Digestive System

Honors Anatomy & Physiology

I. Overview of Digestive SystemA. Alimentary Canal

1. Continuous, muscular digestive tube2. Digests – breaks food down into smaller, simpler

substances3. Absorbs - digested substances pass through lining into

blood

B. Digestive Processes1. Ingestion – taking food into mouth2. Propulsion

a. Moves food through canalb. Peristalsis – alternate waves of contraction and relaxation;

some mixing occurs here as well3. 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 Digestiona. Series of catabolic steps in which complex food

molecules are chemically broken down into building blocks

5. Absorptiona. Passage of digested end products from lumen of

tube through mucosal cells via active or passive transport

b. Small intestine is MAJOR absorptive site6. Defecation

a. Eliminates indigestible substances as feces

C. Digestive System Organ Wall Construction1. Peritoneum

a. Visceral peritoneum – covers external surface of most digestive organs

b. Parietal peritoneum – lines body wallc. Peritoneal space – contains lubricating fluid

d. Mesentery1) Extension between parietal and visceral

peritoneum2) Shaped like a pleated fan with small end

attached to posterior abdominal wall3) Outer edge encloses most of small

intestine4) Provides routes for blood vessels,

nerves, and lymphatics to reach digestive organs

2. Alimentary Canal Wallsa. Mucosa

1) Innermost layer2) Secretion of mucus, digestive enzymes, and

hormones3) Absorption of digestive end products4) Protection against infectious disease5) Simple columnar epithelium + Goblet cells

(mucus)

b. Submucosa1) External to mucosa2) Contains blood vessels, lymphatic vessels,

nerve fibers3) Rich supply of elastic fibers enables stomach to

retain normal shapec. Muscularis

1) Responsible for segmentation and peristalsis2) 2 layers of smooth muscle – circular (inner) and

longitudinal (outer) – at right angles to each other

d. Serosa1) Protective outermost layer – visceral

peritoneum2) Produces peritoneal fluid

II. Anatomy of Digestive SystemA. Oral Cavity

1. Functionsa. Analysis of material before swallowingb. Mechanical processing of foodc. Lubrication by mixing food with salivary secretionsd. Limited digestion of complex carbohydrates with

amylase found in saliva2. Structures

a. Hard palate – part of facial palatine boneb. Soft palate & Uvulac. Used for speechd. Pulled up during swallowing to close off nasal passages

c. Tongue1) Large muscle attached to mandible & hyoid 2) Frenulum – tissue that attaches tongue to

bottom of oral cavity3) Papillae – bumps on surface of tongue; tasting

d. Teeth1) Cuspids – grinding2) Incisors – cutting3) Canines – gripping/tearing

e. Salivary glands1) Parotid

a) Largest; located in front and slightly below earb) Produces saliva rich in amylase

2) Sublinguala) Located under tongueb) Mucus produced in saliva

3) Submandibulara) Located on backside of mandible at base of oral

cavityb) Fairly viscous, mucous saliva

4) Salivaa) 99.4% waterb) 0.6% collection of electrolytes, buffers,

enzymes, etc.

5) Mumps – common childhood illness before 1970a) Viral infection of parotid glandsb) In adult males, virus attacks testes in 25%

of cases, leading to sterility

B. Pharynx1. Transports food to esophagus without further

processing

C. Esophagus1. Hollow muscular tube 25 cm long and 2 cm in

diameter2. Function: Carry food/liquids to stomach3. Folds in mucosa / submucosa allow for

expansion during passage of large amounts of food

D. Stomach1. Functions

a. Bulk storage of ingested foodb. Mechanical breakdown of ingested foodc. Disruption of chemical bonds through actions of

acids and enzymesd. Production of intrinsic factor – glycoprotein

required for vitamin B12 absorption

2. Chymea. Mixture of ingested food plus gastric juiceb. Viscous, highly acidic, soupy mixture

3. Anatomya. Greater/Lesser Curvaturesb. Fundus

1) Portion superior to the cardiac sphincter2) “Overflow”

c. Body1) Largest region2) Mixing tank

d. Pylorus1) Muscular pyloric sphincter regulates release of

chyme – usually open2) Glands secrete mucus and important digestive

enzymes

e. Rugae = folds1) Mucous membrane is highly folded so that it

can expand during food ingestion

f. Muscle Layers1) Longitudinal2) Circular3) Oblique

g. Gastric Glands1) Parietal – Makes HCl and intrinsic factor2) Goblet cells – Makes bicarbonate-rich mucus to

coat and protect stomach lining3) Chief cells – makes pepsinogen

h. Digestion1) HCL denatures dietary protein2) 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 Activity1) Cephalic Phase – brief phase

a) Starts with sight, smell, taste or thought of foodb) Directed by the CNSc) Production of gastric juice begins: ~ 500 mL/hr

2) Gastric Phase – lasts several hoursa) Begins with arrival of food

3) Intestinal Phasea) Chyme enters duodenum

j. Absorption1) Aspirin2) Alcohol

E. Small Intestine1. Functions

a. Primary role in digestion, absorption, and propulsionb. 90% of nutrients absorbed in small intestine

2. Anatomya. Size – 6 m long; 4 cm (duodenum) to 2.5 cm (ileum) in

diameterb. Cross-section

1) Plicaea) Transverse foldsb) Permanentc) Increases surface area

2) Intestinal villia) Covered by simple columnar epitheliumb) Microvilli – brush border

c. Subdivisions1) Duodenum ** Majority of digestion

occursa) Length: 25 cm closest to the stomachb) Receives chyme & digestive enzymes from

pancreas and liverc) In response to presence of fat in chyme,

duodenum produces cholecystokinin to trigger bile release

d) Mucous glands secrete alkaline mucuse) Buffers change pH of chyme from pH of 1-2

to a pH of 7-8 by end of duodenum

2) Jejunuma) Length: 2.5 mb) Nutrient absorption occurs herec) Villi are very prominent in proximal

half d) Villi get smaller in distal halfe) Drastic weight loss measure –

remove a significant portion of jejunum to reduce normal absorption

3) Ileuma) Length: 3.5 mb) Villi – scattered and stumpy (conical)c) Large lymphoid nodules protect

ileum from bacteria present in large intestine

F. Accessory Organs1. Pancreas

a. Anatomy1) Location: lies to left of & partially behind

stomach2) Shape: elongated, pink/gray organ →

“feathery”3) Length: 15 cm4) Pancreatic duct delivers enzymes to duodenum

b. Functions1) Endocrine – secretes insulin and

glucagon into blood for glucose metabolism

2) Exocrinea) Digestion – Alkaline pancreatic juice

arrives before chyme is delivered to duodenum

b) Pancreatic juicei. 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. Livera. Anatomy

1) 1.5 kg – largest visceral organ2) Divided into four lobes3) Communicates with duodenum via bile duct

b. Function1) 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 Digestiona) Bile salts break fat globules into smaller

pieces = EMULSIFICATIONi. Increases surface area: volume ratioii. Makes fat breakdown more efficient

b. Jaundice1) 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 Intestine1. 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 defecation2. Structure **LACKS VILLI**

a. Size: Length – 1. 5 m; Width - 7.5 cmb. 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. Cecum1) Expanded pouch: ~5-8 cm long2) Collects and stores chyme3) Vermiform appendix

a) Attached to posterior medial surface of cecum

b) Primary function: Immune protectionc) Inflammation = appendicitis

e. Colon1) Larger diameter & thinner wall than small

intestine2) Regions

a) Ascendingb) Transversec) Descendingd) Sigmoid – “S” shaped

3) Walls of colon elongate and distend for feces storage

4) Colon cancera) Relatively commonb) Start screening at age 50c) Mortality remains high

f. Rectum1) The last 15 cm2) Expandable organ for temporary

storage of fecal material3) Anal canal is last few centimeters;

controlled by anal sphincter

3. Physiologya. 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 bacteria1) Vitamin K: Fat soluble vitamin needed by liver

to synthesize 4 clotting factors2) Biotin: water soluble vitamin important in

glucose metabolism3) Vitamin B5: 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 cecume. Toxins

1) Bacterial metabolism generates ammonia, indole, skatol (odor of feces), hydrogen sulfide gas (rotten egg odor)

4. Motilitya. Peristalsis occurs 2 -3 times / dayb. Constipation: Food the remains in large intestine

too long becomes overly dehydrated → Difficult to pass

c. Diarrhea1) Watery stools2) Any condition that rushes food residue through

the large intestine not allowing water reabsorption

III. DigestionA. Mechanical

1. Physically breaks food down into smaller pieces or mixes ita. Teeth, tongue, hard palate, stomach “churning”,

segmentation, peristalsis

2. Deglutition – swallowing3. 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 Digestion1. Salivary amylase & pancreatic amylase

a. Polysaccharide →disaccharideb. Starch →maltose

2. Maltase, sucrose, and lactasea. Small intestine brush border enzymes that break

disaccharides down into monosaccharides

3. Monosaccharidesa. Primarily glucoseb. Absorbedc. BUILDING BLOCK

D. Protein Digestion1. Pepsin

a. Produced in stomachb. Pepsinogen + HCl →Pepsinc. Breaks protein (long chain of amino acids) down into peptide

chunks (shorter amino acid chains)

2. Trypsin a. Produced in pancreas along with chymotrypsinb. Breaks large peptide chunks into smaller fragments

3. Peptidasesa. Brush border enzyme that liberates final amino acids

4. Amino acidsa. Absorbedb. BUILDING BLOCK

A. Fat Digestion1. Occurs in small intestine

a. Bile physically breaks up large fat globules into smaller packages →EMULSIFICATION

2. Pancreatic lipase3. Fatty acids + glycerol (3-carbon alcohol)

a. Absorbed by lacteals (immune system) in villus

b. Transported to blood at right atrium

c.