1. 23The Digestive SystemPharynx* From the mouth, the oro- and laryngopharynx allow passage of:* Food and fluids to the esophagus* Air to the trachea* Lined with stratified squamous epithelium and mucus glands* Has two skeletal muscle layers* Inner longitudinal* Outer pharyngeal constrictorsEsophagus*Muscular tube going from the laryngopharynx to the stomach* Travels through the mediastinum and pierces the diaphragm* Joins the stomach at the cardiac orificeEsophagusEsophageal Characteristics* Esophageal mucosa nonkeratinized stratified squamous epithelium* The empty esophagus is folded longitudinally and flattens when foodis present

2. * Glands secrete mucus as a bolus moves through the esophagus*Muscularis changes from skeletal (superiorly) to smooth muscle(inferiorly)Digestive Processes in the Mouth* Food is ingested*Mechanical digestion begins (chewing)* Propulsion is initiated by swallowing* Salivary amylase begins chemical breakdown of starch* The pharynx and esophagus serve as conduits to pass food from themouth to the stomachDeglutition (Swallowing)* Coordinated activity of the tongue, soft palate, pharynx, esophagus,and 22 separate muscle groups* Buccal phase bolus is forced into the oropharynxDeglutition (Swallowing)Pharyngeal-esophageal phase controlled by the medulla and lowerpons* All routes except into the digestive tract are sealed off* Peristalsis moves food through the pharynx to the esophagusDeglutition (Swallowing)Deglutition (Swallowing)Deglutition (Swallowing)Deglutition (Swallowing) 3. Deglutition (Swallowing)Stomach* Chemical breakdown of proteins begins and food is converted tochyme* Cardiac region surrounds the cardiac orifice* Fundus dome-shaped region beneath the diaphragm* Body midportion of the stomach* Pyloric region made up of the antrum and canal which terminates atthe pylorus* The pylorus is continuous with the duodenum through the pyloricsphincterStomach* Greater curvature entire extent of the convex lateral surface* Lesser curvature concave medial surface* Lesser omentum runs from the liver to the lesser curvature* Greater omentum drapes inferiorly from the greater curvature to thesmall intestineStomach* Nerve supply sympathetic and parasympathetic fibers of theautonomic nervous system* Blood supply celiac trunk, and corresponding veins (part of thehepatic portal system) 4. Microscopic Anatomy of the Stomach*Muscularis has an additional oblique layer that:* Allows the stomach to churn, mix, and pummel food physically* Breaks down food into smaller fragmentsMicroscopic Anatomy of the Stomach* Epithelial lining is composed of:* Goblet cells that produce a coat of alkaline mucus* The mucous surface layer traps a bicarbonate-rich fluid beneath it* Gastric pits contain gastric glands that secrete gastric juice, mucus,and gastrinMicroscopic Anatomy of the StomachMicroscopic Anatomy of the StomachMicroscopic Anatomy of the StomachGlands of the Stomach Fundus and Body* Gastric glands of the fundus and body have a variety of secretory cells* Mucous neck cells secrete acid mucus* Parietal cells secrete HCl and intrinsic factorGlands of the Stomach Fundus and Body* Chief cells produce pepsinogen* Pepsinogen is activated to pepsin by:* HCl in the stomach* Pepsin itself via a positive feedback mechanism 5. * Enteroendocrine cells secrete gastrin, histamine, endorphins,serotonin, cholecystokinin (CCK), and somatostatin into the laminapropriaStomach Lining* The stomach is exposed to the harshest conditions in the digestivetract* To keep from digesting itself, the stomach has a mucosal barrier with:* A thick coat of bicarbonate-rich mucus on the stomach wall* Epithelial cells that are joined by tight junctions* Gastric glands that have cells impermeable to HCl* Damaged epithelial cells are quickly replacedDigestion in the Stomach* The stomach:* Holds ingested food* Degrades this food both physically and chemically* Delivers chyme to the small intestine* Enzymatically digests proteins with pepsin* Secretes intrinsic factor required for absorption of vitamin B12Regulation of Gastric Secretion* Neural and hormonal mechanisms regulate the release of gastric juice* Stimulatory and inhibitory events occur in three phases* Cephalic (reflex) phase: prior to food entry* Gastric phase: once food enters the stomach* Intestinal phase: as partially digested food enters the duodenum 6. Cephalic Phase* Excitatory events include:* Sight or thought of food* Stimulation of taste or smell receptors* Inhibitory events include:* Loss of appetite or depression* Decrease in stimulation of the parasympathetic divisionGastric Phase* Excitatory events include:* Stomach distension* Activation of stretch receptors (neural activation)* Activation of chemoreceptors by peptides, caffeine, and rising pH* Release of gastrin to the bloodGastric Phase* Inhibitory events include:* A pH lower than 2* Emotional upset that overrides the parasympathetic divisionIntestinal Phase* Excitatory phase low pH; partially digested food enters theduodenum and encourages gastric gland activity* Inhibitory phase distension of duodenum, presence of fatty, acidic,or hypertonic chyme, and/or irritants in the duodenum* Initiates inhibition of local reflexes and vagal nuclei 7. * Closes the pyloric sphincter* Releases enterogastrones that inhibit gastric secretionRelease of Gastric Juice: Stimulatory EventsRelease of Gastric Juice: Inhibitory EventsRegulation and Mechanism of HCl Secretion* HCl secretion is stimulated by ACh, histamine, and gastrin throughsecond-messenger systems* Release of hydrochloric acid:* Is low if only one ligand binds to parietal cells* Is high if all three ligands bind to parietal cells* Antihistamines block H2 receptors and decrease HCl releaseRegulation and Mechanism of HCl SecretionResponse of the Stomach to Filling* Stomach pressure remains constant until about 1L of food is ingested* Relative unchanging pressure results from reflex-mediated relaxationand plasticityResponse of the Stomach to Filling* Reflex-mediated events include:* Receptive relaxation as food travels in the esophagus, stomachmuscles relax* Adaptive relaxation the stomach dilates in response to gastric filling* Plasticity intrinsic ability of smooth muscle to exhibit the stress-relaxationresponseGastric Contractile Activity 8. * Peristaltic waves move toward the pylorus at the rate of 3 per minute* This basic electrical rhythm (BER) is initiated by pacemaker cells(cells of Cajal)Gastric Contractile Activity*Most vigorous peristalsis and mixing occurs near the pylorus* Chyme is either:* Delivered in small amounts to the duodenum or* Forced backward into the stomach for further mixingGastric Contractile ActivityRegulation of Gastric Emptying* Gastric emptying is regulated by:* The neural enterogastric reflex* Hormonal (enterogastrone) mechanisms* These mechanisms inhibit gastric secretion and duodenal fillingRegulation of Gastric Emptying* Carbohydrate-rich chyme quickly moves through the duodenum* Fat-laden chyme is digested more slowly causing food to remain inthe stomach longerRegulation of Gastric EmptyingSmall Intestine: Gross Anatomy* Runs from pyloric sphincter to the ileocecal valve* Has three subdivisions: duodenum, jejunum, and ileumSmall Intestine: Gross Anatomy 9. * The bile duct and main pancreatic duct:* Join the duodenum at the hepatopancreatic ampulla* Are controlled by the sphincter of Oddi* The jejunum extends from the duodenum to the ileum* The ileum joins the large intestine at the ileocecal valveSmall Intestine: Microscopic Anatomy* Structural modifications of the small intestine wall increase surfacearea* Plicae circulares: deep circular folds of the mucosa and submucosa* Villi fingerlike extensions of the mucosa* Microvilli tiny projections of absorptive mucosal cells plasmamembranesDuodenum and Related OrgansSmall Intestine: Microscopic AnatomySmall Intestine: Histology of the Wall* The epithelium of the mucosa is made up of:* Absorptive cells and goblet cells* Enteroendocrine cells* Interspersed T cells called intraepithelial lymphocytes (IELs)* IELs immediately release cytokines upon encountering AgSmall Intestine: Histology of the Wall* Cells of intestinal crypts secrete intestinal juice* Peyers patches are found in the submucosa 10. * Brunners glands in the duodenum secrete alkaline mucusIntestinal Juice* Secreted by intestinal glands in response to distension or irritation ofthe mucosa* Slightly alkaline and isotonic with blood plasma* Largely water, enzyme-poor, but contains mucusLiver* The largest gland in the body* Superficially has four lobes right, left, caudate, and quadrate* The falciform ligament:* Separates the right and left lobes anteriorly* Suspends the liver from the diaphragm and anterior abdominal wallLiver* The ligamentum teres:* Is a remnant of the fetal umbilical vein* Runs along the free edge of the falciform ligamentLiver: Associated Structures* The lesser omentum anchors the liver to the stomach* The hepatic blood vessels enter the liver at the porta hepatis* The gallbladder rests in a recess on the inferior surface of the rightlobeLiver: Associated Structures* Bile leaves the liver via: 11. * Bile ducts, which fuse into the common hepatic duct* The common hepatic duct, which fuses with the cystic duct* These two ducts form the bile ductGallbladder and Associated DuctsLiver: Microscopic Anatomy* Hexagonal-shaped liver lobules are the structural and functional unitsof the liver* Composed of hepatocyte (liver cell) plates radiating outward from acentral vein* Portal triads are found at each of the six corners of each liver lobuleLiver: Microscopic Anatomy* Portal triads consist of a bile duct and* Hepatic artery supplies oxygen-rich blood to the liver* Hepatic portal vein carries venous blood with nutrients fromdigestive visceraLiver: Microscopic Anatomy* Liver sinusoids enlarged, leaky capillaries located between hepaticplates* Kupffer cells hepatic macrophages found in liver sinusoidsLiver: Microscopic Anatomy* Hepatocytes functions include:* Production of bile* Processing bloodborne nutrients* Storage of fat-soluble vitamins 12. * Detoxification* Secreted bile flows between hepatocytes toward the bile ducts in theportal triadsMicroscopic Anatomy of the LiverComposition of Bile* A yellow-green, alkaline solution containing bile salts, bile pigments,cholesterol, neutral fats, phospholipids, and electrolytes* Bile salts are cholesterol derivatives that:* Emulsify fat* Facilitate fat and cholesterol absorption* Help solubilize cholesterol* Enterohepatic circulation recycles bile salts* The chief bile pigment is bilirubin, a waste product of hemeThe Gallbladder* Thin-walled, green muscular sac on the ventral surface of the liver* Stores and concentrates bile by absorbing its water and ions* Releases bile via the cystic duct, which flows into the bile ductRegulation of Bile Release* Acidic, fatty chyme causes the duodenum to release:* Cholecystokinin (CCK) and secretin into the bloodstream* Bile salts and secretin transported in blood stimulate the liver toproduce bile* Vagal stimulation causes weak contractions of the gallbladder 13. Regulation of Bile Release* Cholecystokinin causes:* The gallbladder to contract* The hepatopancreatic sphincter to relax* As a result, bile enters the duodenumRegulation of Bile ReleaseRegulation of Bile ReleaseRegulation of Bile ReleaseRegulation of Bile ReleaseRegulation of Bile ReleaseRegulation of Bile ReleaseRegulation of Bile Release