Chapter 26 Physiology of the Digestive System

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  Digestive tract is functionally an extension of external environment— material does not truly enter body until it is absorbed into internal environment

Mechanical Digestion  Movements of the digestive tract  Change ingested food from large particles into minute particles, facilitating chemical digestion  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

Mechanical Digestion  Mastication  Reduces size of food particles  Mixes food with saliva in preparation for swallowing  Deglutition (Figure 26-2)  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 pharynx to esophagus, the mouth, nasopharynx, and larynx must be blocked  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

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  Peristalsis  Wavelike ripple of the muscle layer of a hollow organ  Progressive motility that produces forward movement of matter along the GI tract  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

Mechanical Digestion  Regulation of motility  Gastric motility  Emptying the stomach takes approximately 2 to 6 hours  Stomach - food is churned and mixed with gastric juices to form chyme  Chyme is ejected about every 20 seconds into the duodenum  Gastric emptying is controlled by hormonal and nervous mechanisms  Hormonal mechanism—fats in duodenum stimulate release of gastric inhibitory peptide, which acts to decrease peristalsis of gastric muscle and slows passage of chyme into duodenum  Nervous mechanism—enterogastric reflex; receptors in duodenal mucosa are sensitive to presence of acid and to distention; impulses over sensory and motor fibers in vagus nerve cause a reflex inhibition of gastric peristalsis

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

Chemical Digestion  Changes in chemical composition of food as it travels through the digestive tract  Result of hydrolysis  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

Chemical Digestion  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, which are found in the cell membrane of epithelial cells covering the villi that line intestinal lumen

Chemical Digestion  Protein compounds are made up of twisted chains of amino acids  Proteases catalyze hydrolysis of proteins into intermediate compounds and, finally, into amino acids  Main proteases: pepsin in gastric juice, trypsin in pancreatic juice, peptidases in intestinal brush border

Chemical Digestion  Fat digestion  Fats must be emulsified by bile in small intestine before being digested (Figure 26-11)  Pancreatic lipase is the main fat-digesting enzyme  Residues of digestion—some compounds of food resist digestion and are eliminated as feces

Extra Slides Not Covered In Class

Secretion  Saliva—secreted by salivary glands  Mucus lubricates food and, with water, facilitates mixing  Amylase is an enzyme that begins digestion of starches; small amount of salivary lipase released, function uncertain  Sodium bicarbonate increases the pH for optimal amylase function

Secretion  Gastric juice—secreted by gastric glands  Pepsin (secreted as inactive pepsinogen by chief cells) is a protease that begins the digestion of proteins  Hydrochloric acid (HCl, secreted by parietal cells)  HCl decreases the pH of chyme for activation and optimal function of pepsin (Figure 26-13)  Released actively into the gastric juice by H-K pumps (proton pumps)  Vesicles in the resting parietal cell move to the apical surface when the cell becomes active—thus increasing the surface area for the process of secretion (Figure 26-14)  Intrinsic factor (secreted by parietal cells) protects vitamin B 12 and later facilitates its absorption (Figure 26-15)  Mucus and water lubricates, protects, and facilitates mixing of chyme

Secretion  Pancreatic juice—secreted by acinar and duct cells of 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 (Figures and 26-17)

Secretion  Bile—secreted by the liver; stored and concentrated in 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 liver and eventually eliminated in 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

Control of Digestive Gland 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  Gastric secretion—three phases (Figure 26-18)  Cephalic phase—“psychic phase,” because mental factors activate 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)  Gastric phase—when products of protein digestion reach pyloric portion of stomach, they stimulate release of gastrin; gastrin accelerates secretion of gastric juice, ensuring enough enzymes are present to digest food  Intestinal phase—various mechanisms seem to adjust gastric secretion as chyme passes to and through intestinal tract; endocrine reflexes involving gastric inhibitory peptide, secretin, and CCK inhibit gastric secretions

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  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

Absorption  Process of absorption  Passage of substances through intestinal mucosa into blood or lymph (Figure 26-19)  Most absorption occurs in small intestine  Mechanisms of absorption  For some substances such as water, absorption occurs by simple diffusion or osmosis  Other substances are absorbed through more complex mechanisms (Figures and 26-21)  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 lumen to absorbing cells of the villi  After food is absorbed, it travels to the liver via the portal system  In summary, most absorption occurs in the small intestine (Figure 26-22)

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 (Figure 26-23)  Constipation—contents of lower 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 small intestine, causing decreased absorption of water and electrolytes and a watery stool

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

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