1. Dr. Kim Wilson

2.  Primary function of the digestive system: to bring essential nutrients into the internal environment so they are available to every cell in the body  Mechanisms used to accomplish the primary function of the digestive system ◦ Ingestion: food is taken in ◦ Digestion: breakdown of complex nutrients into simple nutrients ◦ Motility of the gastrointestinal (GI) wall: physically breaks down large chunks of food material and moves food along the tract ◦ Secretion of digestive enzymes allows chemical digestion Overview of Digestive Function

4. Mechanisms of 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  The digestive tract is functionally an extension of the external environment; material does not truly enter the body until it is absorbed into the internal environment

5. Mechanical Digestion  Mechanical digestion: movements of the digestive tract ◦ Function - Breakdown large particles of food to small particles  chemical digestion ◦ How?  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

6. Deglutition ◦ Deglutition: process of swallowing; complex process requiring coordinated, rapid movements ◦ 3 stages:  Oral stage (mouth to oropharynx):  voluntarily controlled  formation of a food bolus in the middle of the tongue  tongue presses bolus against the palate and food is then moved into the oropharynx  Pharyngeal stage (oropharynx to esophagus):  involuntary movement;  to propel bolus from the pharynx to the esophagus, the mouth, nasopharynx, and larynx must be blocked;  a combination of contractions and gravity move bolus into esophagus  Esophageal stage (esophagus to stomach):  involuntary movement;  contractions and gravity move bolus through esophagus and into stomach

7. Peristalsis ◦ Two main types of motility produced by the smooth muscle of the GI tract  Peristalsis: wavelike ripple of the muscle layer of a hollow organ  Progressive motility that produces forward movement of matter along the GI tract

8.  Segmentation: mixing movement  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

9.  Gastric motility ◦ Takes approximately 2 to 6 hours to empty stomach ◦ Food is churned (propulsion and retropulsion) and mixed with gastric juices to form chyme  Ejected approximately every 20 seconds into the duodenum  Gastric emptying is controlled by hormonal and nervous mechanisms ◦ Hormonal mechanism: fats in duodenum stimulate the release of gastric inhibitory peptide, which decreases peristalsis of gastric muscle and slows passage of chyme into duodenum ◦ Nervous mechanism: enterogastric reflex; receptors in the duodenal mucosa are sensitive to presence of acid and distention; impulses over sensory and motor fibers in the vagus nerve cause a reflex inhibition of gastric peristalsis

10. Intestinal Motility  Intestinal motility includes peristalsis and segmentation  Segmentation in duodenum and upper jejunum mixes chyme with digestive juices from the pancreas, liver, and intestinal mucosa  Rate of peristalsis picks up as chyme approaches end of jejunum, moving it through the rest of the small intestine into the large intestine  After leaving stomach, chyme normally takes approximately 5 hours to pass through the small intestine  Peristalsis: regulated in part by intrinsic stretch reflexes; stimulated by cholecystokinin (CCK)

11. Chemical Digestion  Chemical digestion: changes in chemical composition of food as it travels through the digestive tract  Mechanism = hydrolysis (water is split into hydrogen ions) ◦ Digestive enzymes  Extracellular, organic (protein) catalysts  Principles of enzyme action  Specific in their action  Function optimally at a specific pH  Most enzymes catalyze a chemical reaction in both directions  Enzymes are continually being destroyed or eliminated from the body and must continually be synthesized  Most digestive enzymes are synthesized as inactive proenzymes

13. Carbohydrate Digestion ◦ Carbohydrates are saccharide compounds  Polysaccharides are hydrolyzed by amylases to form disaccharides  Final steps of carbohydrate digestion are catalyzed by sucrase, lactase, and maltase  Found in the cell membrane of epithelial cells covering the villi that line the intestinal lumen

14.  Protein compounds are composed of twisted chains of amino acids  Proteases catalyze hydrolysis of proteins into intermediate compounds and, finally, amino acids  Main proteases: ◦ Pepsin in gastric juice ◦ Trypsin in pancreatic juice ◦ Peptidases in intestinal brush border

15. Fat Digestion ◦ Fats must be emulsified by bile in small intestine before being digested  Pancreatic lipase is the main fat-digesting enzyme ◦ Residues of digestion: some compounds of food resist digestion and are eliminated as feces

16.  Emulsification involves the formation of micelles and breaking fat drops into small droplets to make them soluble in water.  Micelles are spherical shaped shells formed of lecithin molecules based on their solubility in water.

17. Secretions that Aid in Digestion  Saliva: secreted by salivary glands ◦ Mucus lubricates food and, with water, facilitates mixing ◦ Amylase = enzyme that begins digestion of starches; a small amount of salivary lipase is released, its function uncertain ◦ Sodium bicarbonate increases the pH for optimal amylase function  Gastric juice: secreted by gastric glands ◦ Pepsin (secreted as inactive pepsinogen by chief cells) is a protease that begins the digestion of proteins

18. ◦ Hydrochloric acid: secreted by parietal cells  Decreases the pH of chyme for activation and optimal function of pepsin  Released actively into the gastric juice by hydrogen- potassium pumps (proton pumps) Secretions that Aid in Digestion

19.  Intrinsic factor (secreted by parietal cells) protects vitamin B12 and later facilitates its absorption  Mucus and water lubricate, protect, and facilitate mixing of chyme Secretions that Aid in Digestion

20. Structures that Aid Digestion  Vesicles in the resting parietal cell move to the apical surface when the cell becomes active  Result = increased surface area for the process of secretion

21.  Pancreatic juice: secreted by acinar and duct cells of the pancreas ◦ Proteases (e.g., trypsin and chymotrypsin) are enzymes that digest proteins and polypeptides ◦ Lipases are enzymes that digest emulsified fats ◦ Nucleases are enzymes that digest nucleic acids such as DNA and RNA ◦ Amylase is an enzyme that digests starches ◦ Sodium bicarbonate increases the pH for optimal enzyme function; its manufacture also helps restore normal pH of blood Secretions that Aid in Digestion

22.  Where? Pancreatic duct cells  Bicarbonate (HCO 3 ) is produced by the dissociation of carbonic acid (H 2 CO 3 )  Bicarbonate ions are exchanged for Cl ions = “chloride shift”  Electrical gradient is created from outward movement of negative bicarbonate ions into lumen of the pancreatic ducts ◦ Positive Na ions from interstitial fluid are drawn into pancreatic juice

23.  HCL secretion moves hydrogen ions into the digestive tract to decrease the pH of chyme  Gastric parietal cells move bicarbonate ions into the blood to increase the pH of blood plasma  Duct cells function to counterbalance the effects of HCL secretion and bicarbonate ion transport  Duct cells secrete HCO 3 - into GI lumen to make blood pH return to normal

24.  Bile: secreted by the liver; stored and concentrated in the gallbladder ◦ Lecithin and bile salts emulsify fats by encasing them in shells to form tiny spheres called micelles ◦ Sodium bicarbonate increases pH for optimal enzyme function ◦ Cholesterol, products of detoxification, and bile pigments (e.g., bilirubin) are waste products excreted by the liver and eventually eliminated in the feces  Intestinal juice: secreted by cells of intestinal exocrine cells ◦ Mucus and water lubricate and aid in continued mixing of chyme ◦ Sodium bicarbonate increases pH for optimal enzyme function Secretions that Aid in Digestion

25.  Salivary secretion ◦ Only reflex mechanisms control the secretion of saliva ◦ Chemical and mechanical stimuli come from the presence of food in the mouth ◦ Olfactory and visual stimuli come from the smell and sight of food Secretions that Aid in Digestion

26.  Three phases: 1.Cephalic phase  Called psychic phase because mental factors activate the mechanism  Parasympathetic fibers in branches of the vagus nerve conduct stimulating efferent impulses to the glands  Stimulate production of gastrin (by G cells in the stomach) 2. Gastric phase  When products of protein digestion reach the pyloric portion of the stomach, they stimulate release of gastrin  Gastrin accelerates secretion of gastric juice, ensuring enough enzymes are present to digest food 3.Intestinal phase ◦ Various mechanisms seem to adjust gastric secretion as chyme passes to and through the intestinal tract ◦ Endocrine reflexes involving gastric inhibitory peptide, secretin, and Cholecystokinin (CCK) inhibit gastric secretions Gastric Secretion

28. Pancreatic Secretion  Stimulated by several hormones released by intestinal mucosa ◦ Secretin evokes production of pancreatic fluid low in enzyme content but high in bicarbonate ◦ Cholecystokinin (CCK): several functions  Causes increased exocrine secretion from the pancreas  Opposes gastrin, thus inhibiting gastric hydrochloric acid secretion  Stimulates contraction of the gallbladder so that bile is ejected into the duodenum

29.  Bile Secretion ◦ Bile secreted continually by the liver ◦ Secretin and CCK stimulate ejection of bile from the gallbladder  Intestinal secretion ◦ Little is known about how intestinal secretion is regulated ◦ Suggested that the intestinal mucosa is stimulated to release hormones that increase the production of intestinal juice

30.  Process ◦ Involves passage of substances through the intestinal mucosa into the blood or lymph ◦ Intestinal villi and microvilli help increase absorption by increasing surface area ◦ Most absorption occurs in the small intestine

31. Mechanisms of Absorption  For some substances such as water, absorption occurs by simple diffusion or osmosis  Secondary active transport: how sodium is transported  Sodium cotransport (coupled transport): how glucose is transported  Fatty acids, monoglycerides, and cholesterol are transported with the aid of bile salts from the lumen to absorbing cells of the villi  Transcellular absorption moves nutrient particle through cells (as described above), and paracellular absorption moves particles between cells  After food is absorbed, it travels to the liver by the portal system

32. Absorption of Glucose, Na, and AA  Form of secondary active transport  Each involves two carriers  Mechanism involves a sodium gradient (basal side), which permits passive transport of sodium and another molecule into the GI lumen via a passive carrier molecule on the luminal side of the cell

34.  Sites ◦ Stomach ◦ Small intestine ◦ Colon ◦ Rectum  Amount of absorption of some substances varies depending on the site of absorption ◦ Ex. Alcohol  20% in stomach  80% in small intestine

35.  Definition: the expulsion of feces from the 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 the rectum is distended  Constipation: contents of the lower part of the colon and rectum move at a slower than normal rate; extra water is absorbed from the feces, resulting in a hardened stool  Diarrhea: result of increased motility of the small intestine, causing decreased absorption of water and electrolytes and a watery stool

36. Digestion – The Big Picture  Primary contribution of the 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 and nervous system, are important in producing spoken language ◦ Gastric acids help the immune system by destroying potentially harmful bacteria

37.  To accomplish its functions, the digestive system needs other system contributions ◦ Regulation of digestive motility and secretion requires the nervous system and endocrine system ◦ Oxygen for digestive activity depends on proper functioning of the respiratory and circulatory systems ◦ Integumentary and skeletal systems support and protect the digestive organs ◦ Muscular system is needed for ingestion, mastication, deglutition, and defecation to occur normally Digestion – The Big Picture