Digestive System Honors Anatomy & Physiology

I.Overview of Digestive System A.Alimentary Canal 1.Continuous, muscular digestive tube 2.Digests – breaks food down into smaller, simpler substances 3.Absorbs - digested substances pass through lining into blood

B.Digestive Processes 1.Ingestion – taking food into mouth 2.Propulsion a.Moves food through canal b.Peristalsis – alternate waves of contraction and relaxation; some mixing occurs here as well 3.Mechanical Digestion – Physical processing a.Chewing, mixing of food with saliva with tongue; churning of food in stomach b.Segmentation – rhythmic local constriction of the intestine (mixes food with digestive enzymes)

4.Chemical Digestion a.Series of catabolic steps in which complex food molecules are chemically broken down into building blocks 5.Absorption a.Passage of digested end products from lumen of tube through mucosal cells via active or passive transport b.Small intestine is MAJOR absorptive site 6.Defecation a.Eliminates indigestible substances as feces

C.Digestive System Organ Wall Construction 1.Peritoneum a.Visceral peritoneum – covers external surface of most digestive organs b.Parietal peritoneum – lines body wall c.Peritoneal space – contains lubricating fluid

d.Mesentery 1)Extension between parietal and visceral peritoneum 2)Shaped like a pleated fan with small end attached to posterior abdominal wall 3)Outer edge encloses most of small intestine 4)Provides routes for blood vessels, nerves, and lymphatics to reach digestive organs

2.Alimentary Canal Walls a.Mucosa 1)Innermost layer 2)Secretion of mucus, digestive enzymes, and hormones 3)Absorption of digestive end products 4)Protection against infectious disease 5)Simple columnar epithelium + Goblet cells (mucus)

b.Submucosa 1)External to mucosa 2)Contains blood vessels, lymphatic vessels, nerve fibers 3)Rich supply of elastic fibers enables stomach to retain normal shap e c.Muscularis 1)Responsible for segmentation and peristalsis 2)2 layers of smooth muscle – circular (inner) and longitudinal (outer) – at right angles to each other

d.Serosa 1)Protective outermost layer – visceral peritoneum 2)Produces peritoneal fluid

II.Anatomy of Digestive System A.Oral Cavity 1.Functions a.Analysis of material before swallowing b.Mechanical processing of food c.Lubrication by mixing food with salivary secretions d.Limited digestion of complex carbohydrates with amylase found in saliva 2.Structures a.Hard palate – part of facial palatine bone b.Soft palate & Uvula c.Used for speech d.Pulled up during swallowing to close off nasal passages

c.Tongue 1)Large muscle attached to mandible & hyoid 2)Frenulum – tissue that attaches tongue to bottom of oral cavity 3)Papillae – bumps on surface of tongue; tasting d.Teeth 1)Cuspids – grinding 2)Incisors – cutting 3)Canines – gripping/tearing

e.Salivary glands 1)Parotid a)Largest; located in front and slightly below ear b)Produces saliva rich in amylase 2)Sublingual a)Located under tongue b)Mucus produced in saliva 3)Submandibular a)Located on backside of mandible at base of oral cavity b)Fairly viscous, mucous saliva

4)Saliva a)99.4% water b)0.6% collection of electrolytes, buffers, enzymes, etc. 5)Mumps – common childhood illness before 1970 a)Viral infection of parotid glands b)In adult males, virus attacks testes in 25% of cases, leading to sterility

B.Pharynx 1.Transports food to esophagus without further processing C.Esophagus 1.Hollow muscular tube 25 cm long and 2 cm in diameter 2.Function: Carry food/liquids to stomach 3.Folds in mucosa / submucosa allow for expansion during passage of large amounts of food

D.Stomach 1.Functions a.Bulk storage of ingested food b.Mechanical breakdown of ingested food c.Disruption of chemical bonds through actions of acids and enzymes d.Production of intrinsic factor – glycoprotein required for vitamin B 12 absorption 2.Chyme a.Mixture of ingested food plus gastric juice b.Viscous, highly acidic, soupy mixture

3.Anatomy a.Greater/Lesser Curvatures b.Fundus 1)Portion superior to the cardiac sphincter 2)“Overflow” c.Body 1)Largest region 2)Mixing tank d.Pylorus 1)Muscular pyloric sphincter regulates release of chyme – usually open 2)Glands secrete mucus and important digestive enzymes

e.Rugae = folds 1)Mucous membrane is highly folded so that it can expand during food ingestion f.Muscle Layers 1)Longitudinal 2)Circular 3)Oblique g.Gastric Glands 1)Parietal – Makes HCl and intrinsic factor 2)Goblet cells – Makes bicarbonate-rich mucus to coat and protect stomach lining 3)Chief cells – makes pepsinogen

h.Digestion 1)HCL denatures dietary protein 2)Protein breakdown a)Pepsinogen in presence of HCl is converted into → Pepsin (active enzyme) b)Long amino acid chain broken down into peptide fragments i.Regulation of Gastric Activity 1)Cephalic Phase – brief phase a)Starts with sight, smell, taste or thought of food b)Directed by the CNS c)Production of gastric juice begins: ~ 500 mL/hr

2)Gastric Phase – lasts several hours a)Begins with arrival of food 3)Intestinal Phase a)Chyme enters duodenum j.Absorption 1)Aspirin 2)Alcohol

E.Small Intestine 1.Functions a.Primary role in digestion, absorption, and propulsion b.90% of nutrients absorbed in small intestine 2.Anatomy a.Size – 6 m long; 4 cm (duodenum) to 2.5 cm (ileum) in diameter b.Cross-section 1)Plicae a)Transverse folds b)Permanent c)Increases surface area 2)Intestinal villi a)Covered by simple columnar epithelium b)Microvilli – brush border

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

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

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

F.Accessory Organs 1.Pancreas a.Anatomy 1)Location: lies to left of & partially behind stomach 2)Shape: elongated, pink/gray organ → “feathery” 3)Length: 15 cm 4)Pancreatic duct delivers enzymes to duodenum

b.Functions 1)Endocrine – secretes insulin and glucagon into blood for glucose metabolism 2)Exocrine a)Digestion – Alkaline pancreatic juice arrives before chyme is delivered to duodenum b)Pancreatic juice i.Composition: water, ions, & digestive enzymes

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

2.Liver a.Anatomy 1)1.5 kg – largest visceral organ 2)Divided into four lobes 3)Communicates with duodenum via bile duct b.Function 1)Role in digestion – Metabolism of carbohydrates by regulating blood glucose; proteins; lipids 2)Bile – Produced in liver; 97% water + bile salts and bilirubin (pigment produced from hemoglobin breakdown)

3)Gallbladder – Stores bile, concentrates it and releases it into duodenum in response to fat in chyme 4)Packaging of fats = Mechanical Digestion a)Bile salts break fat globules into smaller pieces = EMULSIFICATION i.Increases surface area: volume ratio ii.Makes fat breakdown more efficient

b.Jaundice 1)Blocked bile duct causes bile to be absorbed into blood – gallstones can cause this 2)Yellowish skin – discoloration that results from buildup of yellow bile pigments

G.Large Intestine 1.Functions a.Reabsorption of water and compaction of chyme into feces b.Absorption of important vitamins produced by bacterial action c.Storage of fecal matter prior to defecation 2.Structure **LACKS VILLI** a.Size: Length – 1. 5 m; Width cm b.Secretions 1)Abundance of goblet cells – mucus needed for lubrication 2)No digestive enzymes produced

c.Ileocecal valve – controls movement of chyme into large intestines d.Cecum 1)Expanded pouch: ~5-8 cm long 2)Collects and stores chyme 3)Vermiform appendix a)Attached to posterior medial surface of cecum b)Primary function: Immune protection c) Inflammation = appendicitis

e.Colon 1)Larger diameter & thinner wall than small intestine 2)Regions a)Ascending b)Transverse c)Descending d)Sigmoid – “S” shaped 3)Walls of colon elongate and distend for feces storage 4)Colon cancer a)Relatively common b)Start screening at age 50 c)Mortality remains high

f.Rectum 1)The last 15 cm 2)Expandable organ for temporary storage of fecal material 3)Anal canal is last few centimeters; controlled by anal sphincter

3.Physiology a.Absorption of water 1)Roughly 1500 mL of material enters colon each day and only 200 mL of feces ejected b.Absorption of vitamins produced by colonic bacteria 1)Vitamin K: Fat soluble vitamin needed by liver to synthesize 4 clotting factors 2)Biotin: water soluble vitamin important in glucose metabolism 3)Vitamin B 5 : Water soluble vitamin required in manufacturing of steroid hormones

c.Urobilinogens – Bilirubin is broken down by bacteria into urobilinogen = pigment in feces d.Bile salts are reabsorbed in the cecum e.Toxins 1)Bacterial metabolism generates ammonia, indole, skatol (odor of feces), hydrogen sulfide gas (rotten egg odor)

4.Motility a.Peristalsis occurs 2 -3 times / day b.Constipation: Food the remains in large intestine too long becomes overly dehydrated → Difficult to pass c.Diarrhea 1)Watery stools 2)Any condition that rushes food residue through the large intestine not allowing water reabsorption

III.Digestion A.Mechanical 1.Physically breaks food down into smaller pieces or mixes it a.Teeth, tongue, hard palate, stomach “churning”, segmentation, peristalsis 2.Deglutition – swallowing 3.Emulsification of fats by bile B.Chemical 1.Catabolic process in which large food molecules are broken down into their building blocks 2.Building blocks small enough to be absorbed across the membrane

C.Carbohydrate Digestion 1.Salivary amylase & pancreatic amylase a.Polysaccharide →disaccharide b.Starch →maltose 2.Maltase, sucrose, and lactase a.Small intestine brush border enzymes that break disaccharides down into monosaccharides 3.Monosaccharides a.Primarily glucose b.Absorbed c.BUILDING BLOCK

D.Protein Digestion 1.Pepsin a.Produced in stomach b.Pepsinogen + HCl →Pepsin c.Breaks protein (long chain of amino acids) down into peptide chunks (shorter amino acid chains) 2.Trypsin a.Produced in pancreas along with chymotrypsin b.Breaks large peptide chunks into smaller fragments 3.Peptidases a.Brush border enzyme that liberates final amino acids 4.Amino acids a.Absorbed b.BUILDING BLOCK

A.Fat Digestion 1.Occurs in small intestine a.Bile physically breaks up large fat globules into smaller packages →EMULSIFICATION 2.Pancreatic lipase 3.Fatty acids + glycerol (3-carbon alcohol) a.Absorbed by lacteals (immune system) in villus b.Transported to blood at right atrium