1. Chapter 5 Digestion, Absorption, and Metabolism 1

2. Ch 5 Key Concepts: Whole foods must be changed to simpler substances in order to be useful to the body. The body must free the food nutrients and re-form and re-route them to meet special life needs. Special organ structures and functions conduct these tasks through the successive parts of the overall system. 2

3. Objectives Identify the principles of digestion Describe the absorption process Identify the human water balance system Identify human acid – base balance system 3

4. Digestion Basic principles: Food can not be used as eaten Food must be changed to a simpler form for absorption Digestion is the process in which food is broken down in the gastrointestinal tract Absorption is the process in which these nutrients are taken into the cell lining the gastrointestinal tract 4

5. Digestion 5 Principle of Wholeness The parts of this overall process of change do not occur separately but make one continuous whole Mechanical and chemical changes – Food must go through a series of mechanical and chemical changes Together these two types of actions make up digestion: Mechanical digestion: gastrointestinal motility Chemical digestion: gastrointestinal secretions

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8. Mechanical Digestion  Motility = the ability to move spontaneously  Starting in the mouth, muscles and nerves in the walls of the GI tract coordinate their actions to provide the needed motility for digestion to proceed 8

9. Mechanical Digestion Muscles – Layers of muscles provide two types of movement Muscle Tone - tonic contraction ensures the continuous passage of the food mass and valve control along the way Periodic muscle contraction and relaxation – rhythmic waves that mix the food mass and move it forward 9

10. Mechanical Digestion Nerve - Complex nerves in the gastrointestinal wall extend from the esophagus to the anus Control muscle tone in the wall Regulate the rate and intensity of muscle contractions Coordinate all of the various movements 10

11. Chemical Digestion  A number of secretions work together to make chemical digestion possible  5 types of substances involved:  Hydrochloric acid and buffer ions – needed to produce the correct pH which is necessary for enzyme activity  Digestive Enzymes – digestive enzymes are proteins, specific in kind and quantity for breaking down specific nutrients  Mucus- lubricate and protect the mucosal tissues; help mix food mass 11

12. Chemical Digestion Water and electrolytes – products of digestion are carried and circulated through tract and into tissues by water and electrolytes Bile – made in liver and stored in gallbladder, bile breaks down fats into smaller pieces to assist fat enzymes 12

13. Chemical Digestion  Secretory Cells: in the GI tract produce each of the preceding substances.  The action of these cells is stimulated by:  Presence of food  Nerve impulses  Hormones specific for certain nutrients 13

14. Digestion in the Mouth and Esophagus Mechanical Digestion: Mastication (chewing) breaks down food into small particles Muscles at the base of the tongue facilitates swallowing Food is swallowed and passes down the esophagus through peristaltic waves and gravity 14

15. Digestion in the Mouth and Esophagus Gastroesophageal sphincter at the stomach entrance relaxes, allowing food to enter, then constricts to retain food GERD (Gastroesphageal Reflux Disease) – Reflux – Heartburn If the gastroesophageal sphincter is not working properly, it may allow the acid- mixed food to seep back into the esophagus  a uncomfortable feeling of “heartburn” 15

16. Digestion in the Mouth and Esophagus GERD cont. “Heartburn” – unrelated to cardiac Called this because the sensations are perceived as originating in the area of the heart 16

17. Digestion in the Mouth and Esophagus Chemical Digestion Salivary glands secrete material containing salivary amylase or ptyalin – a “starch- splitting” enzyme in the mouth Ebner’s serous glands at the back of the tongue secrete a lingual lipase 17

18. Digestion in the Mouth and Esophagus Salivary glands also secret a mucous material to lubricate and bind food particles, facilitating the swallowing of the food bolus (lump) Secretions from the mucous glands in the esophagus help move food toward the stomach 18

19. Digestion in the Stomach  Mechanical digestion:  Under sphincter control, the food enters the upper portion (fundus) of the stomach as individual bolus lumps  The muscles knead, store, mix, and propel the food mass forward in a slow controlled movement 19

20. Digestion in the Stomach Mechanical digestion cont.  By the time the food mass reaches the lower portion of the stomach, it is a semi-liquid acid/food mix called chyme.  Chyme is released slowly into the first section of the small intestine (duodenum) by the pyloric valve 20

21. Digestion in the Stomach  Chemical digestion:  Gastric secretions contain acid, mucus and enzymes which aid in digestion  Hydrochloric Acid – parietal cells secrete HCL to activate enzymes  Mucus – protect stomach lining from the erosive effect of the acid, and bind and mix the food mass and help it move along 21

22. Digestion in the Stomach Chemical digestion cont.  Enzymes - inactive form (pepsinogen) is secreted by the stomach cells and activated by HCl to become pepsin, a protein-splitting enzyme 22

23. Digestion in the Small Intestine  The structural parts, synchronized movements and array of specific enzymes of the small intestine are highly developed for the final task of mechanical and chemical digestion. 23

24. Digestion in the Small Intestine Mechanical Digestion:  Under the control of nerve impulses, stomach walls stretch from the food mass and the intestinal muscles produce several types of movements:  Peristaltic waves – push food forward  Pendular movements –small, local sweeps back and forth, stirring the chyme 24

25. Digestion in the Small Intestine Mechanical digestion cont. Segmentation rings – alternating contractions and relaxations of circular muscles Longitudinal rotation – long muscles roll slowly moving food in spiral motion Surface villi motions – mix chyme at intestinal walls – exposing additional nutrients for absorption 25

26. Digestion in the Small Intestine Chemical digestion Pancreatic enzymes Carbohydrate Pancreatic amylase converts starch to disaccharides, maltose and sucrose Protein – Trypsin and chymostrypsin split large protein molecules into smaller peptide fragments and single amino acid Fat – Pancreatic lipase converts fats to glycerides and fatty acids 26

27. Napping: what you do after lunch! 27

28. Digestion in the Small Intestine Intestinal enzymes : Carbohydrate Disaccharidases (maltase, lactase, and sucrase) convert their respective disaccharides (maltose, lactose, and sucrose) to monosaccharides (glucose, fructose, and galactose) 28

29. Digestion in the Small Intestine Lactose Intolerance  A large percentage of the world’s population does not produce enough lactase to digest lactose (milk sugar  As a result, these individuals cannot tolerate milk and milk products unless they are in the predigested form – e.g. yogurt, buttermilk, aged cheese, or lactase-treated milk. May also take commercially available oral lactase 29

30. Digestion in the Small Intestine  Protein  The enzyme enterokinase activates trypsinogen from the pancreas and turns it into trypsin (the protein- splitting enzyme).  Fat  Intestinal lipase splits fat in to glycerides and fatty acids 30

31. Digestion in the small Intestine Mucus- large quantities of mucus protect the mucosal lining from irritations and erosion caused by the acidic gastric contents Bile – emulsifying agent - aids fat digestion and absorption [made in the liver, stored in the GB] 31

32. Digestion in the small Intestine Hormones Secretin – produced by the mucosal glands – this controls the acidity and secretion of enzymes from the pancreas. Cholecystokinin – secreted by intestinal mucosal glands when fat is present; triggers the release of bile from gallbladder to emulsify the fat 32

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35. Accessory Organs  The “accessory organs” (to the duodenum) make up the “biliary system”  The liver is sometimes called the “Metabolic Capital” of the human body  Metabolizes all converging nutrients 35

36. Role of the Liver Processes blood immediately after it leaves the gastrointestinal tract Bile production Synthesis of protein and blood clotting factors Regulation of blood glucose Metabolism of hormones and medications 36

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38. Absorption and Transport When digestion is complete, food has been changed into simple end-products that are ready for cell use Transport- is the movement of nutrients through the circulatory system from one area of the body to another 38

39. Absorption and Transport “Simple” end-products: Carbohydrates: reduced to simple sugars (glucose, fructose, galactose) Fats: changed into fatty acids and glycerides Proteins: changed into single amino acids Vitamins and minerals: liberated from food 39

40. Absorption in the Small Intestine Special absorbing structures – [p. 72, Fig.5-6]  Mucosal folds - surface of the small intestine piles into folds  Villi – small finger-like projections, cover the mucosal folds, increasing the area of exposed intestinal surface  Microvilli – electron microscope reveals a covering of multiple small projections on the surface of the villi. This is the brush border because it looks like bristles on a brush 40

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42. Absorption Processes  Moving vital nutrients across the inner intestinal wall into body circulation.  Simple diffusion – the force by which particles move outward in all directions, from an area of greater concentration to lesser concentration  Facilitated diffusion – simple diffusion with the help of a protein channel for carrier assisted movement of larger items across the mucosal cell wall 42

43. Absorption and Transport  Active transport – Force by which particles move from area of greater concentration to lesser concentration with help from a carrier or ferry  Pinocytosis – Penetration of larger materials by attaching to the thicker cell membrane and being engulfed by the cell 43

44. Absorption in the Large Intestine  Main absorptive task: to take up water  Most water in chyme is absorbed in the first half of the colon  Only a small amount remains to form and eliminate the feces  Dietary fiber  Not digested  Contributes bulk to food mass  Helps form feces 44

45. Water: good for digestion 45

46. Transport Nutrients must be transported to cells Vascular system (blood, circulation) – Most of the products of digestion are water-soluble nutrients  can be absorbed into the vascular system directly from the intestinal cells Composed of veins and arteries and is responsible for supplying the entire body with nutrients and oxygen necessary for life. Transports waste, such as carbon dioxide and nitrogen, to the lungs and kidneys for removal 46

47. Transport  Lymphatic system – route for fatty materials which are not water soluble  Fat molecules pass into the lymph vessels in the villi, flow into the larger lymph vessels of the body and eventually enter the blood stream 47

48. Metabolism  Macronutrients in food have been broken down through digestion (-> monosaccharides, fatty acids, amino acids) and now must be absorbed into blood stream  They will be converted to energy or stored for later use. 48

49. Metabolism  Energy for Fuel - the sum of chemical reactions occurring within a living cell to maintain life  2 types of metabolism:  Catabolism – the breaking down of larger substances into smaller units  Eg. The process of breaking down large carbohydrate and protein chains into their smaller building blocks – monosaccharides and amino acids- is a catabolic reaction  Anabolism – the process in which cells build large substances from smaller particles 49

50. Help for Anabolism 50

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52. Metabolism  Energy for storage  If the amount of food consumed yields more energy than is needed, the “leftover’ energy is stored for later use in the body.  E.g. Excess glucose can be stored as glycogen in the liver and muscles for quick energy at a later time 52

53. Metabolism and fight/flight 53