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-Biology 55
Digestive System
DIVISIONS OF THE DIGESTIVE SYSTEM
Gastrointestinal (GI) Tract – path that food follows
mouth, pharynx, esophagus, stomach, small intestine,
large intestine, rectum, anus
Accessory Digestive Organs – salivary glands, liver, gallbladder, pancreas
FUNCTIONS OF THE DIGESTIVE SYSTEM
1) ingestion – eating and drinking
2) digestion – breaking down food into molecules that can be absorbed
3) absorption – passage of nutrient molecules from GI tract into blood or lymph
4) defecation – elimination of feces (undigested or unabsorbed materials)
ANATOMY OF THE DIGESTIVE SYSTEM
Abdominal (peritoneal) Cavity – contains most of GI tract
peritoneum – serous membrane lining cavity, organs, and forming double-layered
mesentery supporting organs and containing vessels and nerves to organs
greater omentum – fold of mesentery overlying intestines
peritonitis – inflammation of peritoneum
Layers of the GI Tract
mucosa – mucous membrane lining inner lumen (central cavity)
stratified squamous epithelium – protects mouth, pharynx, esophagus, and
anal canal from abrasion
simple columnar epithelium – secretes and absorbs materials in the stomach
and small and large intestines
submucosa – connective tissue containing blood and lymphatic vessels and nerves
muscularis – layers of smooth muscle that move contents
serosa – visceral peritoneum covering organs in abdominal cavity
MOUTH (ORAL CAVITY) – ingests food, chews it, and mixes it with secretions to
form a bolus (moist ball of food)
Hard and Soft Palates – separate mouth from nasal cavity
hard palate– maxillae and palatine bones
soft palate – muscle that closes nasal cavity when swallowing
Lips, Cheeks, and Tongue – skeletal muscle structures that manipulate food to
hold it in the mouth and push it between the teeth to be chewed
tongue also has taste buds to determine palatability of food
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Salivary Glands – secrete saliva that lubricates and chemically digests food
mucus – lubricates mouth and food
salivary amylase – digests starch
lysozyme – kills bacteria
Teeth – cut and grind food into smaller particles (mastication)
crown – visible portion above gumline
enamel – extremely hard, calcium material protecting crown
root – portion embedded in alveolus of maxilla or mandible
periodontal ligament – fibrous CT that anchors tooth
gingiva (gums) – mucosa attached to and protecting root of tooth
pulp – connective tissue with blood vessels and nerves in center of tooth
Deciduous Teeth – 20 baby teeth that fall out and are replaced
Permanent Teeth – 32 adult teeth
Types of Teeth:
1) incisors – chisel-shaped front teeth that cut food
2) cuspids (canines) – pointed teeth that tear food
3) bicuspids (premolars) – flat teeth that grind food
4) molars – flat, back teeth that crush and grind food
caries (cavities) – holes in teeth caused by acids from foods and bacteria
PHARYNX (throat) – muscular tube that carries food from mouth to esophagus
(also carries air from nasal cavity or mouth to larynx)
Swallowing – movement of food from mouth to stomach
voluntary phase – tongue pushes bolus back into pharynx
tongue moves up and back pushing bolus into pharynx
swallowing reflex – initiated by touch sensations in pharynx
soft palate closes nasal cavity (keeps food out of nose)
epiglottis closes larynx (keeps food out of lungs)
respiratory center inhibited (breathing stops)
esophagus opens to allow entry of bolus
ESOPHAGUS – muscular tube that carries food from pharynx to stomach
peristalsis – waves of contraction and relaxation of circular and longitudinal
muscles move bolus down esophagus
esophageal sphincter – circular muscle that opens so food can enter stomach,
then closes again to prevent gastroesophageal reflux
heartburn – acid from stomach damages esophageal mucosa
STOMACH (gastric/gastro-) – muscular and glandular sac that stores food and mixes it
with gastric secretions to form a soupy mixture called chyme
hydrochloric acid (HCl) – kills bacteria, denatures proteins, activates enzymes
pepsinogen (active form – pepsin) – digests proteins
intrinsic factor – needed to absorb vitamin B12
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mucus – protects mucosa of stomach from HCl
ulcers – erosions in stomach lining due to inadequate protective mucus
Anatomy of Stomach
1) cardia – superior portion near esophageal sphincter
2) fundus – rounded portion that stretches to store large meals
3) body – main central portion that churns food and gastric juice
4) pylorus – narrowed part attached to intestine
pyloric sphincter – circular muscle that opens to allow chyme to
pass into intestine
rugae – folds in mucosa of empty stomach; disappear as stomach fills
SMALL INTESTINE – muscular and glandular tube that is the main site of chemical
digestion and absorption of nutrients
Anatomy of Small Intestine
1) duodenum – first portion that is attached to stomach
2) jejunum – middle portion
3) ileum – last portion that is attached to large intestine
ileocecal valve – opening into cecum; regulated by a sphincter
structures that increase surface area for digestion and absorption
circular folds in mucosa and submucosa
villi – finger-like projections of mucosa
contain many capillaries and lacteals (lymphatic capillaries)
microvilli – finger-like projections of epithelial cell membrane
brush border (fuzzy appearance) with attached enzymes
Peyer’s patches – large lymphatic nodules in ileum
Functions of Small Intestine
alkaline mucus – neutralizes HCl from stomach
brush border enzymes – digest carbohydrates, proteins, nucleic acids
duodenum receives pancreatic enzymes (from pancreas)
digest carbohydrates, proteins, lipids, nucleic acids
duodenum receives bile (from liver) – emulsifies fats
segmentation – contractions that mix intestinal contents
absorption
into blood capillaries
monosaccharides (carbohydrates)
amino acids (proteins)
water and ions
into lymphatic lacteals
fatty acids, glycerol (lipids)
peristalsis – slow waves move nondigested contents to large intestine
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LARGE INTESTINE – large tube composed of sacs called haustra that functions
mainly in absorbing water and forming indigestible materials into feces
bacterial flora – normal, symbiotic bacteria living in GI tract
have enzymes that digest materials our enzymes cannot
produces gases and odor
produce vitamins – K, B vitamins
absorption
water, vitamins (K,B), bile salts
mass movement – rapid contraction drives undigested materials into rectum
feces – bacteria, unabsorbed or indigestible materials (fiber)
Anatomy of the Large Intestine
1) cecum – blind pouch attached to ileum
appendix – finger-like extension containing lymphoid nodules
2) ascending colon – right abdomen
3) transverse colon – crosses from right to left abdomen
4) descending colon – left abdomen
5) sigmoid colon – S-shaped entrance to rectum
6) rectum – expands to store feces
7) anus – exit for feces
internal sphincter – involuntary (smooth muscle)
external sphincter – voluntary (skeletal muscle)
PANCREAS
endocrine function – pancreatic islets regulate blood glucose level
exocrine function – secretes pancreatic juice into small intestine
pancreatic duct – carries pancreatic juice to duodenum
sodium bicarbonate – helps neutralize HCl from stomach
pancreatic enzymes (many)
pancreatic amylase – digests carbohydrates
trypsin, chymotrypsin – digest proteins
lipase – digests lipids
nuclease – digests nucleic acids
LIVER
hepatic portal system carries blood and absorbed materials from GI tract to
liver for processing before they enter general circulation
liver regulates composition of the blood
Anatomy of Liver – composed of lobes, divided into lobules (groups of liver cells)
Portal Triad – 3 structures next to lobules
hepatic artery – supplies oxygenated blood to liver cells
bile duct – collects bile secreted by liver cells
hepatic portal vein – carries venous blood from GI tract to liver cells
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Functions of Liver
1) maintains blood glucose level
glucose stored as glycogen after meals
glycogen stores broken down to glucose between meals
2) breaks down amino acids
ammonia (toxic by-product) converted to urea
3) synthesizes plasma proteins
4) removes or breaks down many toxins in blood
5) produces and secretes bile
bile pigments (yellow-green) derived from bilirubin
bile salts – formed from cholesterol
emulsify fats in small intestine – breaks hydrophobic clumps into
smaller droplets that can be digested by lipase and absorbed
6) stores iron and vitamins (A,D,E,K)
7) phagocytosis and antigen presentation
Kupffer cells – macrophages that remove old blood cells, debris, microbes
GALLBLADDER – pear-shaped sac that stores bile produced by liver
common bile duct – empties bile into duodenum
REGULATION OF DIGESTIVE ACTIVITY
cephalic phase (head) – taste, smell, sight, thought of food
causes parasympathetic stimulation which increases digestive secretions
and smooth muscle contractions(motility)
gastric phase – food entering stomach
stretch receptors cause nervous reflexes that increase activity of stomach
and small intestine
nutrients in stomach cause secretion of gastrin (hormone from stomach)
which increases activity of the stomach
intestinal phase – chyme entering duodenum
chyme in duodenum cause secretion of hormones
secretin – stimulates liver and pancreas functions
cholecystokinin (CCK) – stimulates pancreas and gallbladder
gastrocolic reflex – stretch of stomach and duodenum causes reflex mass
movement in large intestine
defecation reflex – triggered by stretch receptors in wall of rectum
peristalsis in rectum and relaxation of internal sphincter
conscious control over external sphincter (skeletal muscle)
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