Digestive System

CHAPTER 14

Digestive System

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Functions of Digestion

� Ingestion - intake of food� Propulsion

� a. Peristalsis - alternating contracting & relaxation

� b. Segmentation - movement of food back & forth across an organ mixing with digestive juices

� Mechanical Digestion - breaking food into smaller fragments� mouth & stomach

� mastication- chewing

� Chemical Digestion - breakdown large molecules into their building blocks; occurs in mouth, stomach & small intestine

� Polysaccharides –Starch – composed of monosaccharides(glucose)

� Lipids – Fatty Acids & Glycerol

� Proteins – Amino Acid Chains

� Nucleic Acids –Nucleotide chains

� Absorption - transport of nutrients into the blood

� Defecation - elimination of waste in a solid form (feces)

Divisions of Digestive System

� Alimentary Canal or Gastrointestinal Tract (GI)

� coiled hollow tube with 2 openings

� mouth, pharynx, esophagus, stomach, small intestine, large intestine

� Accessory Organs -assist in breakdown of molecules

� teeth, tongue, salivary glands, pancreas, liver, gall bladder

Organs of the Alimentary Canal

� 1. Mouth or oral cavity� lips (labia) - protect oral cavity opening

� cheeks - forms lateral walls� hard palate - forms anterior roof

� soft palate - forms posterior roof� uvula - fleshy fingerlike projection; closes

nasal cavity when swallowing� vestibule - area between lips & cheeks and

teeth & gums

� 2. Pharynx - common throat� oropharynx - posterior

to oral cavity� nasopharynx - part of

respiratory passageway� laryngopharynx -

continues to esophagus; larynx -voice box

� 3. Esophagus - gullet; runs from pharynx through the diaphragm to the stomach

� 25 cm long� smooth muscle� propels food to stomach

through peristalsis� http://www.westga.edu/~

lkral/peristalsis/

� 4. Stomach � A. Structure

� Cardiac – surrounds junction between esophagus & stomach

� Fundus – superior portion� Body – main portion; lined with folded walls

called rugae that disappear when stomach is full

� Pylorus – terminal portion

� B. Valves or sphincters� Cardioesophageal (cardiac) – leads food

into stomach� Pyloric – leads partial digested food

(chyme) into small intestine

C. Curvatures:� Greater – outer (lateral) curve� Lesser – inner (medial) curve

� C. Mechanical Digestion

� 3 muscle layers

� segmentation of food� 1st – longitudinal� 2nd – circular� 3rd – oblique

p. 416 Fig 14.4

� D. Chemical digestion� Walls lined with gastric pits that lead to the

gastric gland� Gastric gland contains 3 types of cells� 1. Mucus neck cells – secrete alkaline sticky

mucus to protect stomach walls� 2. Parietal cells – produces HCl which

activates enzyme production� 3. Chief cells – produce protein digestive

enzymes called pepsinogen (inactive form of pepsin)

� Sequence of events:� 1. food enters stomach� 2. Parietal cells secrete HCl which

lowers pH � 3. low pH triggers chief cells to produce

pepsinogen� 4. pepsinogen converted to its active

form of pepsin

� 5. Small Intestine� A. Structure� Extends from the pyloric sphincter to the

ileocecal valve� 7 m long� Mesentary – web like membrane that

coils small intestine & holds it intact

� 3 subdivisions:� Duodenum – curves

around the pancreas; receives chyme from stomach, enzymes from pancreas & bile from liver

� Jejunum – middle portion; bulk of digestion & absorption

� Ileum – terminal portion

� B. Chemical Digestion� 1. pyloric sphincter controls amount of food entering

from stomach� 2. pancreas produces enzymes that are secreted to

small intestines through pancreatic duct� 3. Bile formed in liver is secreted through bile duct� 4. Pancreatic & bile ducts join to form

hepatopancreatic ampulla� 5. together enzymes,bile and bicarbonate (neutralize

acids) enter duodenum through duodenal papilla

� C. Absorption � Peyer’s Patches –

collection of lymphatic tissue that increases toward end of small intestine that prevents absorption of bacteria

� Surface area increased by 3 structures:� Circular folds

(plicae circularis) –deep folds of inner walls

� Villi – fingerlike projections that contain bv & lymphatic duct called the lacteal

� Microvilli – “brush border”; projections of the cell membrane that give a fuzzy appearance

Chemical Influences� Carbohydrates – sugars & starches� Salivary amylase – mouth

� Pancreatic amylase – produced by pancreas & sent to duodeum

� Brush border enzymes – dextrinase, glucoamylase

� Lactase (lactose), maltase (maltose), sucrase (sucrose)

� Produced & act in small intestine

� Proteins� Begins in stomach with pepsin & rennin

(more abundant in children)� Pancreas – trypsin, chymotrypsin,

carboxypeptidase

� Brush border – aminopeptidase, carboxypeptidase & dipeptidase

� Fats� Bile emulsifies fats

� Pancreatic lipases sent to small intestine to complete fat digestion

� Nucleic Acids� Pancreas produces nucleases to

breakdown nucleic acids (DNA, RNA)

� inhibits secretion of gastric juicesChyme from stomachDuodenumGASTRIC INHIBITORY PEPTIDE (GIP)

�Release of pancreatic juices & bile�Relaxes duodenal papillae

Chyme from stomachDuodenumCHOLECYSTOKININ (CCK)

�Release of pancreatic juices & bileChyme from stomachduodenumSECRETIN

�Inhibits secretion of gastric & pancreatic juices�Inhibits emptying of stomach & gall bladder

Food in stomachstomachSOMATOSTATIN

� activates parietal cellsFood in stomachStomachHISTAMINE

� release of gastric juice�Mobility of small intestine�Relaxes ileocecal valve

Food in stomachStomachGASTRIN

ACTIONSTIMULUSSOURCEHORMONE

� 6. Large Intestine

� Extends from ileocecal valve to anus� Absorption of water & elimination of waste

� No villi� Lined with goblet cells that produce mucus for

lubrication

� Outer walls puckered with haustra which aids in contraction

� Cecum – 1st portion ; ileocecal valve that allows indigestible material from sm. Intestine

� Appendix – sac extending from cecum

� Ascending colon –travels up right side of abdominal cavity ; turns at the hepatic flexure

� Transverse colon –travels across the abdomin; turns at the splenic flexture

� Descending colon –travels down left side of abdomen

� Sigmoid colon – S shaped

� Rectum – storage chamber

� Anal sphincter – external & internal; voluntary & involuntary muscles; opens during defecation

ACCESSORY ORGANS

� PANCREAS

� Gland that extends across abdomen from spleen to duodenum

� Located retroperitoneal – behind parietal peritoneum

� Functions: � produces enzymes in a alkaline fluid� Produces insulin ( breaks down glucose) and

glucagon hormone that raises glucose level)

PANCREAS

� LIVER� Largest gland ; 4 lobes� Suspended from diaphragm by falciform

ligament� Produces bile – yellow/green water solution

containing bile salts, bile pigments (bilirubin), cholesterol, phospholipids and electrolytes

� Bile emulsifies fat into small globules� Right & left hepatic ducts collect bile � Fuse into the common hepatic duct

� GALL BLADDER� Green sac within lobes of liver� When not digesting food bile backs up

into the cystic duct & is stored in gall bladder

� Bile becomes concentrated in gall bladder due to water absorption

� Gallstones – results from too much water absorption and cholesterol crystallizes

� Jaundice – results from blockage of common hepatic or bile ducts, hepatitis(inflammation of liver) or cirrhosis(hardening of liver)

� SALIVARY GLANDS� Parotid – lies anterior to ears; largest� Submandibular – below jaw� Sublingual – beneath tongue� Empty secretions in floor of mouth� Saliva – mixture of mucus (moistens &

binds food into bolus), amylase (digest starch) and antibodies (inhibit bacteria)

� TEETH� Massication of food� Deciduous teeth – 1st set; 20� Permanent teeth – 2nd set; 32

� Incisors – front; chisel shaped for cutting

� Canines – corner fangs; puncture & pierce

� Premolars / bicuspids –sides; grinding

� Molars – rear; crush & grind

� Crown – exposed area; above gum

� Root – beneath the gum

� Neck – connect root to crown

� Gingiva – gums

� Enamel – hard outer covering of crown

� Dentin – bone like material under enamel; composes bulk of tooth

� Cementum – glue like substance on outer root surface

� Pulp cavity – contains pulp – blood vessels, nerves & connective tissue – provides nutrients to tooth & sensations

� Root canal – extension of pulp into root

� Peridontal ligament –holds tooth in socket