The Digestive System

The Digestive System • Mouth---bite, chew, swallow • Pharynx and esophagus----transport • Stomach----mechanical disruption; absorption of water & alcohol • Small intestine--chemical & mechanical digestion & absorption • Large intestine----absorb electrolytes & vitamins (B and K) • Rectum and anus---defecation

Types of Digestion • Mechanical – mouth, stomach,SI, LI • Chemical – mouth, stomach, SI

Layers of the GI Tract 1. Mucosal layer 2. Submucosal layer 3. Muscularis layer 4. Serosa layer

Mucosa • Epithelium • stratified squamous (in mouth, esophagus & anus) = areas of physical stress • simple columnar in the rest = areas for secretion & absorption • secretes enzymes and absorbs nutrients • specialized cells (goblet) secrete mucous onto cell surfaces • enteroendocrine cells---secrete hormones controlling organ function • Lamina propria • loose connective tissue • contains BVs and lymphatic tissue • Muscularis mucosae • thin layer of smooth muscle • causes folds to form in mucosal layer • responsible for local food movements • increases absorption with exposure to “new” nutrients

Submucosa • Loose connective tissue • containing BV, glands and lymphatic tissue • Meissner’s plexus • part of the enteric nervous system - “brain of the gut” • parasympathetic division only - sensory and motor neurons • vasoconstriction of blood vessels to gut • controls contraction of muscularis mucosa • controls the secretory cells of the mucosal epithelium • connected to the myenteric plexus (in the muscularis layer)

Muscularis • areas of skeletal and smooth muscle found • Skeletal muscle = voluntary control • in mouth, pharynx , upper esophagus and anus • control over swallowing and defecation • Smooth muscle = involuntary control • inner circular fibers & outer longitudinal fibers • mixes, crushes & propels food along by peristalsis • Auerbach’s plexus (myenteric plexus) • both parasympathetic & sympathetic innervation of circular and longitudinal smooth muscle layers • part of the Enteric nervous system • controls overall GI tract motility

Serosa • An example of a serous membrane • Covers all organs and walls of cavities not open to the outside of the body • Covering the digestive organs in the peritoneal cavity = visceral peritoneum • Secretes a serous fluid • Consists of connective tissue covered with simple squamous epithelium

Peritoneum • Peritoneum • visceral layer covers organs • parietal layer lines the walls of body cavity • Peritoneal cavity • potential space containing a bit of serous fluid

The path of food: oral cavity/teeth/salivary glands oropharynx/epiglottis esophagus stomach small intestine: duodenum small intestine: jejunum small intestine: ileum large intestine: ascending colon large intestine: transverse colon large intestine: descending colon sigmoid colon rectum anus

Mouth • site of mechanical & chemical digestion • Oral cavity proper • the roof = hard, soft palate and uvula • floor – geniohyoid, mylohyoid – contains the tongue • lips and cheeks-----contains buccinator muscle that keeps food between upper & lower teeth • Vestibule---area between cheeks and teeth

Mouth • Lined with an oral mucosa (stratified squamous epithelium & lamina propria) • Lining of the cheeks = buccal mucosa • Lining of the maxilla and mandible = alveolar mucosa (gingiva) • Landmarks: lingual frenulum, labial frenulum, uvula • Shortened lingual frenulum can impede movement of the tongue within the mouth

Tongue • physiological functions • manipulation of food for chewing and swallowing • production of digestive enzyme • speech • lamina propria layer of the mucosa contains the lingual glands for the secretion of mucus and lingual lipase • extrinsic muscles – control the movement of the tongue in and out of the mouth, manipulates food, hold the tongue in position and forms the floor of the mouth • intrinsicmuscles – originate from and insert into the connective tissue of the tongue, alter the shape and size of the tongue for speech and swallowing

Salivary Glands • Parotid glandbelow your ear and over the masseter • Submandibular glandis under lower edge of mandible • Sublingual glandis deep to the tongue in floor of mouth • All have ducts that empty into the oral cavity (exocrine glands) • Parotid duct open up near maxillary pre-molars • The submandibular and sublingual glands share the same duct – opens on either side of lingual frenulum

Salivary Glands • minor salivary glands • 600 to 1000 glands • small aggregates of secretory tissue present in the submucosa • not found in the gingiva and anterior hard palate • predominantly mucus glands • except the lingual glands (von Ebner’s glands) found on the tongue – open into troughs surrounding the circumvallate papilla • labial, lingual, palatal, buccal, glossopalatine and retromolar glands

Salivary Glands Sublingual gland • Classified either as: serous or mixed • Serous glands - cells secrete a watery fluid – e.g. parotid • Mixed glands secrete both mucus and a serous fluid – e.g. submandibular & sublingual

Saliva • Components: • 1. water • 2. enzymes: amylase, lipase, lyzozyme • 3. mucins, mucus • composition can vary from gland to gland • parotid gland: watery fluid rich in amylase, proline-rich proteins and glycoproteins • submandibular gland also contains higher levels of mucins • 4. multiple electrolytes: sodium, postassium, chloride, calcium, magnesium • 5. glucose, amino acids, urea, uric acid and lipids • 6. secretory Igs – IgG and IgM and IgA • 7. growth factors & hormones: EGF, insulin

Functions of Saliva • 1. buffering – bicarbonate, phosphate ions • protection against demineralization caused by bacterial acids resulting from the breakdown of sugars • 2. Pellicle formation • many salivary proteins bind to the surface of the teeth and oral mucosa – forms a thin film = salivary pellicle • several of these proteins bind calcium to protect the tooth • other pellicle proteins bind bacteria – initial attachment of organisms that produce plaque

Functions of Saliva • 3. maintenance of tooth integrity • saliva is saturated with calcium and phosphate ions • at the tooth surface the high concentration of calcium and phosphate results in maturation of the enamel – increases surface hardness • re-mineralization of lesions upon caries can also be induced by saliva – enhanced by fluoride in the saliva • repair to large areas of enamel breakdown is not possible • 4. antimicrobial action • barrier function provided by mucins • saliva also contains a spectrum of proteins with antimicrobial activity – histatins, lysozyme, lactoferrin and peroxidase • also the presence of antibodies – IgA is the major salivary Ig (results in agglutination of microbes and prevents their adherence to oral tissues)

Functions of Saliva • 5. tissue repair • variety of growth factors are present in saliva • many promote tissue growth and differentiation • 6. digestion • wets food for swallowing – produces the bolus • chemical digestion via amylase and lipase • 7. taste • solubilizes food substances – allows binding to taste receptors located in taste buds

Salivation • controlled by the ANS • parasympathetic system provides a constant supply of saliva to keep the mucus membranes moist and to lubricate the food • Increased salivation – parasympathetic system • sight, smell, sounds, memory of food, tongue stimulation---rock in mouth • stimulation of taste receptors are conveyed to the cerebral cortex to the salivatory nuclei in brainstem- returning impulses via the parasymp system travel via CN 7 & 9 • Decreased salivation – sympathetic system • dry mouth when you are afraid • also inhibits salivation during dehydration

Mumps • Myxovirus that attacks the parotid gland • Symptoms • inflammation and enlargement of the parotid • fever, malaise & sour throat (especially swallowing sour foods) • swelling on one or both sides • Sterility rarely possible in males with testicular involvement (only one side involved) • Vaccine available since 1967

Digestion in the Mouth • Mechanical digestion = mastication or chewing • breaks food into pieces • mixes food with saliva so it forms a bolus • Chemical digestion • salivary amylase • begins starch digestion at pH of 6.5 or 7.0 found in mouth • when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis ceases • lingual lipase • secreted by glands in lamina propria of tongue • begins breakdown of long and medium-chain triglycerides into a free fatty acids and a diglyceride

Digestion in the Mouth • Mechanical digestion = mastication or chewing • breaks food into pieces • mixes food with saliva so it forms a bolus • performed by the teeth • grinding, tearing and shearing of food • two main divisions:crown and root • crown: above gum-line • covered with a layer of enamel • root: secures the tooth in the jaw socket • entry of nerves and blood vessels -innermost layer - pulp (nerves/blood vessels) -outer covering of specialized calcified connective tissue – dentin

Pharynx • Funnel-shaped tube extending from internal nares to the esophagus (posteriorly) and larynx (anteriorly) • Skeletal muscle lined by mucous membrane • Deglutition or swallowing is facilitated by saliva and mucus • starts when bolus is pushed into the oropharynx • sensory nerves send signals to deglutition center in brainstem (medulla oblongata)

Deglutition: Swallowing • Deglutition or swallowing is facilitated by saliva and mucus • starts when bolus is pushed into the oropharynx • sensory nerves send signals to deglutition center in medulla oblongata

Deglutition: Swallowing • 1. Voluntary phase---tongue pushes food to back of oral cavity • stimulates receptors in the oropharynx • message travels to the deglutition center in the MO and lower pons • 2. Pharyngeal stage (involuntary) • breathing stops & air passages are closed • vocal cords close • epiglottis becomes bent over airway (glottis) as larynx is lifted • controlled by autonomic nervous system

Deglutition: Swallowing • 3. Esophageal phase (involuntary) • Peristalsis pushes food down the esophagus • circular fibers behind bolus contract • longitudinal fibers in front of bolus also contract to shorten the distance of travel and widens the esophagus • Travel time is 4-8 seconds for solids and 1 sec for liquids • Lower esophageal sphincter relaxes as food approaches the stomach

Esophagus • Mucosa = stratified squamous • Submucosa = contains large mucous glands • Muscularis = upper 1/3 is skeletal, middle is mixed, lower 1/3 is smooth • upper & lower esophageal sphincters are prominent rings of circular muscle • Collapsed muscular tube • In front of vertebrae • Posterior to trachea • Posterior to the heart • Pierces the diaphragm at the hiatus • Site for hiatal hernia or diaphragmatic hernia

Gastroesophageal Reflex Disease • If lower esophageal sphincter fails to open • distension of esophagus feels like chest pain or heart attack • If lower esophageal sphincter fails to close • stomach acids enter esophagus & cause heartburn (GERD) • for a weak sphincter---don't eat a large meal and lay down in front of TV • smoking and alcohol make the sphincter relax worsening the situation • Control the symptoms by avoiding • coffee, chocolate, tomatoes, fatty foods, onions & mint • take Tagamet HB or Pepcid AC 60 minutes before eating • neutralize existing stomach acids with Tums

Stomach--Chemical Digestion • Protein digestion begins • HCl denatures (unfolds) protein molecules • HCl transforms pepsinogen into pepsin that breaks peptides bonds between certain amino acids • Fat digestion continues • gastric lipase splits the triglycerides in milk fat • most effective at pH 5 to 6 (infant stomach) • HCl kills microbes in food • Mucous cells protect stomach walls from being digested • 1-3mm thick layer of mucous

Anatomy of Stomach • Size when empty • large sausage • stretches due to rugae • Parts of stomach • cardic portion • fundus • body • pyloric portion – narrows at the pyloric sphincter • bolus mixes with gastric juice in the stomach to form chyme • stomach empties as small squirts of chyme leave the stomach through the pyloric sphincter

Muscularis • Three layers of smooth muscle--outer longitudinal, circular & inner oblique • Permits greater churning & mixing of food with gastric juice Serosa • Simple squamous epithelium over a bit of connective tissue • Also known as visceral peritoneum

Mucosa of the Stomach • simple columnar epithelium with embedded mucus cells • forms columns = gastric glands that open into the stomach lumen through gastric pits • gastric glands - for the secretion of gastric juice – mix of water, HCl, enzymes and hormones

Histology of the Stomach - Mucosa • food can remain in the fundus of the stomach for up to one hour before being mixed with gastric juice • during this time – salivary amylase and lipase continue their digestion • after being mixed with gastric juice – these enzymes become inactivated

Cells of the Mucosa • Chief cells: secretepepsinogen (inactive protease)into the stomach which will become pepsin (active protease)– for protein digestion • Parietal cells – secrete: • 1. H+ and Cl- ions into the stomach – become Hydrochloric acid • 2. Intrinsic factor - for the absorption of vitamin B12 for RBC production • Enteroendocrine cells = G cells • secrete gastrin hormone– increases gastric juice production, gastric motility and increase gastric emptying

Parietal Cells • for the secretion of HCl and Intrinsic factor • Intrinsic factor (parietal cells) • absorption of vitamin B12 for RBC production • Hydrochloric acid converts Pepsinogen (from chief cells) into the enzyme pepsin = protein digestion • HCl from the parietal cells is secreted as H+ and Cl- ions • the H+ comes from the absorption of water into the parietal cell – production of HCO3- and H+ ions • proton pumps actively pump H+ into the lumen of the stomach • causes movement of K+ into the parietal cell • the bicarbonate is pumped into the blood in exchange for Cl-ions • opening of K+ and Cl- channels causes the diffusion of these ions back into the lumen

pepsinogen = zymogen (proenzyme) • activation requires the removal of a 44 amino acid peptide • HCl creates an acidic environment • causes the pepsinogen to unfold and cleave itself

Gastric Mucosa Secretions • Parietal cells: HCl and Intrinsic Factor • Chief cells: pepsin and gastric lipase • pepsin – breaks down proteins into smaller peptides • most efficient at breaking the peptide bonds at the N-terminus end of phenylalanine, tyrosine and tryptophan • gastric lipase – like lingual lipase it removes one fatty acid to create a free fatty acid and a diglyceride • lingual and gastric lipases are known as acidic lipases • they do not require bile to function

Absorption of Nutrients by the Stomach • Water especially if it is cold • Electrolytes • Some drugs (especially aspirin) & alcohol • Gastric mucosal cells contain alcohol dehydrogenase that converts some alcohol to acetaldehyde • more of this enzyme found in males than females • 5 different forms of ADH – some more efficient then others

Gastric phase of digestion • starts once food reaches the stomach • stretching of the stomach begins this phase • neural and hormonal regulation to promote gastric secretion and motility

Gastric phase of digestion: Neural Regulation • distension of the stomach stimulates stretch mechanoreceptors – stimulates production of gastric juices • chemoreceptors alsodetect elevated pH (from bolus) – adds to the stimulation • impulses travel via the parasympathetic neurons of the Meissner’s plexus and the Auerbach’s plexus to the cells of the mucosa • stimulates the flow of gastric juice, causes waves of peristalsis to mix the food with the juice and move food into the SI • chemoreceptors monitor pH of the stomach chyme as juice is being made • as the pH of the stomach chyme decreases (becomes more acidic) –creates a negative feedback loop • as the food leaves the stomach and the stretching of the wall lessens – this inhibits this path

Gastric phase of digestion: Hormonal Phase • hormonal regulation: follows the neural phase • distension of the stomach activates the parasympathetic system (red arrows) • parasympathetic system activates the G cells  gastrin secretion • secretion of gastrin accelerates the emptying of the stomach via increased muscular contractions (yellow arrows) • gastrin also stimulates the chief cells secretion of pepsinogen

Anatomy of the Small Intestine • 20 feet long----1 inch in diameter • Large surface area for majority of absorption • 3 parts • duodenum---10 inches • jejunum---8 feet • ileum---12 feet • ends at ileocecal valve

Small Intestine • three structures that increase surface area: • plica circularis • permanent ½ inch tall folds in the mucosal layer • not found in lower ileum • cannot stretch out like rugae in stomach • villi • 1 Millimeter tall projections of the mucosa • Core is lamina propria of mucosal layer • Contains vascular capillaries and lacteals (lymphatic capillaries) • absorptive cells with microvilli • cell surface feature known as brush border • The key to the function of the small intestine is its largeSURFACE AREA for absorption

Small intestine mucosa: Intestinal glands Epithelial layer • Absorptive cells– digest and absorb nutrients in the chyme • Goblet cells - Unicellular glands that are part of simple columnar epithelium • Enteroendocrine cells • within the intestinal glands • secretin • cholecystokinin • gastric inhibitory peptide • Paneth cells • secrete lysozyme • kill bacteria

SI mucos: the Lymphatic System • lamina propria of the SI contains areolar connective tissue • plus an abundance of mucosa-associated lymphatic tissue – MALT • solitary lymphatic nodules in the distal part of the ileum • groups of nodes in the ileum – Peyer’s patches

Intestinal glands: Absorptive cells • for the production of brush-border enzymes (“intestinal juice”) • 1 to 2 liters per day • BB enzymes are made and inserted into the microvilli of the absorptive cells • enzymatic digestion by these enzymes occurs externally on these microvilli - rather than in the lumen of the SI • BB enzymes: enzymes for the digestion of carbohydrates, proteins and nucleic acids • as the older cells of the villus are sloughed off and die – burst open and release more enzymes that digest more food in the chyme

Small Intestine: Chemical Digestion -protein and carbohydrate digestion via synthesis of the brush border enzymesby the intestinal glands -enterokinase -maltase, sucrase, lactase, a-dextrinase = smallcarbohydrates and disaccharides -aminopeptidase, dipeptidase = small peptides and dipeptides into amino acids -phosphatases and nucleosidases = nucleotides into nucleosides -resulting monosaccharides, amino acids and nucleotides are internalized by the absorptive cell -SO carbohydrate and protein digestion stops in the interior of the absorptive cell

Small Intestine: Chemical Digestion • BUT - duodenum is also the site for secretion of the pancreatic juice: • 1-2 qt./day------ at pH 7.6 • Pancreas: 5" long by 1" thick • Head close to curve in C-shaped duodenum • pancreatic duct joinscommon bile duct from liver • Opens 4" below pyloric sphincter

Histology of the Pancreas • Acini- dark clusters • 99% of gland • produce pancreatic juice • Islets of Langerhans • 1% of gland – 1 to 2 million • Alpha cells (20%) produce glucagon • Beta cells (70%) produce insulin • Delta cells (5%) produce somatostatin • F cells produce pancreatic polypeptide

The Digestive System