ANATOMY AND PHYSIOLOGY thin outer layer of smooth muscle called the muscularis mucosae DIGESTIVE SYSTEM − − Also known as the Gastrointestinal Tract (GIT) A complex set of organs, glands, and ducts that work together to transform food into nutrients for cells II. FUNCTIONS o o o o o o o o INGESTION – consumption of solid or liquid food, usually through the mouth MASTICATION – the breakdown of large food particles into many small ones PROPULSION MIXING SECRETION DIGESTION – breakdown of large organic molecules into smaller molecules that can be absorbed; occurs through mechanical & chemical means ABSORPTION – movement of molecules out of the digestive tract &b into the blood or lymphatic system ELIMINATION – removal of undigested material, such as fiber from food, plus other waste products from the body as feces OVERVIEW OF THE DIGESTIVE SYSTEM SUBMUCOSA o o o III. MUSCULARIS o o o IV. 1) 2) 3) 4) 5) 6) 7) 8) 9) Oral Cavity (Mouth) Pharynx Esophagus Stomach Small Intestine Large Intestine Rectum Anal Canal Anus Digestive Tract – consists of an inner layer of circular smooth muscle & an outer layer of longitudinal smooth muscle Nerve plexus, innervated by the ANS lies between the 2 muscle layers Nerve plexuses of the muscularis & submucosa compose the Enteric Nervous System (ENS) SEROSA/ADVENTITIA o o o o A. DIGESTIVE TRACT/ALIMENTARY CANAL Lies just outside the mucosa A thick layer of connective tissue containing nerves, blood vessels, & small glands An extensive network of nerve cell processes forms a plexus within this tunic Outermost layer of the digestive tract Either a serosa or an adventitia Serosa – consists of the peritoneum, a smooth epithelial layer & its underlying connective tissue Adventitia – covers the regions of the digestive tract which isn’t covered by peritoneum; connective tissue layer; continuous with the surrounding connective tissue NEURAL INNERVATION OF THE GASTROINTESTINAL TRACT B. ACCESSORY ORGANS 1) 2) 3) 4) 5) 6) Salivary Glands Teeth Tongue Liver Pancreas Gallbladder 1) ENTERIC NERVOUS SYSTEM o Myenteric plexus – plexus of Auerbach o Submucosal plexus – plexus of Meissner 2) AUTONOMIC NERVOUS SYSTEM HISTOLOGY I. o o MUCOSA Innermost layer or tunic Consists of 3 layers: (1) inner mucous epithelium, (2) a loose connective tissue called the lamina propria, & (3) a CHEMICAL REGULATION OF THE DIGESTIVE SYSTEM 1) ACETYLCHOLINE – stimulates GI tract motility & secretions 2) NOREPINEPHRINE – inhibits GI tract motility & secretions 3) SEROTONIN – stimulates GI tract motility 4) GASTRIN – stimulates gastric glands to secrete large amounts of gastric juice 5) SECRETIN – stimulates the flow of pancreatic juice that is rich in bicarbonate (HCO3-) ions 5) MESOCOLON − PERITONEUM − − Bind the transverse colon (transverse mesocolon) & sigmoid colon (sigmoid mesocolon) of the large intestine to the posterior abdominal wall ORGANS Largest serous membrane of the body Consist of simple squamous epithelium (mesothelium) with an underlying areolar connective tissue Contains large folds that weave between the viscera Consists of the: (1) Parietal Peritoneum & the (2) Visceral Peritoneum − − I. − 5 MAJOR PERITONEAL FOLDS 1) MESENTERY o o o o o Fan-shaped fold of the peritoneum Largest peritoneal fold; laden with fat Consist of 2 layers of serous membranes with a thin layer of connective tissue between them Hold many of the organs in place within the abdominal cavity Provides a route for blood vessels & nerves from the abdominal wall to the organs − − − − − − 2) GREATER OMENTUM o o o ORAL CAVITY (MOUTH) 1st part of the digestive tract a) CHEEKS Form the lateral walls of the oral cavity Contains the buccinator muscles which flatten the cheeks against the teeth Play a role in mastication Help form words during mastication b) LIPS Muscular structures mostly formed by the orbicularis oris Covered by keratinized stratified epithelium & is continuous with the moist stratified squamous epithelium of the mucosa in the oral cavity Mesentery extending as a fold from the greater curvature & then to the transverse colon Also known as the fatty apron Omental Bursa – cavity within the greater omentum 3) FALCIFORM LIGAMENT o Attaches the liver to the anterior abdominal wall & diaphragm 4) LESSER OMENTUM o Mesentery connecting the lesser curvature of the stomach & the proximal end of the duodenum to the liver & diaphragm − − − − c) TEETH There are 32 teeth in the normal adult mouth located in the mandible & the maxillae Teeth of adults are called permanent teeth or secondary teeth These replace your primary/deciduous teeth or milk/baby teeth, which consists of 20 teeth Each tooth has 3 regions: (1) the crown, (2) neck, & (3) the root ➢ Crown – has one or more cusps (points); the visible portion of a tooth ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢ Neck – the small region between the crown & the root Root – the smallest region of the tooth & anchors it in the jawbone Pulp Cavity – found in the center of the tooth; filled with blood vessels, nerves, & connective tissue, pulp Dentin – living, cellular, calcified tissue which surrounds the pulp cavity Enamel – extremely hard, acellular substance which covers the dentin of the tooth crown; protects the tooth against abrasion & acids produced by bacteria in the mouth Cementum – covers the surface of the dentin in the root; helps anchor the tooth in the jaw Alveoli – holds the teeth in place; along the alveolar process of the mandible & maxillae Gingiva/Gums – covers the alveolar processes; dense fibrous connective tissue & moist stratified squamous epithelium Periodontal Ligaments – secure the teeth in the alveoli by embedding into the cementum SALIVA − − − − − − − − − − − − − − − − − − − − − − − d) TONGUE A large muscular organ that occupies most of the oral cavity Major attachment of the tongue is in the posterior part of the mouth The anterior part of the tongue is relatively free ➢ Frenulum – a thin fold of tissue which attaches the tongue to the floor of the mouth Anterior two-thirds of the tongue is covered by papillae, some of which contain taste buds Posterior one-third of the tongue is devoid of papillae & has only a few scattered taste buds Posterior portion contains a large amount of lymphatic tissue which helps form the lingual tonsil Helps hold food in place during mastication & plays a role in swallowing e) SALIVARY GLANDS Has 3 major pairs ➢ Parotid Gland – the largest of the salivary glands; located just anterior to each ear; parotid ducts enter the oral cavity adjacent to the 2nd upper molars ➢ Submandibular Gland – produce more serous than mucous secretions; can be felt as a soft lump along the inferior border of the mandible; submandibular ducts open into the oral cavity on each side of the frenulum of the tongue ➢ Sublingual Gland – the smallest of the 3 pairs; produce primarily mucous secretions; lie immediately below the mucous membrane in the floor of the oral cavity; each gland has 10 – 12 small ducts opening onto the floor of the oral cavity Compound alveolar glands Have branching ducts with clusters of alveoli, resembling grapes, at the ends of the ducts Produce saliva − − − − II. − − − III. − − − − − − Helps keep the oral cavity moist & contains enzymes that begin the process of digestion Produced at the rate of 1.0 – 1.5L/day 99.5% water & 0.5% solutes Sodium, potassium, chloride, bicarbonate, & phosphate Dissolved gases & various organic substances including urea, uric acid, mucus, immunoglobin A, lysozyme, & salivary amylase ➢ Salivary Amylase – a digestive enzyme found in the serous part of saliva; breaks down starch; breaks the covalent bonds between glucose molecules in starch & other polysaccharides, this then produces disaccharides such as maltose & isomaltose, which have a sweet taste Prevents bacterial infections in the mouth by washing the mouth with a mildly antibacterial enzyme, lysozyme Neutralizes the pH in the mouth, which reduces the harmful effects of bacterial acids on tooth enamel The mucous secretions of the submandibular & sublingual glands contain a large amount of mucin ➢ Mucin – a proteoglycan that gives a lubricating quality to the secretions of the salivary glands Salivary gland secretion is regulated primarily by the ANS ➢ Parasympathetic Nervous System – control salivation; most important ➢ Sympathetic Nervous System – increases mucous content of saliva when stimulated; dominates during stress f) VESTIBULE Area between the teeth, lips, & cheeks Divides into sulci: (1) labial, & (2) buccal Communicates with the surface of the body by the rima or orifice of the mouth g) ORAL CAVITY PROPER Bounded at the sides and in front by the alveolar process and at the back by the isthmus of the fauces Enclosed by teeth h) TONSILS Located in the lateral posterior walls of the oral cavity, in the nasopharynx, & in the posterior surface of the tongue i) PALATE Separates the oral cavity from the nasal cavity & prevents food from passing into the nasal cavity while chewing & swallowing ➢ Hard Palate – anterior part containing bones ➢ Soft Palate – posterior portion consisting of skeletal muscles & connective tissue ➢ Uvula – a posterior extension of the soft palate PHARYNX Connects the mouth with the esophagus Consists of 3 parts: (1) the nasopharynx, (2), oropharynx, & laryngopharynx Normally, only the oropharynx & laryngopharynx carry food to the esophagus ➢ Pharyngeal Constrictors (Superior, Middle, & Inferior) – forms the posterior walls of the oropharynx & laryngopharynx ESOPHAGUS Muscular tube lined with moist stratified squamous epithelium About 25cm long Lies anterior to the vertebrae & posterior to the trachea within the mediastinum Upper two-thirds has skeletal muscle in its wall Lower one-third has smooth muscle in its wall Transports food from the pharynx to the stomach ➢ Upper & Lower Esophageal Sphincters – regulate the movement of food into & out of the esophagus ➢ The lower esophageal sphincter is sometimes called the cardiac sphincter DEGLUTITION STRUCTURE ACTIVITY Pharyngeal deglutition Pharynx Esophagus stage of • Relaxation of upper esophageal sphincter • Esophageal stage of deglutition (peristalsis) Relaxation of lower esophageal sphincter Secretion of mucus • • IV. RESULT Moves bolus from oropharynx to laryngopharynx & into esophagus; closes air passageways • Permits entry of bolus from laryngopharynx into esophagus • Pushes bolus down esophagus • Permits entry of bolus down esophagus • Lubricates esophagus for smooth passage of bolus STOMACH − − − − − Primarily houses food for mixing with hydrochloric acid & other secretions An enlarged segment of the digestive tract in the left superior part of the abdomen Internal volume of about 50mL when empty 1.0 – 1.5L after a typical meal Up to 4L when extremely full & will extend nearly as far as the pelvis − − − − − − Takes approximately 4 hours to clear a meal Antrum holds 30mL 3mL of chyme is released into the duodenum per contraction Receives parasympathetic fibers from vagus & sympathetic fibers from the celiac ganglia Protected from the harsh acidic & enzymatic environment it creates through 3 ways: (1) mucous coat, (2) tight junctions, & (3) epithelial cell replacement Divided into 4 regions: ➢ Cardiac Region (Cardia) – around the gastroesophageal opening; small area within about 3cm of the cardiac orifice ➢ Fundic Region (Fundus) – most superior part of the stomach; dome-shaped portion superior to esophageal attachment ➢ Body (Corpus) – makes up the greatest part of the stomach; turns to the right, forming a greater curvature & a lesser curvature ➢ Pyloric Region – narrower pouch at the inferior end i. Pyloric Opening – opening into the small intestine ii. Pyloric Sphincter – thick ring of smooth muscle surrounding the pyloric opening ➢ MOVEMENT − − − HISTOLOGY − − − − − − − The muscularis layer of the stomach is different from the other regions of the stomach in that it has 3 layers: (1) an outer longitudinal layer, (2) a middle circular layer, & (3) an inner oblique layer These muscular layers produce a churning action in the stomach The submucosa & mucosa of the stomach are thrown into large folds called rugae when the stomach is empty The rugae allow the mucosa & submucosa to stretch & the folds disappear as the stomach is filled Stomach is lined with simple columnar epithelium Mucosal surface forms numerous tube-like gastric pits which are the openings for the gastric glands There are 5 groups of epithelial cells in the stomach: 1) Surface mucous cells – coats & protects the stomach lining 2) Mucous neck cells – produce mucus 3) Parietal cells – produce hydrochloric acid & intrinsic factor 4) G cells/Endocrine cells – produce regulatory chemicals; gastrin 5) Chief cells – produce pepsinogen SECRETIONS OF THE STOMACH − − As food enters the stomach, the food is mixed with stomach secretions to become a semifluid mixture called chyme Stomach secretions from the gastric glands include: ➢ Hydrochloric Acid – produces a pH of about 2.0 in the stomach; kills microorganisms & activates the enzyme, pepsin ➢ Pepsin – converted from its inactive form pepsinogen; breaks covalent bonds of proteins to form smaller peptide chains; exhibits optimum enzymatic activity at a pH of about 2.0 ➢ Mucus – lubricates the epithelial cells of the stomach wall & protects them from the damaging effect of the acidic chyme & pepsin Intrinsic Factor – binds with vitamin B12 & makes it more readily absorbed in the small intestine − − 2 types of stomach movement aid digestion& help move chyme through the digestive tract: (1) mixing waves & (2) peristaltic waves Both result from smooth muscle contractions in the stomach wall Contractions occur every 20 seconds ➢ Mixing Waves – from relatively weak contractions; thoroughly mix ingested food with stomach secretions to form chyme ➢ Peristaltic Waves – from stronger contractions; force the chyme toward & through the pyloric sphincter; pyloric sphincter usually remains closed because of mild tonic contraction, but each peristaltic contraction is sufficiently strong to cause partial relaxation of this & to pump a few millimeters of chyme through it & into the duodenum; increased by low blood glucose levels ➢ Hunger Pangs – caused by peristaltic waves when the stomach is empty; occur for about 2 – 3 minutes & can build in strength; begin 12 – 24 hours after the previous meal; growling If the stomach empties too fast, the efficiency of digestion & absorption in the small intestine is reduced If the rate of emptying is too slow, the highly acidic contents of the stomach may damage the stomach wall REGULATION OF STOMACH SECRETION 1) Cephalic Phase − Stomach secretions are increased in anticipation of incoming food − Vagus nerves carry parasympathetic action potentials to the stomach where enteric plexus neurons are activated − Postganglionic neurons stimulate secretion by parietal & chief cells & stimulate gastrin & histamine secretion by endocrine cells − Gastrin is carried through the blood back to the stomach, along with histamine ➢ Gastrin – stimulates additional secretory activity ➢ Histamine – most potent stimulator of hydrochloric acid secretion 2) Gastric Phase − Greatest volume of gastric secretion occurs; activated by presence of food in the stomach − Distention of stomach stimulates mechanoreceptors & activates a parasympathetic reflex; action potentials generated by the mechanoreceptors are carried by the vagus nerves to the medulla oblongata − Medulla oblongata increases action potentials in the vagus nerves that stimulate secretions by parietal & chief cells & stimulate gastrin & histamine secretion by endocrine cells − Distention of the stomach also activates local reflexes that increase stomach secretions − Gastrin is carried through the blood back to the stomach, along with histamine 3) Intestinal Phase − Primarily inhibits gastric secretions; controlled by the entrance of acidic chyme into the duodenum − Chyme in the duodenum with a pH less than 2 or containing lipids inhibits gastric secretions by 3 mechanisms − Chemoreceptors in the duodenum are stimulated by H+ (low pH) or lipids; action potentials generated are carried by the vagus nerves to the medulla oblongata where they inhibit parasympathetic action potentials, thereby decreasing gastric secretions − Secretin & cholecystokinin produced by the duodenum decrease gastric secretions in the stomach ➢ ➢ V. o Secretin – inhibits gastric secretions; from duodenum Cholecystokinin – inhibits gastric secretions; from duodenum MOVEMENT SMALL INTESTINE − VI. − − − − − About 6m long Consists of 3 parts: (1) the duodenum (25cm), (2) the jejunum (2.5m), & (3) the ileum (3.5m) Common bile duct & pancreatic duct join & empty into the duodenum Chyme passes through the small intestine in a span of 3 – 5 hours Has 3 modifications that increase its surface area about 600-fold: (1) circular folds, (2) villi, & (3) microvilli ➢ Ileocecal Junction – the site where the ileum connects to the large intestine; contains ileocecal sphincter & ileocecal valve ➢ Peyer Patches – clusters of lymphatic nodules; numerous in the ileum HISTOLOGY − − − − − − − − − Mucosa & submucosa form a series of circular folds Finger-like projections of the mucosa form villi Most of the cells composing the surface of the villi have microvilli Each villus is covered by simple columnar epithelium Within the loose CT core of each villus are a blood capillary network & a lacteal The mucosa of the small intestine is simple columnar epithelium with 4 major cell types: ➢ Absorptive cells – have microvilli; produce digestive enzymes, & absorb digested food ➢ Goblet cells – produce a protective mucus ➢ Granular cells – may help protect the intestinal epithelium form bacteria ➢ Endocrine cells – produce regulatory hormones Epithelial cells are located within tubular glands of the mucosa, called intestinal glands or crypts of Lieberkuhn at the base of the villi The submucosa of the duodenum contains mucous glands, called duodenal glands Epithelial cells in the walls of the small intestine have enzymes, bound to their free surfaces ➢ Peptidases – digest proteins ➢ Disaccharidases – digest small sugars, specifically disaccharides FUNCTIONS o o Segmentations mix chyme with digestive juices & bring food into contact mucosa for absorption; peristalsis propels chyme through small intestine Completes digestion of carbohydrates, proteins, & lipids; begins & completes digestion of nucleic acids Mixing & propulsion of chyme are the primary mechanical events that occur in the small intestine ➢ Peristaltic Contractions – proceed along the length of the intestine for variable distances & cause the chyme to move along the small intestine ➢ Segmental Contractions – propagated for only short distances & mix intestinal contents LARGE INTESTINE − − − Absorbs about 90% of nutrients & water that pass through digestive system − − − − − − − − − Divided into 4 regions: the (1) cecum, (2) colon, (3) rectum, & (4) the anal canal a) CECUM Proximal end of the large intestine where it joins with the small intestine at the ileocecal junction A sac that extends inferiorly about 6cm past the ileocecal junction ➢ Appendix – 9cm long tube attached to the cecum b) COLON Divided into 4 regions: (1) the ascending colon, (2) the transverse colon, the (3) descending colon, & (4) the sigmoid colon 1.5 – 1.8 m long Mucosal lining of the colon contains numerous straight, tubular glands called crypts, which contain many mucusproducing goblet cells The longitudinal smooth muscle layer does not completely envelop the intestinal wall but forms 3 bands called teniae coli c) RECTUM A straight, muscular tube that begins at the termination of the sigmoid colon & ends at the anal canal Muscular tunic is composed of smooth muscle & is relatively thick in the rectum compared to the rest of the digestive tract d) ANAL CANAL The last 2 – 3cm of the digestive tract Begins at the inferior end of the rectum & ends at the anus The smooth muscle layer is even thicker than that of the rectum ➢ Internal canal sphincter – formed by smooth muscle at the superior end of the canal ➢ External canal sphincter – formed by skeletal muscle at the inferior end of the anal canal FUNCTIONS − − − − − 18 – 24 hours are required for material to pass through the large intestine Chyme is converted into feces Feces formation is due to the absorption of water & salts, the secretion of mucus, & extensive action of microorganisms Colon stores the feces until they are eliminated by the process of defecation Microorganisms constitute about 30% of the dry weight of the feces ➢ Mass movements – occur every 8 – 12 hours; several strong contractions that the large parts of the colon undergo ➢ Defecation reflex – occurs when feces distend the rectal wall; consists of local & parasympathetic reflexes VIII. LIVER − − − − − − − − Produces bile Largest internal organ of the body Weighs about 1.36kg Located in the upper right quadrant of the abdomen Consists of 2 major lobes: (1) right lobe, (2) left lobe The 2 lobes are separated by a connective tissue septum called the falciform ligament 2 smaller liver lobes: the (1) caudate lobe & the (2) quadrate lobe can be seen from the inferior view The porta, the gate through which blood vessels, ducts, & nerves enter or exit the liver, can also be seen from the inferior view HISTOLOGY − VII. − − − PANCREAS Located retroperitoneal, posterior to the stomach in the inferior part of the left upper quadrant Has a head near the midline of the body & a tail that extends to the left where it touches the spleen Composed of both endocrine & exocrine tissues that perform several functions ➢ Pancreatic islets (islets of Langerhans) – contained in the endocrine glands ➢ Islet cells – produce the hormones insulin & glucagon ➢ Acinar gland – the exocrine part ➢ Acini – produce digestive enzymes; connected by small ducts which join to form larger ducts, & the larger ducts join to form the pancreatic duct − − − − − − Many delicate connective tissue septa divide the liver into lobules with portal triads at their corners The portal triads contain 3 structures: (1) the hepatic artery, (2) hepatic portal vein, & (3) the hepatic duct Hepatic cords, formed by plate-like groups of liver cells called hepatocytes, are located between the center & the margins of each lobule The hepatic cords are separated from one another by blood channels called the hepatic sinusoids The sinusoid epithelium contains phagocytic cells that help remove foreign particles from the blood ➢ Bile canaliculus – is a cleft-like lumen between the cells of each hepatic cord Bile flows through the bile canaliculi to the hepatic ducts in the portal triads Macrophages are called Kupffer cells FUNCTIONS − − − The increased pH resulting from the secretion of HCO3- stops pepsin digestion but provides the proper environment for the function of pancreatic enzymes Exocrine secretions include bicarbonate ions (HCO3-) & digestive enzymes called pancreatic enzymes ➢ Bicarbonate ions – neutralize the acidic chyme that enters the small intestine from the stomach ➢ Pancreatic enzymes – are important in digesting all major classes of food The major protein-digesting (proteolytic) enzymes are: (1) trypsin, (2) chymotrypsin, & (3) carboxypeptidase ➢ Pancreatic amylase – continues the polysaccharide digestion that began in the oral cavity ➢ Nucleases – enzymes that degrade DNA & RNA to their component nucleotides ➢ Lipase – a lipid-digesting enzyme FUNCTIONS o o o o o o o o Bile Production Detoxification of Drugs Carbohydrate Metabolism Lipid Metabolism Protein Metabolism Storage Phagocytosis Activation of Vitamin D BILE − − − Secreted by the liver at the rate of 600 – 1000mL a day Dilutes & neutralizes stomach acid Dramatically increases the efficiency of fat digestion & absorption − Bile salts emulsify fats, breaking the fat globules into smaller droplets − Also contains excretory products, such as cholesterol, fats, & bile pigments ➢ Bilirubin – a bile pigment that results from the breakdown of hemoglobin ➢ Gallstones – may form if the amount of cholesterol secreted by the liver becomes excessive & is not able to be dissolved by the bile salt IX. GALLBLADDER − − A small sac on the interior surface of the liver Stores concentrated bile
