Chapter 26 Physiology of the Digestive System

1. Chapter 26Physiology of the Digestive System

2. Overview of Digestive Function • Primary function of digestive system— to bring essential nutrients into the internal environment so that they are available to each cell of the body • Mechanisms used to accomplish primary function of digestive system: • Ingestion— food is taken in • Digestion— breakdown of complex nutrients into simple nutrients • Motility of gastrointestinal (GI) wall— physically breaks down large chunks of food material and moves food along the tract • Secretion of digestive enzymes allows chemical digestion • Absorption— movement of nutrients through the GI mucosa into the internal environment • Elimination— excretion of material that is not absorbed • Regulation— coordination of the various functions of the digestive system

3. Mechanical digestion • Mechanical digestion— movements of the digestive tract • Change ingested food from large particles into minute particles, facilitating chemical digestion by increasing surface area • Churn contents of the GI lumen to mix with digestive juices and come in contact with the surface of the intestinal mucosa, facilitating absorption • Propel food along the alimentary tract, eliminating digestive waste from the body • Mastication— chewing movements • Reduces size of food particles • Mixes food with saliva in preparation for swallowing

4. Mechanical digestion • Deglutition— process of swallowing • complex process requiring coordinated and rapid movements • Oral stage (mouth to oropharynx) • voluntarily controlled • formation of a food bolus in the middle of the tongue • tongue presses bolus against the palate  food is then moved into the oropharynx

5. Mechanical digestion • Oral  pharyngeal • Pharyngeal stage (oropharynx to esophagus) • involuntary movement • to propel bolus from pharynx to esophagus, the mouth, nasopharynx, and larynx must be blocked • combination of contractions and gravity move bolus into esophagus

6. Mechanical digestion • Esophageal stage (esophagus to stomach) • involuntary movement • contractions and gravity move bolus through esophagus and into stomach Oral  pharyngeal  esophageal

7. Mechanical digestion • Peristalsis and segmentation • two main types of motility produced by the smooth muscle of GI tract • can occur together, in an alternating fashion

8. Peristalsis • Peristalsis • wavelike ripple of the muscle layer of a hollow organ • produces forward movement of matter along the GI tract

9. Peristalsis • Rate of peristalsis picks up as chyme approaches end of jejunum  large intestine • after leaving stomach, normally takes approximately 5 hours for chyme to pass all the way through small intestine • regulated in part by intrinsic stretch reflexes • stimulated by cholecystokinin (CCK) • chol = bile, cyst = bladder/sac, kinin = move • Stimulates contraction of gallbladder  bile flowing into duodenum

10. Previously contracted regions relax and adjacent regions now contract, effectively “chopping” the contents of each segment into smaller chunks. Segmentation • digestive reflexes cause a forward-and-backward movement with a single segment of the GI tract • helps break down food particles, mixes food and digestive juices, and brings digested food in contact with intestinal mucosa to facilitate absorption • mixes chyme with digestive juices

11. Regulation of motility • Gastric motility • emptying the stomach takes approximately 2 to 6 hours • while in the stomach, food is churned and mixed with gastric juices to form chyme (milky substance) • chyme is ejected about every 20 seconds into the duodenum

12. Regulation of motility Gastric emptying is controlled by hormonal and nervous mechanisms • Hormonal mechanism • Fats in duodenum stimulate release of gastric inhibitory peptide (GIP)  decrease peristalsis of gastric muscle  slows passage of chyme into duodenum • Nervous mechanism • “enterogastric reflex” = overload in small intestine  decreased gastric motility • receptors in duodenal mucosa are sensitive to presence of acid (pH 3-4) and to distention • Sensory and motor fibers  reflex inhibition of gastric peristalsis

13. Chemical Digestion • Chemical digestion • changes in chemical composition of food as it travels through the digestive tract • these changes are the result of hydrolysis

14. Digestive enzymes • Extracellular, organic (protein) catalysts • Principles of enzyme action • Very specific • Function optimally at a specific pH • Enzymes are continually being destroyed or eliminated from the body and must continually be synthesized • Most digestive enzymes are synthesized as inactive proenzymes

15. Carbohydrate Digestion Carbohydrates are saccharide compounds (C6H12O6) • Polysaccharides are hydrolyzed by amylases (saliva and pancreatic juice)  disaccharides • Disaccharides catalyzed by sucrase, lactase, and maltase (epithelial cells covering the villi= brush border)  monosaccharides (glucose, fructose and galactose) *Sucking on bread (starch) creates maltose (malt sugar) to make a malt shake

16. Protein Digestion Protein compounds are made up of twisted chains of amino acids • Proteases catalyze hydrolysis of proteins   amino acids • Main proteases: • pepsin in gastric juice • trypsin & chymotrypsin in pancreatic juice • peptidases in intestinal brush border

17. Fat Digestion Fats must be emulsified by bile and lecithin in small intestine before being digested  micelles • Hydrophilic heads (polar) = outer surface • Hydrophobic tails (nonpolar) = inner surface • Pancreatic lipase is the main fat-digesting enzyme • Colipase = coenzyme that anchors lipase to the inner face of a micelle

18. Residues of digestion • Some compounds of food resist digestion and are eliminated as feces • Humans lack the enzymes to hydrolyze them • Includes: • Cellulose • Connective tissue from meat • Undigestable fats (combined with Mg+ or Ca+2)

19. Secretion • Saliva— secreted by salivary glands • Mucus • Lubricates food and, with water, • Facilitates mixing • Protects digestive mucosa • Amylase = enzyme that begins digestion of starches  maltose • Sodium bicarbonate increases the pH for optimal amylase function

20. Secretion • Gastric juice— secreted by gastric glands • Pepsin • secreted as inactive pepsinogen by chief cells • protease that begins the digestion of proteins • Hydrochloric acid (HCl) • secreted by parietal cells • decreases the pH of chyme for activation and optimal function of pepsin • Released actively into the gastric juice • Mucus and water lubricates, protects, and facilitates mixing of chyme • Parietal cells also absorb vitamin B12 for production of new RBC

21. Secretion • Pancreatic juice— secreted by exocrine cells • Proteases (trypsin and chymotrypsin) = enzymes to digest proteins and polypeptides • Trypsinogen activated by enterokinase  Trypsin activates chymotrypsin, nucleases and amylase • Lipases = enzymes to digest emulsified fats • Nucleases = enzymes to digest nucleic acids (DNA and RNA) • Amylase = enzyme that digests starches • Sodium bicarbonate increases the pH for optimal enzyme function • its manufacture also helps restore normal pH of blood

22. Secretion • Bile • secreted by the liver • stored and concentrated in gallbladder • Bile salts & lecithin emulsify fats by encasing fats in shells to form tiny spheres called micelles • Sodium bicarbonate increases pH for optimal enzyme function • Cholesterol, products of detoxification, and bile pigments are waste products excreted by liver  eliminated in feces

23. Secretion • Intestinal juice— secreted by cells of intestinal exocrine cells • Largely small intestine • Mucus and water lubricate and aid in continued mixing of chyme • Sodium bicarbonate increases pH for optimal enzyme function

24. Refer to Table 26-3, Page 779 • Other great tables: Table 26-4 and Table 26-5 (P. 782)

25. Control of Salivary Secretion • Salivary secretion • Only reflex mechanisms control secretion of saliva • Chemical and mechanical stimuli come from presence of food in the mouth • Olfactory and visual stimuli come from the smell and sight of food

26. Control of Gastric Secretion • Gastric secretion— three phases • Cephalic phase—“psychic phase” • Parasympathetic system (“rest and digest”) • Stimulates the gastric glands • stimulates production of gastrin (in the stomach)  stimulates gastric secretion

27. Control of Gastric Secretion • Gastric phase • when products of protein digestion reach pylorus  stimulate release of gastrin  accelerates secretion of gastric juice, ensuring enough enzymes are present to digest food

28. Control of Gastric Secretion • Intestinal phase— various mechanisms adjust gastric secretion as chyme passes to the intestinal tract • endocrine reflexes involving GIP, secretin, and CCK inhibit gastric secretions • Also inhibited by enterogastric reflex

29. Overall Gastric Control • Anticipation of swallowing  stomach prepares itself by increasing secretion of enzymes and acid • can then be adjusted according to amount of food present and whether it contains proteins (the only food that can be chemically digested by gastric juice) • Gastric secretion can be slowed when the duodenum becomes full • This prevents the stomach from finishing its task before the small intestine is ready to receive the chyme

30. Control of Digestive Gland Secretion • Pancreatic secretion stimulated by several hormones released by intestinal mucosa • Secretin evokes production of pancreatic fluid low in enzyme content but high in bicarbonate • CCK— several functions • Causes increased exocrine secretion from pancreas • Opposes gastrin, thus inhibiting gastric HCl secretion • Stimulates contraction of gallbladder so that bile is ejected into duodenum • Secretion of bile • secreted continually by liver • secretin and CCK stimulate ejection of bile from gallbladder

31. Control of Digestive Gland Secretion • Intestinal secretion • little is known about how intestinal secretion is regulated • suggested that intestinal mucosa is stimulated to release hormones that increase production of intestinal juice

32. Absorption • Process of absorption • Passage of substances through intestinal mucosa into blood • Most absorption occurs in small intestine • Mechanisms of absorption • For some substances such as water, absorption occurs by diffusion (osmosis) • Other substances are absorbed through more complex mechanisms • Active transport— transports sodium • Sodium cotransport (coupled transport)— transports glucose • Fatty acids, monoglycerides, and cholesterol are transported with the aid of bile salts from lumen to absorbing cells of the villi • After food is absorbed, it travels to the liver Most absorption occurs in the small intestine!

33. Elimination • Elimination • expulsion of feces from digestive tract • act of expelling feces is called defecation • Defecation occurs as a result of a reflex brought about by stimulation of receptors in the rectal mucosa that is produced when rectum is distended • Constipation • contents of lower colon and rectum move at a slower rate • extra water is absorbed from the feces  hardened stool • Diarrhea • result of increased motility of small intestine • causing decreased absorption of water and electrolytes  watery stool

34. The Big Picture: Digestion and the Whole Body • Primary contribution of digestive system to overall homeostasis is to provide a constant nutrient concentration in the internal environment • Secondary roles of digestive system • Absorption of nutrients • Teeth and tongue, along with respiratory system and nervous system, important in producing spoken language • Gastric acids aid the immune system by destroying potentially harmful bacteria Useful Map (that may have more than you need to know)

35. GI Problems The Big Picture: Digestion and the Whole Body • To accomplish its functions, digestive system needs other systems to contribute • Regulation of digestive motility and secretion requires nervous system and endocrine system • Oxygen for digestive activity needs proper functioning of respiratory and circulatory systems • Integumentary and skeletal systems support and protect digestive organs • Muscular system is needed for ingestion, mastication, deglutition, and defecation to occur normally Digestive Quiz!