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Chapter 24
The Digestive System
Digestive Processes
 Ingestion
 Movement of food
 Digestion
 Mechanical digestion
 Chemical digestion
 Absorption
 Defecation
General Structure
 Digestive organs divided
into 2 main groups
 GI (alimentary) tract
 Accessory structures
 teeth, tongue, salivary
glands
 liver, gallbladder, pancreas
General Histology of GI Tract
 Same histological organization from inside to outside
and from end to end
Salivary Glands
 3 pairs salivary glands
 Parotid glands
 Submandibular glands
 Sublingual glands
Salivary Glands
 Composition of Saliva
 99.5% water, 0.5% solutes
 Na+, K+, Cl-, HCO3-, and PO4-, proteins, waste
products
 lysozyme
 salivary amylase
 Saliva composition differs from each gland
 parotid - watery saliva, amylase
 submandibular - thicker mucous, amylase
 sublingual - mostly mucous, little amylase
Salivary Glands
 Function of Saliva
 Water dissolves food for taste, digestion
 Mucous lubricates food
 Cl- activates amylase
 HCO3-, and PO4- buffer mouth acids
 Wastes are removed
Salivary Glands
 Secretion of Saliva - 1-1.5 L l day
 Primarily under nervous control
 PNS normal salivary secretions
 saliva swallowed
 most reabsorbed
 SNS, dehydration decrease flow
 Food (mechanically, chemically) stimulates saliva
 behavioral – memories from cortex
 starts digestion
 continues long after eating
 Irritating foods or nausea
Physiology of Digestion in Mouth
 Mechanical digestion
 Chewing, mastication
 Food mixed w/ saliva
 Shaped in bolus
 Chemical digestion
 Salivary amylase
 Polysaccharide (starches) to disaccharides (maltose)
Physiology of Deglutition


Swallowing
 Moving bolus from mouth to stomach
 Facilitated by saliva, mucous
Three phases
1. Voluntary stage - bolus to oropharynx
2. Pharyngeal stage - bolus through pharynx into
esophagus
 involuntary
 receptors in oropharynx stimulate brain to close off
airway
3. Esophageal stage
 upper esophageal sphincter closes
 esophagus controls movement
Stomach
 Mucosa - simple columnar epithelium w/ gastric pits
Stomach
 Mucosa
 Gastric glands w/
secretory cells
 chief cells –
pepsionogen
(pepsin)
 parietal cells – H+
ions
 mucous cells mucous
 G cells - gastrin
 Secretions = 2-3
Ll day
Stomach
 Physiology of digestion - Chemical digestion
 Chief cells secrete pepsinogen
 cleaves proteins into peptides
 only effective at lo pH, activates itself and by acid
 Parietal cells secrete HCl active transport w/ CA
 kills microbes, denatures proteins
 stimulates secretion of hormones for bile, pancreatic juice
 Mucous cells secrete mucous - 1-3 mm mucous
layer in stomach prevents self-digestion
 Gastric lipase
 splits short chain triglycerides
 limited role in digestion since works best at pH 5-6
Stomach
 Absorption
 Impermeable to passage of most materials to blood
 Does absorb:
 water
 electrolytes
 drugs (aspirin)
 alcohol
Stomach

Regulation of gastric secretion
and motility
 Regulated by combination of

nervous and hormonal factors
3 phases of secretion
1. cephalic
2. gastric
3. intestinal
Stomach
1. Cephalic phase
 Stimulus
 smell, taste of food
 thoughts of food
 Response
 cortex and hypothalamus
 parasympathetic impulses
 stimulate juice and peristalsis
Stomach
2. Gastric phase
 Stimulus - Neural
 stretch receptors, chemoreceptors
 distension or increase in pH
stimulate submucosal plexus
 Response
 increased peristalsis
 increase gastric juice production
 As food passed on, pH ,
distension , stimulus
disappears
Stomach
2. Gastric phase (cont.)
 Stimulus - Endocrine
 distension, partly digested proteins,
caffeine stimulate G cells
 gastrin secretion
 inhibited at pH < 2
 stim when pH rises
 Response
 enteroendocrine (look it up!)
 gastrin in blood to gastric glands
 stimulate secretion gastric juices
 contraction of lower esophageal


sphincter
 motility
relax pyloric ileocecal sphincter
Stomach
2. Gastric phase (cont.)
 Control of HCl secreting cells
 stimulated by three chemicals
 gastrin
 acetylcholine
 histamine
 work in conjunction with
carbonic anhydrase
 All three needed for strong
secretion
 presence of one results in
small production of HCl
 blockage of the histamine
receptor decreases secretion –
different from allergic response
 Tagamet
 Zantac
Stomach
3. Intestinal phase
 Stimulus - stretch receptors,
chemoreceptors
 Response
 inhibitory - Start enterogastric reflex
 stretch receptors, chemoreceptors


inhibit gastric emptying
slows chyme exit
ensures proper intestinal
digestion/absorption
 excitatory - Chemoreceptors
 detect FA’s, glucose in small intestine
 stimulate enteroendocrine cells in
small intestine
 cholecystokinin (CCK) - activity
 secretin - secretions
Stomach
 Hormonal/neuronal
reflexes regulate gastric
emptying
 Stomach emptying
inhibited by
enterogastric reflex, CCK
Pancreas
 Accessory organ -
aids small intestine
with food digestion
Pancreas
 Histology - clusters of
glandular epithelial cells
 1% of cells in pancreatic
islets (islets of
Langerhans)
 endocrine pancreas
 glucagon, insulin,
somatostatin, pancreatic
polypeptide
 99% in acini
 exocrine portion of
pancreas
 secrete pancreatic juice
Pancreas
 2 large ducts collect juice
 Larger - pancreatic duct
 joins common bile duct from
liver, gallbladder
 enter duodenum at
hepatopancreatic ampulla with
common duct
 Smaller accessory duct
empties into small intestine
upstream
Pancreas
 Pancreatic juice
 1.2-1.5 L/day
 Mostly water some salts, bicarbonate, enzymes
 alkaline, pH 7.1-8.2
 buffers acidic gastric juice, stops pepsin, creates proper
pH for enzymes in intestine
 Enzymes include
 pancreatic amylase
 trypsin, chymotrypsin, carboxypeptidase - secreted in
inactive protein forms
 pancreatic lipase
 ribonuclease and deoxyribonuclease
Pancreas
 Regulation of
pancreatic secretions
 Nervous mechanisms PNS
 Hormonal mechanisms
 secretin
 CCK
Liver
 Liver
 General
 heaviest body organ, 3
lbs
 2nd largest organ of
body
 Anatomy
 covered in peritoneum
 large R lobe, smaller L
lobe separated by a
ligament
Liver
 Histology
 Hepatic ducts join cystic duct
from gallbladder form common
bile duct
 Gallbladder
 Common bile, pancreatic duct
form hepatopancreatic ampulla
 Empty into duodenum
Liver
 Blood Supply
 Two sources
 hepatic artery- oxygenated
blood from aorta
 hepatic portal vein deoxygenated blood from
intestine w/ newly
absorbed nutrients
 Both carry blood into
sinusoids
 Kupffer's cells phagocytize
microbes, bits of foreign
matter
 nutrients, toxins stored
 Substances from liver
cells secreted into blood
Liver
 Bile
 800-1000 ml/day
 Yellow, brownish, or olive-green liquid
 pH 7.6-8.6, mostly water, bile salts, bile acids,
cholesterol, lecithin (phospholipid), bile pigments,
ions
 Part excretory product, part digestive secretion
 bile salts help in emulsification
 bilirubin
Liver
 Regulation of bile secretion
 Nervous
 Hormonal
Liver
 Physiology of the liver - vital
 Carbohydrate metabolism - blood glucose levels
 glycogenesis
 glycogenolysis
 gluconeogenesis
 Lipid metabolism  stores, metabolizes some triglycerides
 cholesterol for bile salt production
 Protein metabolism
 deamination
 Remove, detoxify NH3 group from AA’s
 AA's used for ATP production or changed to carbos, fats
 synthesizes most plasma proteins
 can convert AA's from one to another (transamination)
Liver
 Physiology of the liver - vital
 Removal of drugs, hormones
 detoxify or secrete into bile several drugs
 alter thyroid, steroid hormones
 Excretion of bile - bilirubin
 Synthesis of bile salts
 Storage - stores vitamins, iron
 Phagocytosis
 Activation of Vitamin D (?)
Gall Bladder
 Pear shaped sac, 710 cm long
 Physiology
 stores, concentrates bile
 CCK stimulates release
 when small intestine
empty sphincter closes
forcing bile into
gallbladder
Summary: Digestive Hormones
 Gastrin
 Secretin
 Cholecystokinin
Small Intestine
 Site of digestion,
absorption
 Anatomy - divided into 3
segments
 duodenum
 jejunum
 Ileum
 Other parts
 Pyloric sphincter
 Ileocecal sphincter
 Plicae circulares (circular
folds)
Small Intestine
Small Intestine
 Intestinal juice




Secreted from epithelium
1-2 L/day, pH 7.6
mostly water and mucous
w/ pancreatic juice provide
neutralization and medium for
absorption
 Brush border enzymes
 enzymes on surface of epithelial cells
 brush border enzymes digest some
chyme
 Carbohydrates
 Peptides
 Nucleotides
 brush border cells rubbed off, break
open releasing enzymes
Small Intestine
 Physiology of digestion in small intestine
 Mechanical digestion
 controlled by autonomic nervous system
 2 phases
 segmentation
 primary action of sm.int. where food is present
 alternate contraction, relaxation of circular muscle around
intestine
 peristalsis
 propels chyme onward
 weak and only after most has been absorbed
Small Intestine
 Physiology of digestion (cont.)
 Chemical digestion
 Complete digestion a function of pancreatic juice, bile,
intestinal juice
 Prior to small intestine
 mouth - amylase
 stomach
 pepsin
 lingual lipase
Small Intestine
 Physiology of digestion (cont.)
 Chemical digestion (cont.)
 In small intestine:
 carbohydrate digestion
 pancreatic amylase
 other disaccharides, monosaccharides broken down at
brush border
 protein digestion
 pancreatic juices
 finished by proteases in brush border membrane
 lipid digestion
 bile salt emulsification
 pancreatic lipase
 nucleic acid digestion
 ribonuclease and deoxyribonuclease in pancreatic juices
 brush border enzymes digest nucleotides
Small Intestine
 Regulation of intestinal secretion and motility
 Local reflexes in response to chyme (Ach)
 Vasoactive Intestinal Peptide (VIP) stimulates
production of intestinal juice
 Segmentation movements initiated by distension
 pacemakers
 local reflexes and PNS reflex return  motility
 w/ absorption,  distension and  peristalsis
Small Intestine
 Physiology of Absorption
 About 90% of all
absorption takes place in
small intestine
 Occurs by
 diffusion
 facilitiated diffusion
 osmosis
 active transport
 secondary active transport
 driven by active transport
Small Intestine
Small Intestine
 Water absorption
 Total volume into small
intestine/day - 9.3 L
 2.3 L from ingestion
 7 L from secretions
 Small intestine absorbs 
8.3 L /day
 passive absorption following
nutrients
 osmosis
 Rest passes to large
intestine where most is
reabsorbed (0.9 L)
Small Intestine
 Vitamin absorption
 Fat-soluble vitamins (A, D, E, K) included w/
micelles
 Water soluble (B, C) absorbed by diffusion - B12
absorbed w/ intrinsic factor
Large Intestine
 Functions
 Completion of
absorption
 Manufacture certain
vitamins
 Formation/expulsion
of feces
Large Intestine
 Physiology of digestion in the large intestine
 Mechanical digestion
 chyme passage regulated by sphincter
 valve generally closed - slow passage
 following meal gastroileal reflex - ileal motility, chyme to
cecum
 when cecum full sphincter contracts
 colon movements start when chyme passes sphincter
 haustral churning
 haustra relaxed, distended until full
 then contract, squeeze contents into next haustra
 peristalsis - slow
 mass peristalsis (gastrocolic reflex)
 during or immediately following a meal, 3-4 times day
 strong peristaltic wave from middle of transverse colon
 pushes contents into rectum
Large Intestine
 Physiology of digestion in the large intestine
 Chemical digestion
 mucous but no enzymes secreted
 digestion by bacteria in colon
 final breakdown of substances, mostly carbohydrates
 produce some vitamins, B and K
Large Intestine
 Absorption and feces formation
 Chyme
 3-10 hours in large intestine becomes solid (due to
water reabsorption) feces
 large intestine absorbs water, electrolytes and some
vitamins
 Feces
 water, inorganic salts, sloughed off epithelial cells,
bacteria, products of bacterial decomposition,
undigested parts of food
 most water reabsorbed in small intestine but large
intestine also important in water reabsorption
Large Intestine
 Physiology of defecation
 Mass peristalsis
 pushes fecal matter into rectum
 distension stimulates stretch receptors initiating reflex
for defecation
 PNS stimulated by stretch receptors
 stimulates contraction of rectum
 shortens and  pressure in it
 PNS stimulation opens internal sphincter
 feces expelled