Chapter 15 - The Digestive System

Irregular tube; open at both ends, called “Alimentary canal” or “Gastrointestinal (GI) Tract” 29 feet long (adults) - 9 meters Food & other substances that enter tube are not really

inside body Passageway of food: broken down (digested) and

absorbed thru walls < entering body - cells

Both - Mechanical & Chemical Digestion

Break Down of Food

Teeth- first physical breakdownStomach-churning of food (physical)Mouth- first chemical breakdown (salvia)Digestive enzymes throughout GI tractDigestion - Process where large food particles

reduced to absorbable molecules Absorption - Process of small molecules passing

thru digestive system walls into body

Key Organs of the GI Tract

Know Main & Accessory organs, Table 16-2; page 476

Small Intestine : Duodenum, Jejunum, & IleumLarge Intestines (elimination > feces): Cecum, Colon: Ascending, Transverse, Descending,

Sigmoid

Wall of Digestive Tract -

Mouth to anus Four layers of tissue; surrounding the hollow

space within the tube “lumen” May vary in structure in different organs Mucosa or mucous membrane - tough in

esophagus, delicate, for absorption or secretion in rest of tract

Submucosa - connective tissue, blood vessels & nerves

Muscularis - 2 layers, responsible for

wavelike, rhythmic contractions (peristalsis), moves contents, assists in mixing & mechanical breakdown

Serosa - outermost covering, composed of visceral peritoneum

Mesentery - double folded peritoneal tissue, anchors loops of digestive tract to posterior wall of abdominal cavity

Mouth - Oral cavity - hollow chamber (roof, a floor, & walls) Entrance of food; digestion begins immediately Mucous membranes > mucus, protects against

digestive juices & lubricates food passage This mucous protects & lubricates Hard palate - bony structure, front portion Soft palate - posterior, chiefly muscles Uvula - cone-shaped process hanging down from

soft palate. W/ help of soft pal., prevents food or liquid from entering nasal cavity

Floor of the mouth - Tongue - skeletal muscular structure, covered w/

mucous membrane Anchored to bones in skull > hyoid bone

Frenulum- thin membrane; attaches tongue to floor of mouth Tongue-tied: too short

Papillae: small elevations on surface Vallate type - largest, inverted V-shaped row of

about 10-12 mushroomlike elevations - tastebuds

Teeth - Four major types -

Incisors - (sharp/cutting) Canines - cuspids (pierce/tear) Premolars - bicuspids & Molars - tricuspids

(grinding/crush)Mastication > chewing of foodForms a bolus > ready for swallowingBy age 2 - full set 20 teeth (cut 1st - 2 yrs.)By age 17 to 24 - 32 permanent teeth (cut 1st - 6

yrs.)

Typical Tooth -Three main parts -

Crown - visible, covered w/ enamel (hardest tissue in body)

Neck - narrow portion surrounds by gum tissue (gingiva)

Root - fitted into socket in upper or lower jaw, lined by fibrous, periodontal membrane

Inside Structure - Enamel on outside, Dentin, Pulp cavity (blood vessels & nerves) moving inward

Salivary Glands - 3 Pairs - ducts drain saliva into oral cavity,

secretes about 1 liter/day Parotid - in front of each ear (mumps -

tender) Submandibular - ducts by fernullum Sublingual - ducts into floor of mouth

Saliva contents - salivary amylase (begins CHO digestion), mucus (moistens food)

Pharynx - Behind nasal cavity & mouth Tubelike structure made of muscles, lined

w/ mucous membrane Part of respiratory & digestive systems

Esophagus - Passage for food to stomach Tube-like structure, 10 inches long Mucous lined GERD - often caused by hiatal hernia

Stomach - Upper part of abdominal cavity, under

diaphragm Pouch for food, hollow, expands (can push up

on diaphragm > discomfort) Lower esophageal sphincter (LES) or cardiac

sphincter - rings of muscle tissue at end of esophagus - keeps food from reentering the esophagus when the stomach contracts

Chyme - semi-solid mixture, produced by contraction of stomach muscles that mixes food w/ gastric juices

Stomach contractions - Created by 3 layers of muscles, run

lengthwise, around, obliquelyMakes stomach one of strongest organs >

peristalsisBreaks food into tiny particles

Mucous membranes line stomach -

contains gastric glands > secrete gastric juice & hydrochloric acid

When empty, wrinkled folds - rugaeThree divisions of stomach -

Fundus, body, pylorusPyloric sphincter - holds food in stomach,

empties contents slowly into small intestine

Ulcer - carterlike wound or sore in membrane of stomach• 1 in 10 persons suffer in USA• Helicobacter pylori bacterium (H. pylori)

Small Intestines - Portion of digestion tract that extends from the pylorus

to the ileocecal valve 12- 20 feet in length, coiled, convoluted, and occupies

most of the abdominal cavity Intestinal glands - secrete digestive juices Smooth muscle wall - contracts > peristalsis

Plicea - circular folds covered w/ villi, increases surface area > absorption

In or on the villi - Blood capillaries - absorb CHO & protein end

products (glucose & amino acids) Lacteal - lymphatic vessel - absorb lipids Microvilli - brushlike border, > surface more Chemical digestion - most occurs in 1st

subdivision of duodenum Minor & major duodenal papillae - ducts where

pancreatic enzymes & bile enter small intestine

Liver - Large organ, fills R upper abdominal cavity Exocrine gland - secretes bile into ducts Hepatic - means liver Bile - essential for breaking up or emulsification

of fats CCK (cholecystokinin) - hormone secretion

triggered by lipids in chyme > makes gallbladder contract & release bile

Drains from common bile duct into duodenum

Gallbladder - concentrates & stores bile

Pancreas -C-shaped, exocrine gland that lies behind the

stomach & duodenumPancreatic juice - most important digestive juice

- contains enzymes for all 3 food groupsSodium bicarbonate (alkaline substance) -

neutralizes hydrochloric acidEnters small intestine thru same duct as bileIslets of Langerhans - hormones producedPancreatitis - inflammation (blockage, CF)

Large Intestine -Begins with the ending of the ileum at the

ileocecal valve - called the cecumApproximately 5 feet in length, much larger in

diameter than small intestineContents - not called chymeFunction - reabsorb water & saltsMaterial acted on by bacteria > more nutrients

from cellulose & other fibers Synthesis Vit. K needed for blood clotting, Production of some B-complex vit.

Not as well suited for absorption as small intestine - no villi

Normal passage of material thru large intestine - 3 to 5 days

Subdivisions - flow in GI Tract one-wayCecum - pouchlike areaAscending colon - right side of body

Bends at hepatic or right colic flexure

Transverse colon - extends across front Bends at splenic or left colic flexure

Descending colon - left side abdomen

Sigmond colon - S-shaped segment,

terminates in rectumAnal canal - terminal end of rectum, ends at

external opening - anusInner anal sphincter - involuntary, smooth

muscle, keeps anus closed except during defecation

Outer anal sphincter - striated, voluntary muscle

Appendix - Vermiform appendix - “worm-shaped”, tubular

structure, blind tube No important digestive fnc. - digest cellulose Appendicitis - inflammation

Peritoneum - Large, moist, slippery sheet of serous membrane Peritoneal space - small space between parietal &

visceral layers - surfaces slide freely Retroperitoneal - organs outside peritoneum Extensions of peritoneum-mesentary, greater omentum

- both assist in anchoring abd. contents

Digestion -Chemical & mechanical

breakdownCHO - amylase in mouth, slight effectamylase from pancreas - into small intestine

Absorption of simple sugars (glucose)

Proteins - stomach (HCL/pepsinogen> pepsin) Finished in small intestine by pancreatic (trypsin)

& peptidases in intestinal juice Amino acids - basic protein units

Fats - in small intestine

Bile emulsification of fats > pancreatic lipase > fatty acids & glycerol

Key digestive juices & enzymes* page 410 Table 15-2Absorption - taking food, breaking it

down in form for utilization of bodyJust as important as digestion