The Digestive System Chapter 16 The main function of the digestive system is to breakdown and absorb nutrients, water, and electrolytes This system has four functions: Motility is the muscular contractions that mix and move the contents of the digestive tract. Secretion of digestive juices by exocrine glands into the digestive tract. Digestion via chemical change (hydrolysis) of large molecules (e.g., carbohydrates, proteins, and fats) into their smaller subunits Absorption of nutrients, along with water, vitamins, and electrolytes, into the blood and lymph. The digestive system Mouth consists of a tract plus Salivary glands the accessory organs. Pharynx The tract is a continuous tube that consists of the Esophagus mouth, pharynx, esophagus, Stomach stomach, small intestine, large intestine, and anus. Liver The lumen of this tube Gallbladder is continuous with the Pancreas external environment. Large The accessory organs Intestine are the salivary glands, Small Intestine exocrine glands, & biliary system (liver & gallbladder). Rectum Anus Other facts of the four main digestive processes are: The smooth muscle in the walls of the digestive tract maintains a low level of contraction called tone. There are two types of digestive Lumen motility: propulsive movements & mixing movements. Duct cells Digestive secretions consist of water, Exocrine gland electrolytes, & organic cells Secretory constituents such as product enzymes, bile, salts, & mucus. They are released by hormonal or neural stimulation. Capillary Most absorption occurs in the small intestine where it is completed. Polysaccharides (e.g., starch and glycogen) are chemically changed into disaccharides, which are changed into monosaccharides. Intermediate End Products Category of foodstuffs Products of Digestion Carbohydrates Monosaccharides Polysaccharides (starch and glycogens) Maltose (glucose, galactose, fructose) Amylase Maltase Disaccharides Sucrose Sucrase Absorbable Lactose Units = Glucose = Fructose Lactase = Galactose Proteins are chemically changed into polypeptides, which are changed into amino acids. Proteins Peptide Fragments Amino Acids Carboxypeptidase Pepsin & Aminopeptidase & Chymotrypsin Trypsin The end products of fat digestion are monoglycerides and free fatty acids. Fats Free Triglyceride Glycerol fatty acids Lipase Monoglyceride Macromolecules are broken down via hydrolysis. Maltose Glucose Glucose The wall of the digestive tract consists of four layers. Body wall The mucosa lines the Peritoneum Muscularis luminal surface. externa Mesentery It has an inner Serosa Outer longitudinal muscle epithelial layer. Inner circular muscle The lamina propria Mucosa is a middle layer of Mucous membrane connective tissue. Lamina propria Muscularis mucosa The muscularis mucosa is a sparse Lumen Submucosa layer of smooth muscle. Duct of The submucosa is accessory Myenteric plexus made of connective digestive gland Submucous plexus tissue The muscularis externa is the The serosa is the outer main smooth layer of the tract. connective tissue layer. Digestive motility and secretion are regulated by four factors. 1) Smooth muscle cells display rhythmic, spontaneous variations in membrane potentials induced by pacesetter cells. Peristalsis and segmentation depend on pacesetters 2) The myenteric plexus and submucous plexus are the enteric nervous system Acetylcholine release promotes muscle contraction 3) Extrinsic nerves of the autonomic nervous system innervate digestive structures from the outside. Parasympathetic signals induce digestive activity. 4) Endocrine glands within the mucosa release hormones that signal digestive responses. Sensory information comes from chemoreceptors, mechanoreceptors, and osmoreceptors The oral cavity is the entrance to the digestive tract. Its opening is formed by the muscular lips. The palate separates the mouth & nasal passages. The tongue consists of voluntary skeletal muscles. The pharynx is the cavity at the rear of the throat. Mastication by the teeth starts digestion. This grinds and breaks as well as mixing it with saliva and stimulating the taste buds. Salivary secretion begins chemical digestion of starch via amylase Saliva also keeps the mouth and teeth clean. Lysozyme in the saliva lyses bacteria. Salivary secretion is continuous but can be reflexly increased. Multiple factors control salivation Cerebral cortex Other inputs Salivary center Conditioned in medulla reflex Pressure receptors Simple reflex Autonomic nerves & chemoreceptors in mouth Salivary glands Salivary secretions External Local changes in influence digestive tract Receptors in digestive tract Intrinsic Extrinsic Gastro- nerve automatic intestinal plexuses nerves hormones Self- Smooth muscle contraction excitable Exocrine gland cells Producing digestive = Short reflex fluids = Long reflex Endocrine gland cells digestive hormones) = Hormonal pathway Swallowing Nasal passages mechanisms Soft palate Uvula Hard palate force food into Bolus the esophagus. Tongue Pharynx Glottis Epiglottis Swallowing is initiated when Esophagus Trachea bolus contacts the pharynx. Swallowing center Afferent impulses are sent to the inhabits respiratory swallowing center in the medulla. center in brain stem Elevation of uvula prevents food from entering nasal passages Position of tongue prevents food from re-entering mouth Food is prevented from entering the respiratory tract by closing the glottis and tilting of the epiglottis. Motility associated with the pharynx and esophagus is swallowing. The bolus passes the Bolus pharyngoesophageal sphincter which prevents air from entering the digestive tract during breathing & also prevents eructation (burping). A swallowing center initiates a primary peristaltic wave. Peristalic waves push food through the esophagus. The gastroesophageal sphincter prevents reflux of gastric contents. Esophageal secretion is entirely protective. The stomach stores food & begins protein This J-shaped chamber has a digestion. fundus, body, and antrum. Its terminal part has a pyloric Esophagus Smooth muscle sphincter. The most important function Fundus of the stomach is to store Gastro- food. esophageal Body The body of the stomach sphincter acts a holding area Stomach Pyloric folds The stomach also sphincter secretes pepsinogen & HCl to begin Antrum protein digestion Oxyntic Occurs mostly in mucosa the antrum. Pyloric gland area Esophagus The stomach accommodate a twenty-fold increase in Gastro- esophageal volume by relaxation. sphincter Peristalic waves pass Pyloric sphincter down the stomach about 3 times/min The mixing movements of the stomach Peristaltic contraction produces chyme which is passed on in small amounts to the doudenum. Most of this mixing occurs in the antrum as food pushes against the pyloric sphincter. Gastric emptying is largely controlled by factors in the duodenum. Peristaltic action pushes stomach chyme into the duodenum. Distension of the stomach increases gastric motility. Signaling by the vagus nerve and the hormone gastrin also increases motility. Gastric emptying is inhibited by the neural response and hormonal responses. Secretin and CCK are enterogastrones that inhibit this process. Duodenal factors that limit gastric emptying 1) undigested fat 2) pH 3) osmolarity 4) distension Vomiting relfex. The stomach does not actively participate in vomiting. It arises from contraction of the respiratory muscles. The stomach sphincters are relaxed & the stomach is squeezed to evacuate the food during vomiting. This activity is coordinated by the vomiting center in the medulla. Causes of vomiting include tactile stimulation, irritation/distension of the stomach and duodenum, elevated intracranial pressure, chemical agents, & psychogenic factors. Severe vomiting can lead to dehydration and circulatory problems. Gastric digestive juice is Gastric pit secreted by glands Mucosa located a the base of gastric pits. Submucosa In oxyntic mucosa Surface epithelial cells Gastric In pyloric pit Mucosa cells mucus gland area Chief cells HCl Gastric G cells gland Parietal cells Pepsinogen gastrin Enterochomaffin-like D cells (ECL) cells Histamine somatostatin Production of HCl by parietal cells Plasma Parietal cell Gastric lumen Cellular metabolism Chief cell = Active cell ca = Carbonic anhydrase Functions of HCl in Autocatalysis the stomach include: Pepsinogen Pepsin Digestion Pepsin formation, Protein The breakdown of Gastric connective tissue & HCI lumen Peptide fragments muscle fibers, The denaturation of proteins, Killing of most microorganisms. A mucus lining on the surface of the gastric mucosa. is protective. Control of stomach parietal & chief cell activity Acetylcholine from the intrinsic nerve plexuses & the vagus nerve stimulate secretion. Histamine increases on parietal cells for HCl release Gastrin activates parietal and ECL cells Somatostatin inhibits secretions from parietal cells G & ECL cells. The control of gastric secretion involves 3 phases. During the cephalic phase stimuli from the head (cheweing and thinking about food) increase the secretion of HCl & pepsinogen. By the gastric phase stimuli (proteins & distension) in the stomach initiate gastric secretions through efferent pathways. The intestinal phase is inhibitory, shutting off the flow of gastric juices. Other facts about stomach activity include: Salivary amylase continues carbohydrate digestion in the body of the stomach. Proteins digestion begins in the antrum where food is mixed with gastric secretions. The stomach lining is protected from gastric secretions by the gastric mucosal barrier. Gastric secretion gradually decreases as food empties from the stomach into the intestine. Mucus coating Impermeable to HCI Cells lining gastric mucosa Tight junction The pancreas contains exocrine & endocrine cells. Endocrine cells of the islets of Langerhans secrete hormones—insulin & glucagon. The exocrine pancreas secretes digestive enzymes and an aqueous alkaline fluid. The alkaline fluid has sodium carbonate. The enzymes are proteolytic enzymes, pancreatic amylase, & pancreatic lipase. Trypsin, Chymotrypsin and carboxypeptidase are the proteolytic enzymes. Pancreatic exocrine secretion is regulated by secretin and CCK, hormones from the small intestine Secretin signals the secretion of NaHCO3. CCK induces the secretion of digestive enzymes. Bile duct from liver Stomach Hormones (insulin, glucagon) Blood Duct cells Endocrine portion Acinar cells secrete of pancreas secrete digestive NaHCO3 (Islets of enzymes solution Langerhans) Exocrine portion of panaceas Acid in Neutralizes Fat and protein Digests duodenal products in lumen duodenal lumen Secretion release CCK release from duodenal from duodenal mucosa mucosa (via bloodstream) (via bloodstream) Pancreatic duct Pancreatic cells acinar cells Secretion of aqueous Secretion of NaHCO3 solution into pancreatic digestive duodenal lumen enzymes into duodenal lumen Heart Blood from the digestive tract carries it contents to the liver. Inferior vena cava Aorta Hepatic Hepatic vein Blood enters the liver from vein the digestive tract by the Liver sinusoids hepatic portal system. The portal vein of this Liver Arteries to system breaks into a digestive tract capillary network, the liver Hepatic sinusoids. portal vein Digestive capillaries Digestive tract The liver carries out numerous metabolic functions. It carries out the metabolic processing of nutrients. It stores substances such as glycogen and fats. It detoxifies or degrades body wastes. It synthesizes plasma proteins. It activates vitamin D. It removes bacteria and worn-out RBCs. It excretes cholesterol and bilirubin. The liver also produces bile, which is essential for the digestion of fats Anatomy and flow through the liver. The liver lobules are delineated by vascular sinusoids. Hepatocytes secrete bile into these canniculli which converge to form bile ducts Central Sinusoids Branch of Bile vein hepatic portal vein canaliculi Bile Branch of duct hepatic artery Kupffer cell (Macrophage) Bile canaliculi Sinusoids Bile Central duct Hepatic vein Hepatic portal vein artery Hepatic plate Bile is stored in the gallbladder Bile salts Cholesterol between meals. After a meal the liver and gallbladder Liver secrete bile into the small intestine for fat digestion. Its secretion Portal Gallbladder circulation is induced by Sphincter of Oddi chemical (bile salts being returned to the liver), Colon Duodenum hormonal (secretin), & neural Terminal ileum (cephalic-vagal) mechanisms. Bilirubin is a waste product excreted in the bile. The detergent action of bile emulsifies fats. Fat globules are broken into smaller droplets, increasing surface area to facilitate enzymatic attack (pancreatic lipase). Pancreatic lipase is anchored to a fat droplet by the polypeptide colipase. Negativity charged H2O-soluble portion (a carboxyl group at the end of a glycine or taurine chain) Small lipid (fat) droplet with bile Lipid-soluble portion salt molecules (derived from cholesterol) absorbed on its surface Large fat droplet Through action of bile salts Lipid emulsion Micelle formation. Tiny micelles are water soluable and Hydrophobic core can be transported Hydrophilic shell through aqeous pathways Lipids including Cholesterol Water-soluble portion Water-soluble portion Lipid-soluble portion Lipid-soluble portion Bile salt Lecithin The small intestine is where most digestion and absorption occur. Its three segments are the duodenum, jejunum, & ileum. It does not secrete digestive enzymes. The pancreas secretes enzymes into the tract. The small intestine enzymes complete digestion intracellularly. These include the disaccharidases and aminopeptidases. The process of segmentation mixes and slowly propels the food. Segmentation contractions are initiated by BER cells. The circular smooth muscle responsiveness is influenced by the distension of the intestine, gastrin, and extrinsic nerve activity. Segmentation mixes chyme with secretions and slowly moves the contents through the tract. The migrating motility complex is an internal housekeeper sweeping the intestine clean between meals. The small intestine has adaptations to maximize absorption: The mucosal lining has a large surface area due to its circular folds and fingerlike projections called villi. The epithelial cells also have microvilli. A villus has a cover of epithelial cells, a connective tissue core, a capillary network, and the terminal lymphatic vessel. During absorption molecules produced by digestive enter the capillary or lymphatic vessel. The mucosal lining has a rapid turnover. The crypts of Liberkuhn have stem cells for cell regeneration. Epithelial cell Capillaries Central lacteal Mucous cell Circular fold Crypt of Lieberkühn Arteriole Venule Villus Lymphatic vessels Microvilli The epithelial cells in the small intestine have a variety of transport mechanisms. Na+ is pumped from the tract lumen into the interstitial fluid. From their it enters capillaries by diffusion. The transport of sodium creates an osmotic pressure drawing in water as N+ it is absorbed. Glucose and galactose are moved by secondary active transport. They are cotransported with sodium. Fructose is absorbed by passive facilitated diffusion. Amino and small peptides are are also absorbed across intestinal cells by secondary active transport. Lumen cotransport carrier Na+ & Epithelial energy cell dependent Facilitated diffusion of villus secondary active transport Capillary Lumen Na+- and energy- dependent Energy absorption required Epithelial cell of villus Capillary Digested fat is absorbed passively and enters the lymph. Triglycerides are converted to monoglycerides & free fatty acids are produced by hydrolysis. These water-insoluble products are carried to the inside of water-soluble micelles. On the mucosal surface these molecules leave the micelle and passively diffuse through the lipid bilayer of the luminal membrane. They are resynthesized into triglycerides inside the epithelial cells. There they associate with carrier protiens to create water-soluble chylomicrons which leave the cells by exocytosis. They enter the central lacteals, lymphatic vessels. Lipid emulsion Micelles Lumen diffusion Lumen Micelle Micelles Microvillus Fatty acids, monoglycerides Aggregate and Passive coated with Short or lipoprotien medium absorption chain Basement Epithelial membrane cell of villus (Exocytosis) Central lacteal Capillary Other facts on adsorption include: Vitamin adsorption is mainly passive. Water-soluble vitamins are absorbed with water. Fat-soluble vitamins are absorbed in micelles. Iron and calcium absorption is regulated. Transferrin carries some iron to the bone marrow. While the rest is stored in the ferritin pool. Most Ca++ is absorption is increased by parathyroid hormone mediated activation of vitamin D Most absorbed nutrients immediately pass through the liver for processing. The liver monitors nutrient molecules and controls their concentration in the blood or lymph leaving the liver. Communication between the stomach, pancreas, and small intestine balance absorption secretion. Iron not absorbed by cells Dietary Iron lost iron Iron lost in feces Absorbable iron as cell is sloughed Lumen Iron absorbed into cell Ferritin – pool of iron not absorbed into blood Epithelial cell of villus Iron absorbed into Excess Plasma blood; bound to iron in transferrin blood Digestive tract lumen Stomach parietal cell Blood Maintance of pH balance Pancreatic duct cell Intestinal epithelial cell New Ascending meal colon Gastrin Ileocecal valve Ileocecal sphincter The ileocecal juncture, between Pushes valve open and relaxes sphincter the small and Pushes valve closed large intestine, and contracts Ileum prevents sphincter contamination of Cecum the small intestine Appendix by colonic bacteria. The large intestine Transverse colon is a drying & storage organ. Ascending colon Most digestion & Taeniae coli absorption has Haustra been accomplished in the small Ileocecal intestine. valve The colon extracts water and salt and Cecum eliminates the feces. Appendix Rectum The colon contains many beneficial External anal bacteria including Internal anal sphincter some that generate sphincter (skeletal muscle) (smooth muscle) vitamin K Anal canal Movement through the large intestine. The large-intestine secretion is entirely protective and consists of an alkaline mucus solution Contractions of the haustrae slowly shuffle the colonic contents back and forth. Mass movements propel colonic contents long distances. They drive the feces into the distal part of the large intestine. Material is stored here until eliminated by defecation. Muscle contractions and the relaxation of two sphincter muscles eliminate the feces. Intestinal gases are absorbed or expelled. There are several important gastrointestinal hormones. 1) Gastrin is produced by G cells in the stomach increases the secretion of hydrochloric acid and pepsinogen and enhances gastric motility. 2) Secretin is produced in the duodenum inhibits gastric emptying and gastric secretion, stimulates the pancreas to produce NaHCO3, stimulates the liver to produce bile. 3) CCK is produced in the duodenum inhibits gastric emptying and gastric secretion, stimulates the pancreas to produce NaHCO3, signals the gallbladder to secrete bile. 4) GIP promotes metabolic processing of nutrients once they are absorbed.
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