Digestive System

Digestive System Honors Anatomy & Physiology

Overview of Digestive System • Alimentary Canal • Continuous, muscular digestive tube • Digests – breaks food down into smaller, simpler substances • Absorbs - digested substances pass through lining into blood

Digestive Processes • Ingestion – taking food into mouth • Propulsion • Moves food through canal • Peristalsis – alternate waves of contraction and relaxation; some mixing occurs here as well • Mechanical Digestion – Physical processing • Chewing, mixing of food with saliva with tongue; churning of food in stomach • Segmentation – rhythmic local constriction of the intestine (mixes food with digestive enzymes)

Chemical Digestion • Series of catabolic steps in which complex food molecules are chemically broken down into building blocks • Absorption • Passage of digested end products from lumen of tube through mucosal cells via active or passive transport • Small intestine is MAJOR absorptive site • Defecation • Eliminates indigestible substances as feces

Digestive System Organ Wall Construction • Peritoneum • Visceral peritoneum – covers external surface of most digestive organs • Parietal peritoneum – lines body wall • Peritoneal space – contains lubricating fluid

Mesentery • Extension between parietal and visceral peritoneum • Shaped like a pleated fan with small end attached to posterior abdominal wall • Outer edge encloses most of small intestine • Provides routes for blood vessels, nerves, and lymphatics to reach digestive organs

Alimentary Canal Walls • Mucosa • Innermost layer • Secretion of mucus, digestive enzymes, and hormones • Absorption of digestive end products • Protection against infectious disease • Simple columnar epithelium + Goblet cells (mucus)

Submucosa • External to mucosa • Contains blood vessels, lymphatic vessels, nerve fibers • Rich supply of elastic fibers enables stomach to retain normal shape • Muscularis • Responsible for segmentation and peristalsis • 2 layers of smooth muscle – circular (inner) and longitudinal (outer) – at right angles to each other

Serosa • Protective outermost layer – visceral peritoneum • Produces peritoneal fluid

Anatomy of Digestive System • Oral Cavity • Functions • Analysis of material before swallowing • Mechanical processing of food • Lubrication by mixing food with salivary secretions • Limited digestion of complex carbohydrates with amylase found in saliva • Structures • Hard palate – part of facial palatine bone • Soft palate & Uvula • Used for speech • Pulled up during swallowing to close off nasal passages

Tongue • Large muscle attached to mandible & hyoid • Frenulum – tissue that attaches tongue to bottom of oral cavity • Papillae – bumps on surface of tongue; tasting • Teeth • Cuspids – grinding • Incisors – cutting • Canines – gripping/tearing

Salivary glands • Parotid • Largest; located in front and slightly below ear • Produces saliva rich in amylase • Sublingual • Located under tongue • Mucus produced in saliva • Submandibular • Located on backside of mandible at base of oral cavity • Fairly viscous, mucous saliva

Saliva • 99.4% water • 0.6% collection of electrolytes, buffers, enzymes, etc. • Mumps – common childhood illness before 1970 • Viral infection of parotid glands • In adult males, virus attacks testes in 25% of cases, leading to sterility

Pharynx • Transports food to esophagus without further processing • Esophagus • Hollow muscular tube 25 cm long and 2 cm in diameter • Function: Carry food/liquids to stomach • Folds in mucosa / submucosa allow for expansion during passage of large amounts of food

Stomach • Functions • Bulk storage of ingested food • Mechanical breakdown of ingested food • Disruption of chemical bonds through actions of acids and enzymes • Production of intrinsic factor – glycoprotein required for vitamin B12 absorption • Chyme • Mixture of ingested food plus gastric juice • Viscous, highly acidic, soupy mixture

Anatomy • Greater/Lesser Curvatures • Fundus • Portion superior to the cardiac sphincter • “Overflow” • Body • Largest region • Mixing tank • Pylorus • Muscular pyloric sphincter regulates release of chyme – usually open • Glands secrete mucus and important digestive enzymes

Rugae = folds • Mucous membrane is highly folded so that it can expand during food ingestion • Muscle Layers • Longitudinal • Circular • Oblique • Gastric Glands • Parietal – Makes HCl and intrinsic factor • Goblet cells – Makes bicarbonate-rich mucus to coat and protect stomach lining • Chief cells – makes pepsinogen

Digestion • HCL denatures dietary protein • Protein breakdown • Pepsinogen in presence of HCl is converted into → Pepsin (active enzyme) • Long amino acid chain broken down into peptide fragments • Regulation of Gastric Activity • Cephalic Phase – brief phase • Starts with sight, smell, taste or thought of food • Directed by the CNS • Production of gastric juice begins: ~ 500 mL/hr

Gastric Phase – lasts several hours • Begins with arrival of food • Intestinal Phase • Chyme enters duodenum • Absorption • Aspirin • Alcohol

Small Intestine • Functions • Primary role in digestion, absorption, and propulsion • 90% of nutrients absorbed in small intestine • Anatomy • Size – 6 m long; 4 cm (duodenum) to 2.5 cm (ileum) in diameter • Cross-section • Plicae • Transverse folds • Permanent • Increases surface area • Intestinal villi • Covered by simple columnar epithelium • Microvilli – brush border

Subdivisions • Duodenum ** Majority of digestion occurs • Length: 25 cm closest to the stomach • Receives chyme & digestive enzymes from pancreas and liver • In response to presence of fat in chyme, duodenum produces cholecystokinin to trigger bile release • Mucous glands secrete alkaline mucus • Buffers change pH of chyme from pH of 1-2 to a pH of 7-8 by end of duodenum

Jejunum • Length: 2.5 m • Nutrient absorption occurs here • Villi are very prominent in proximal half • Villi get smaller in distal half • Drastic weight loss measure – remove a significant portion of jejunum to reduce normal absorption

Ileum • Length: 3.5 m • Villi – scattered and stumpy (conical) • Large lymphoid nodules protect ileum from bacteria present in large intestine

Accessory Organs • Pancreas • Anatomy • Location: lies to left of & partially behind stomach • Shape: elongated, pink/gray organ → “feathery” • Length: 15 cm • Pancreatic duct delivers enzymes to duodenum

Functions • Endocrine – secretes insulin and glucagon into blood for glucose metabolism • Exocrine • Digestion – Alkaline pancreatic juice arrives before chyme is delivered to duodenum • Pancreatic juice • Composition: water, ions, & digestive enzymes

Digestive enzymes: accounts for 70% of total • Pancreatic amylase • Lipase • Peptidases • Proteases • Nucleases • Sodium bicarbonate: produced to neutralize acidic chyme

Liver • Anatomy • 1.5 kg – largest visceral organ • Divided into four lobes • Communicates with duodenum via bile duct • Function • Role in digestion – Metabolism of carbohydrates by regulating blood glucose; proteins; lipids • Bile – Produced in liver; 97% water + bile salts and bilirubin (pigment produced from hemoglobin breakdown)

Gallbladder – Stores bile, concentrates it and releases it into duodenum in response to fat in chyme • Packaging of fats = Mechanical Digestion • Bile salts break fat globules into smaller pieces = EMULSIFICATION • Increases surface area: volume ratio • Makes fat breakdown more efficient

Jaundice • Blocked bile duct causes bile to be absorbed into blood – gallstones can cause this • Yellowish skin – discoloration that results from buildup of yellow bile pigments

Large Intestine • Functions • Reabsorption of water and compaction of chyme into feces • Absorption of important vitamins produced by bacterial action • Storage of fecal matter prior to defecation • Structure **LACKS VILLI** • Size: Length – 1. 5 m; Width - 7.5 cm • Secretions • Abundance of goblet cells – mucus needed for lubrication • No digestive enzymes produced

Ileocecal valve – controls movement of chyme into large intestines • Cecum • Expanded pouch: ~5-8 cm long • Collects and stores chyme • Vermiform appendix • Attached to posterior medial surface of cecum • Primary function: Immune protection • Inflammation = appendicitis

Colon • Larger diameter & thinner wall than small intestine • Regions • Ascending • Transverse • Descending • Sigmoid – “S” shaped • Walls of colon elongate and distend for feces storage • Colon cancer • Relatively common • Start screening at age 50 • Mortality remains high

Rectum • The last 15 cm • Expandable organ for temporary storage of fecal material • Anal canal is last few centimeters; controlled by anal sphincter

Physiology • Absorption of water • Roughly 1500 mL of material enters colon each day and only 200 mL of feces ejected • Absorption of vitamins produced by colonic bacteria • Vitamin K: Fat soluble vitamin needed by liver to synthesize 4 clotting factors • Biotin: water soluble vitamin important in glucose metabolism • Vitamin B5: Water soluble vitamin required in manufacturing of steroid hormones

Urobilinogens – Bilirubin is broken down by bacteria into urobilinogen = pigment in feces • Bile salts are reabsorbed in the cecum • Toxins • Bacterial metabolism generates ammonia, indole, skatol (odor of feces), hydrogen sulfide gas (rotten egg odor)

Motility • Peristalsis occurs 2 -3 times / day • Constipation: Food the remains in large intestine too long becomes overly dehydrated → Difficult to pass • Diarrhea • Watery stools • Any condition that rushes food residue through the large intestine not allowing water reabsorption

Digestion • Mechanical • Physically breaks food down into smaller pieces or mixes it • Teeth, tongue, hard palate, stomach “churning”, segmentation, peristalsis • Deglutition – swallowing • Emulsification of fats by bile • Chemical • Catabolic process in which large food molecules are broken down into their building blocks • Building blocks small enough to be absorbed across the membrane

Carbohydrate Digestion • Salivary amylase & pancreatic amylase • Polysaccharide →disaccharide • Starch →maltose • Maltase, sucrose, and lactase • Small intestine brush border enzymes that break disaccharides down into monosaccharides • Monosaccharides • Primarily glucose • Absorbed • BUILDING BLOCK

Protein Digestion • Pepsin • Produced in stomach • Pepsinogen + HCl →Pepsin • Breaks protein (long chain of amino acids) down into peptide chunks (shorter amino acid chains) • Trypsin • Produced in pancreas along with chymotrypsin • Breaks large peptide chunks into smaller fragments • Peptidases • Brush border enzyme that liberates final amino acids • Amino acids • Absorbed • BUILDING BLOCK

Fat Digestion • Occurs in small intestine • Bile physically breaks up large fat globules into smaller packages →EMULSIFICATION • Pancreatic lipase • Fatty acids + glycerol (3-carbon alcohol) • Absorbed by lacteals (immune system) in villus • Transported to blood at right atrium

Digestive System