fills with feces, the defecation reflex expels the waste products.

37. Accessory Organs of Digestion Organs outside the digestive tract contribute digestive juices through a common bile duct into the small intestine. The organs are: Liver Gallbladder Pancreas Accessory organs of digestion

38. Bile duct from liver Stomach Duodenum Hormones (insulin, glucagon) Blood Endocrine portion of pancreas (Islets of Langerhans) Duct cells secrete aqueous sodium bicarbonate solution Acinar cells secrete digestive enzymes The glandular portions of the pancreas are grossly exaggerated. Exocrine portion of pancreas (Acinar and duct cells)

39. The Liver Determines the metabolic fate of every nutrient we digest and absorb. Performs a wide variety of functions which are essential for life. Produces bile which helps to emulsify fat and improve the efficiency of fat digestion.

41. The Gallbladder A sack attached to the liver where bile is stored and concentrated. Bile empties from the gallbladder into the common bile duct. When chyme with fatty contents enters the small intestine, a hormone stimulates the gallbladder to contract and empty bile into the small intestine.

42. The Pancreas • The Pancreas has both endocrine and exocrine functions. • Endocrine: A term to describe a gland secreting or a hormone being secreted into the blood. • Exocrine: A secretion that is expelled through a duct into a body cavity or onto the surface of the skin.

43. The Pancreas • Endocrine functions include: • Secretion of the hormones insulin and glucagon which regulate blood glucose levels. • Exocrine functions include: • Secretion of digestive enzymes which act on carbohydrates, proteins, fats and bicarbonate. These are secreted into the duodenum.

44. Metabolism Breaking Down Nutrients for Energy: • Nutrients are absorbed from the digestive system into the blood where they converge from capillaries, into veins and then into a single large vein. • This vein carries nutrients to the liver where they break up into a vast capillary network.

45. Between the villi tubular glands secrete enzyme-containing intestinal juice. Stomach Small intestine Folds with villi on them A villus Capillaries The wall of the small intestine Is wrinkled into thousands of folds and is carpeted with villi. Lymphatic vessel Muscle layers beneath folds Artery Vein This photograph shows part of a human intestinal cell with microvilli. Lymphatic vessel Microvilli Three cells of a villus. Each cell is covered with microvilli.

46. Metabolism • Liver cells process the nutrients: • Converts carbohydrate into glucose. • Stores excess glucose as glycogen or fat. • Reassembles fats and packages them for transport or storage. • Alters amino acids as needed to make protein or other amino acids.

47. Metabolism • Nutrients leaving the digestive tract by way of lymph as chylomicrons circulate through the body. • Cells can withdraw fats. • Some find their way into the blood and circulate through to the liver.

48. Metabolism • New products of liver metabolism are released into the bloodstream and circulate to other cells of the body. These products are: • Glucose • Fat packaged with protein (lipoprotein) • Amino acids

49. Metabolism • Surplus fat can be removed by the cells and made ready for storage. • The human body has infinite storage capacity. • Liver glycogen provides a reserve supply of glucose. • The body can draw from this reserve during intervals when glucose is not ingested and absorbed. • This supply lasts about 3 to 6 hours.

50. Metabolism • The storage systems for glucose and fat ensure that the cells will not go without energy nutrients except under extreme conditions. • Other storage systems include: • Liver and fat cells store many vitamins. • Bones provide reserves of calcium and sodium.