Transcript Slide 1
Dr. Kim Wilson 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 ◦ 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 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 ◦ 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 ◦ 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 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 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 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) 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 ◦ 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 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 ◦ 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 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. Saliva: secreted by salivary glands Gastric juice: secreted by gastric 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 ◦ Pepsin (secreted as inactive pepsinogen by chief cells) is a protease that begins the digestion of proteins Secretions that Aid in Digestion ◦ 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 hydrogenpotassium pumps (proton pumps) Secretions that Aid in Digestion Intrinsic factor (secreted by parietal cells) protects vitamin B12 and later facilitates its absorption Mucus and water lubricate, protect, and facilitate mixing of chyme 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 Secretions that Aid in Digestion 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 Where? Pancreatic duct cells Bicarbonate (HCO3) is produced by the dissociation of carbonic acid (H2CO3) 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 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 HCO3into GI lumen to make blood pH return to normal Secretions that Aid in Digestion 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 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 Gastric Secretion 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 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 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 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 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 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 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 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 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 Digestion – The Big Picture 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