IV. Digestion, Absorption and Transport

1. IV. Digestion, Absorption and Transport

2. In order for food to be of any benefit to the body, and for the body to get any nutrients from it, food must be eaten, digested an the resultant nutrients absorbed from the intestinal tract and transported to designated cells throughout the body • This is done through the processes of ingestion, digestion, absorption and transport

3. The Digestive System • Provides two major functions; digestion and absorption • Nutrients such as carbohydrates, fats and proteins are broken down into their basic constituents and then absorbed as sugars, fatty acids and amino acids and finally transported to their designated destinations • The gastrointestinal tract, a hollow tube from mouth to anus, is the organ specifically involved in digestion and absorption

4. Digestion • Involves anatomy, physiology and biochemistry • It is the process of getting food ready for absorption • Includes both the mechanical and chemical processes that break food down into particles that can be absorbed

5. Gastrointestinal (GI) tract • Also called the alimentary canal • Flexible, muscular tube that gets food ready for absorption • Mouth---------> anus - 26 feet • Principle organs - Stomach and Intestine

6. The Complete Digestive Tract

7. Steps Involved in Digestion

8. Before Digestion Begins…….. • Stimulation of the digestive processes begins even before food enters the mouth • The sensory input, sight, sounds and smells, involved in preparing for and anticipating the ingestion of food, play an important role in getting the body ready for taking in (or ingesting) nutrients • This cephalic response (nervous system response) makes the body feel hungry or have the desire to ingest nutrients • Before digestion begins, ingestion (the taking in of nutrients - generally through the mouth) must occur

9. Digestion Begins in the Mouth • Mechanical and chemical processes of chewing and swallowing are under conscious control. The rest of the mechanical processes involved in digestion are under unconscious or involuntary control • Food enters the mouth where it is reduced to a coarse mash through a process called mastication

10. Mastication - the breakdown of food by mandibular movement (movement of the food around the mouth by the tongue, cheeks, lips and lower jaw) and the subsequent breakdown of the food into smaller, swallowable pieces • The mouth contains two fluids that are involved in the initial digestion of food; saliva and mucous

11. Once the food has been chewed, broken down and partially digested, it makes its way to the back of the mouth • The movement of the tongue helps to position the food so that it can be swallowed • The chewed and partially digested food is called a BOLUS (ball of chewed food mixed with saliva)

12. Chemical Processes • Saliva- contains salivary amylase (initial breakdown of carbohydrates) and lingual lipase (initial breakdown) of fats • Mucous - slippery secretion secreted by the mucous membranes lining the mouth; envelopes and lubricates the food, easing passage from the mouth to the rest of the digestive tract

13. Salivary amylase - begins starch breakdown by hydrolyzing the glycosidic linkages (linkages connecting the individual glucose molecules in starch) (yielding monosaccharides, disaccharides and shorter polysaccharides) • Lingual Lipase - secreted by the sublingual salivary gland under the tongue • - begins the breakdown of short chain fatty acids

14. Mouth to the Stomach • As the bolus is swallowed, it enters into the pharynx. • The pharynx is the part of the digestive tract responsible for swallowing and is common to both the digestive and respiratory tracts • Food passes through here on its way to the esophagus and air passes through here on its way to and from the lungs • A valve-like flap, called the epiglottis exists in the pharynx that serves to cover the opening to the lungs when swallowing to prevent food from entering the lungs

15. Sometimes the epiglottis gets stuck, or doesn’t completely cover the opening and consequently food gets stuck or lodged in the airway • When this happens, the person can’t breathe and the food must be dislodged in order to prevent suffocation • If the food can’t be dislodged by coughing then a process called the Heimlich can be used

17. Movement of Food • Once the bolus is swallowed, it slides down the esophagus and is conducted through the diaphragm to the stomach by peristalsis • Peristalsis is the coordinated involuntary muscular contractions and relaxations that aides the movement of the food down the esophagus and also through the intestine • Vomiting - reverse of peristalsis

18. Peristalsis

19. Entrance to the Stomach • Entrance to the stomach is gained through a muscular valve that lies at the end of the esophagus and the beginning of the stomach called the Cardiac Sphincter

20. Cardiac sphincter • Muscle encircling the tube of the digestive tract that acts like a valve opening and closing to allow food in • Under unconscious control • Relaxes to allow the bolus to enter the stomach and closes behind so that it can’t slip back up • Also prevents “reflux” of the stomach contents in to the esophagus thus preventing heartburn • NO CHEMICAL PROCESSES AIDING DIGESTION OCCURS IN THE ESOPHAGUS

21. Stomach • The stomach is a pouch-like expanded portion of the G.I. Tract that serves as a temporary storage place for food. • Approximate 1 litre capacity • Comprised of three sets of muscles; circular, longitudinal and horizontal • The coordinated movement of these muscles results in the “churning action” of the stomach. This mixes the food with the gastric juices to form a semiliquid mass called chyme

23. Digestion occurs via mechanical processes and chemical processes • Mechanical processes involve the churning of the food to mix it with the gastric juices while at the same time breaking it down further • The chemical processes of digestion in the stomach occur via hydrolysis of the food by HCl and the digestive enzymes in the gastric juices • Enteric Coating - mucopolysaccharide (mucous) secreted by the stomach lining that prevents autodigestion of the stomach by the gastric juices

24. Food is retained in the stomach for 2-3 hours (depending on its composition) • After it is thoroughly mixed and liquified, it moves to the bottom of the stomach where it leaves to enter the small intestine by another valve called the pyloric sphincter

25. Regulation of Gastric Secretion • Secretions of the stomach are under both nervous and hormonal control • Governed by signals from the brain, • stomach and small intestine • signals work like an on/off switch - stimulation or inhibition of gastric juice secretion • three phases of gastric secretion • cephalic • gastric • intestinal

26. Cephalic phase • Occurs before food enters the stomach, even before food enters the mouth • nervous signals are sent by the brain to get the stomach ready to receive food • in this stage the gastric secretions are stimulated by the thought,sight, smell and taste of food

27. Gastric phase • Once food enters the stomach it presence (bulk) stimulates the secretion of the gastric juices • The bulk stretches the local nerves at the top of the stomach • This stretching signals the brain that food is present and a hormone, gastrin, is released from the upper part of the stomach • Gastrin stimulates the secretion of the gastric juices containing HCl and the digestive enzymes such as pepsinogen, gastric lipase, etc.

28. Enzymes • Proteins • Enzymes are very important in biochemical reactions • They speed up the rate of a specific biochemical reaction without themselves being altered • Each reaction has its own specific enzyme(s)

30. Components of Gastric Juice • HCl • deactivates salivary amylase (stops starch digestion) (enzyme does not function in an acidic environment • kills many bacteria consumed with the food • provides an acidic environment (pH 1-2) for gastric enzyme function • begins protein digestions by converting pepsinogen (precursor) to pepsin (protein digesting enzyme (enzyme requires an acidic pH)

31. Gastric Lipase • starts fat digestion • fats leave the stomach only partially processed • Bicarbonate • neutralizes the acidity of the chyme before it enters the small intestine • Vitamin B12 • becomes attached to a protein carrier - this protects the vitamin against the harsh environment of the stomach

32. Carbohydrates pass though the stomach to the small intestine as the stomach does not secrete enzymes for carbohydrate digestion and it inactivates amylase from the mouth • Therefore carbohydrates leave first, proteins and fats remain behind for more extensive processing • Fats generally leave last which is why a high fat meal has the “stick to your ribs” quality

33. Intestinal phase • Begins when the chyme passes into the small intestine • Pyloric sphincter (pylorus) • this is a valve between the stomach and the small intestine. It prevents back-up of the intestinal contents into the stomach and regulates the flow of the chyme from the stomach to the small intestine

34. The passage of chyme into the small intestine triggers a series of events that regulates the release of chyme into the small intestine stomach • Motility is slowed, gastric secretions are shut off and the release of food into the small intestine slowed so that thorough digestion can take place

35. Control of Stomach Emptying • The amount of chyme leaving the stomach through the pyloric sphincter is about 1-2 tsp every 30 seconds • Consequently the stomach empties over a period of time rather than as a bulk emptying • Rate is determined by the size and composition of the meal and control from the small intestine

36. Chyme passing through the pyloric sphincter causes the small intestine to distend which then closes the valve (as well as stimulation by the low pH) until the food moves down the intestine after processing thus this will slow stomach emptying • Large meals will take longer to leave the stomach than smaller • The composition (amount of protein, fat and carbohydrate) of the meal will determine the rate of stomach emptying due to the amount of processing required • In addition, out moods can affect stomach emptying (sadness & fear -slow emptying; excitement & aggression-increase emptying)

37. The Small Intestine • 15 - 30 feet long • approximately 95% of all digestion takes place in the small intestine • comprised of the • duodenum (12 inches) • jejunum(8 feet) (most digestion takes place here) • ileum (11 feet) • all three parts have different functions within the small intestine but they are noncompartmentalized

38. Digestion within the small intestine is aided by secretions from itself, the gallbladder and the pancreas • When the chyme enters the small intestine through the pyloric sphincter, the pH is very acidic • the nerve cells sense this and stimulate the pyloric sphincter to close and this stops more chyme from entering • at any given time only about 1-2 tsp enters the small intestine • as it enters the small intestine, it bypasses the bile duct

39. The bile duct secretes the fluids from the gall bladder and the pancreas • Bicarbonate is secreted to neutralize the acidic chyme and to provide an environment conducive for the digestive enzymes to work in • Once the chyme is sufficiently neutralized then the nerve cells will stimulate the pyloric sphincter to relax and more chyme will be allowed to enter until the pH causes the nerve cells to contract the pyloric sphincter

40. Gall bladder • Stores and secretes bile • bile is synthesized in the liver and excess is concentrated and stored in the gall bladder • necessary for fat digestion and absorption • secretes its contents through the bile duct in to the small intestine when required

41. Bile • Contains “surfactants” to help solubilize fats • serves to emulsify fats • makes them partially soluble in water thus allowing for aqueous enzymes to break down fat in an aqueous environment

42. Pancreas • Secretes pancreatic juices into the small intestine via special ducts • pancreatic juices contain enzymes (proteases, lipases and amylase) and bicarbonate • bicarbonate serves to neutralize the HCl from the stomach allowing for the functioning of the enzymes for further digestion

43. Cystic Fibrosis • Inherited disease of infants and children • pancreas is often affected by the thick mucous that blocks the ducts • as a result the cells die • pancreas can’t deliver the enzymes into the small intestine • carbohydrate, fat and protein digestion is affected

44. no amylase, trypsin, lipase, etc. • enzymes must be given orally otherwise the undigested nutrients go to the large intestine • this causes bacterial buildup, acids and gases are produced and the individual is very uncomfortable and in considerable pain • the consequent result is malabsorption and eventually severe malnutrition

45. As well, in the intestine, there are • Various nutrient degrading enzymes are present on the surface of the small intestine (ex. Maltase) • In the neutral pH of the small intestine bacteria thrive • Bacterial action leads to vitamin formation (ex. Vitamin K) and as well, they secrete some enzymes that aid in digestion (ex. Cellulases for the breakdown of cellulose

46. It is difficult for infections to get established in the small intestine due to the high density of “natural” bacteria (Natural flora) • Defending cells (phagocytes) are also present to confer immunity against intestinal diseases • Segmentation - periodic squeezing or partitioning of the intestine by its circular muscles. Tends to force food backward a few inches and allows for better mixing with the juices

47. Pancreatic amylase continues the breakdown of carbohydrate started by salivary amylase • Protein digesting enzymes break protein down into smaller units • The cells lining the small intestine contain enzymes that further break down sugars into monosaccharides and small polypeptides into constituent amino acids • Fat is mixed with bile to emulsify it so that the pancreatic enzymes containing lipases can break down the fat

48. Control of Intestinal Secretions • Two hormones control the release of bile and pancreatic enzymes into the small intestine • Secreted by the mucosal lining of the duodenum • Secretin and Cholecystokinin (CCK) • Work together to control digestion in the small intestine

49. Secretin • Hormone • Secreted by the mucosal lining of the duodenum • Stimulates the pancreas to secrete sodium bicarbonate • Stimulates the liver to secrete bile into the gallbladder

50. Cholecystokinin (CCK) • Hormone • Secreted by the mucosal cells lining the duodenum • Stimulates the pancreas to release its digestive enzymes • Stimulates the gallbladder to contract thus releasing its contents into the duodenum