chapter 23 the digestive system g.r. pitts, ph.d., j.r. schiller, ph.d. and james f. thompson, ph.d....
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Chapter 23The Digestive System
G.R. Pitts, Ph.D., J.R. Schiller, Ph.D. and James F. Thompson, Ph.D.
Use the video clips:CH 23 – Digestive System General Anatomy,
CH 23 – Anatomy of the Mouth and Esophagus, CH 23 – Anatomy of the Stomach, CH 23 – Anatomy of the Pancreas, CH 23 – Anatomy of the Liver, CH 23 – Anatomy of the Small
Intestine and CH 23 – Anatomy of the Large Intestine for a review of digestive system structure
Digestive ProcessesIngestionMovement of foodDigestion
Mechanical digestionChemical digestion
AbsorptionDefecation
General StructureDigestive organs
divided into 2 main groups GI (alimentary)
tract Accessory
structures cheeks, teeth,
tongue, salivary glands
liver, gallbladder, pancreas
Salivary Glands 3 pairs salivary glands
Parotid glands Submandibular glands Sublingual glands
Salivary GlandsComposition of Saliva
99.5% water, 0.5% solutes Na+, K+, Cl-, HCO3
-, and PO4-, proteins,
waste products lysozyme salivary amylase – digests carbohydrates
Saliva composition differs among the 3 glands parotid - watery saliva, amylase submandibular - thicker mucous, amylase sublingual - mostly mucous, a little amylase
Salivary GlandsFunctions of Saliva
Water dissolves food for taste and digestionMucous moistens and lubricates foodMucous lubricates oral surfaces for smooth
actions in swallowing and speechCl- ions activate amylase HCO3
- and PO4- ions buffer bacterial acids
IgA, lysozymes, cyanide, defensins: protect against microorganisms
Salivary GlandsSecretion of Saliva - 1-1.5 L l day
Primarily under nervous controlParasympathetic (ANS) normal salivary
secretions saliva swallowedmost reabsorbed
Sympathetic (ANS) reduced flow (dry mouth)
Food (mechanically, chemically) stimulates salivation behavioral – memories from cortexstarts digestioncontinues after ingestion is completeirritating foods or nausea
Mechanical digestion Chewing = masticationFood mixed with saliva Shaped into a bolus
Chemical digestion – salivary amylase breaks down and converts polysaccharides (starches) to disaccharides (maltose) and monosaccharides (glucose) [no enzymatic action with cellulose which is also a polymer of glucose]
Physiology of Digestion in Mouth
Physiology of Deglutition (Swallowing)
Moving bolus from mouth to stomach Three phases Facilitated by saliva,
mucous secretions Involves mouth,
pharynx, esophagus
1. Buccal phase Voluntary Moves bolus to
oropharynx
Physiology of Deglutition2.Pharyngeal phase
Involuntary
Receptors in oropharynx stimulate medulla and pons to:1.Block mouth with tongue2.Block nasopharynx with
soft palate3.Raise larynx to seal
epiglottis, blocking airways
4.Relax upper esophageal sphincter
Bolus is moved through pharynx into esophagus
Physiology of Deglutition
3. Esophageal stage Upper esophageal
sphincter closes Gastroesopaheal
sphincter opens Esophagus controls
involuntary peristaltic movement
Epiglottis reopens Bolus moves from
esophagus to stomach
EsophagusPeristalsis
Involuntary, rhythmic contraction of muscularis
Controlled by medullary centers
A movement activity: inner circular layer of smooth muscle contracts behind bolus to push it forward; outer longitudinal muscle contracts to pull esophagus wall up
EsophagusPhysiology
Upper esophageal sphincter
Peristalsis Lower esophageal
(“cardiac”) sphincter Sharp transition from
nonkeratinized stratified squamous epithelium to simple columnar epithilium
Esophageal epithelium resistant to abrasion but not to acid and proteolytic enzyme attack – acid reflux disease
StomachPhysiology of digestion - Mechanical digestion
peristaltic movement (mixing waves) back and forth between body and pylorus
3 muscle layers: longitudinal, circular, and oblique chyme
StomachPhysiology of digestion - Chemical
digestionparietal cells secrete intrinsic factor for B12
absorptionparietal cells secrete HCl by active transport
kills microbes, denatures proteinscauses some acid hydrolysis of food moleculesstimulates secretion of hormones for bile &
pancreatic juice flowchief cells secrete pepsinogen (inactive
precursor)activated to pepsin by HCl acid and by other
pepsinsonly an effective protease at acid pHcleaves proteins into smaller peptides
Stomach: MucosaGastric gland chief cells
Secrete pepsinogen (inactive precursor) activated to pepsin by HCl acid
and by other activated pepsin enzymes
only an effective protease at acid pH
cleaves proteins into smaller peptides
Secrete rennin in neonates curdles milk to increase time for
gastric processing
Secrete gastric lipase in neonates splits short chain triglycerides
common in milk limited role in digestion since it
works best at pH 5-6
Stomach: Mucosa Simple columnar epithelium
with goblet cells and gastric pits Secretes 2-3 L l day
Gastric gland parietal cells Secrete intrinsic factor for B12
absorption Secrete HCl by active transport
kills microbes, denatures proteins
causes some acid hydrolysis of food molecules
stimulates secretion of hormones for bile & pancreatic juice flow
Goblet & gastric pit mucous cells secrete mucin 1-3 mm mucus layer in the
stomach prevents self-digestion
Stomach: MucosaGastric gland G cells
(enteroendocrine) Secrete gastrin,
histamine, serotonin, somatostatin
Absorption
Impermeable to diffusion of most molecules into the bloodstream
Absorbs a few lipid soluble compounds: certain drugs (e,g., aspirin) alcohol
Stomach: Regulation of Secretion and Motility
Regulated by combination of neuronal and hormonal factors
3 phases1. Cephalic2. Gastric3. Intestinal
Stomach: Regulation of Secretion and Motility
1.Cephalic phase- Stimuli
- sight
- smell
- taste
- thoughts/memories
- Effect- Parasympathetic
impulses increase gastric secretion
Stomach: Regulation of Secretion and Motility
2.Gastric phase Neural negative feedback
mechanisms
Distension activates stretch receptors causing myenteric and vagovagal reflexes to release Ach
Ach stimulates gastric juice secretion
Chemoreceptors respond to partially digested proteins, caffeine and rising pH
Stimulate gastrin secretion from G cells
Stomach: Regulation of Secretion and Motility
2.Gastric phase (cont.)Gastrin
Inhibited at pH < 2 Gastrin transported in the
blood to the gastric glandsGreatly stimulates HCl
secretionStimulates histamine
secretionSlightly stimulates
pepsinogen secretionContracts lower
esophageal sphincter Increases gastric motilityRelaxes pyloric sphincter
Stomach2. Gastric phase (continued)
Control of HCl secreting parietal cells stimulation by three signal
chemicals gastrin acetylcholine histamine
All three needed for strong H+ secretion
H+ pumps work in conjunction with carbonic anhydrase
blockage of the histamine H2
receptor decreases HCl secretion Tagamet Zantac
HCl
KHCO3
Stomach: Regulation of Secretion and Motility
3.Intestinal phase has excitatory and inhibitory components:
Excitatory Very short phase
Initiated by chyme entry into duodenum
Stretch receptors stimulate release of intestinal (enteric) gastrin
Chemoreceptors detect fatty acids, & glucose in the duodenum Stimulate enteric gastrin
release
Stomach: Regulation of Secretion and Motility3.Intestinal phase (cont.)
InhibitoryEnterogastric reflex: stretch
receptors, chemoreceptors trigger 3 reflexes that1.Inhibit vagoval reflex2.Inhibit myenteric reflex3.Activate sympathetic nervous
system to close pyloric sphincter Inhibit gastric secretion
Enterogastrone secretion Enteroendocrine cells in the small
intestine release:Cholecystokinin (CCK) Gastric inhibitory peptide (GIP) SecretinVasoactive intestinal peptide
(VIP) Hormones inhibit gastric secretion
Stomach: Regulation of Gastric Emptying Food normally passes
through stomach in 4 hours
Hormonal/neuronal reflexes regulate gastric emptying
Large meals and large amounts of liquid increase stomach distension increasing rate of emptying
Stomach emptying inhibited by the enterogastric reflex, enterogastrones, and fat in the duodenum
Stomach
Summary
PancreasPancreatic juice
1.2-1.5 L/day
Mostly water some salts, bicarbonate, enzymesalkaline, pH 7.1-8.2buffers acidic gastric juice, stops pepsin activity,
creates proper alkaline pH for enzymes acting in the intestine
Enzymes include:pancreatic amylasetrypsinogen, chymotrypsinogen,
procarboxypeptidase (inactive zymogens)pancreatic lipaseribonuclease and deoxyribonuclease
Regulation of Pancreatic Secretion Neural control from
parasympathetic division of ANS via vagus nerve
Autoregulation by sensing the presence of fatty acids and amino acids in the acidic chyme
Hormonal control by the secretion of enteroendocrines from duodenum Secretin – stimulates
secretion of water, HCO3-
CCK – stimulates secretion of enzymes
Liver: Blood SupplyTwo sources
Hepatic artery - oxygenated blood from aorta
Hepatic portal vein -deoxygenated blood: absorbed nutrients and toxins
from the stomach and intestines
hormones from the pancreas breakdown products of RBCs
from the spleen
Blood mixes in the sinusoids
Hepatocytes (liver cells) modify and exchange molecules with the blood
LiverNote the
portal triads and
fenestrated capillary sinusoids
hepatic portal venous blood and arterial blood mix in the
sinusoids
Central veins
return blood to
the systemic
circulation via the hepatic
vein and inferior
vena cava
Liver: Bile Secretion Bile from the
hepatocytes enters bile capillaries (canaliculi)
Canaliculi empty into small bile ducts
Hepatic ducts join the cystic duct from the gallbladder to form the common bile duct
Gallbladder stores bile
Common bile duct meets pancreatic duct at the hepatopancreatic ampulla (of Vater)
LiverBile
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 digestive secretion, part excretory productbile salts help in emulsification of ingested fatsbilirubin and other bile pigments are wastes from
lipid catabolism
Liver: Bile Synthesis
Regulation of bile production/secretion nervous control from
parasympathetic division of ANS via vagus nerve
autoregulation by sensing the resence of fatty acids and amino acids in the acidic chyme
hormonal control by the secretion of the enteroendocrines, CCK and secretin, from the duodenum
Cholecystokinin = “gall” “bladder” “moves”
LiverPhysiology of the liver – processes vital to life
Carbohydrate metabolism – regulates blood glucose levelsglycogenesis (insulin)glycogenolysis (glucagon)gluconeogenesis (glucagon)
Lipid metabolism - stores, metabolizes some triglycerides synthesizes new cholesterol degrades excess cholesterol for bile salt production
Protein metabolism -deaminates AA’s by removing amino groups (-NH2) from AA’s
deaminated AA's used for ATP production or changed to carbohydrates or fats as needed
detoxifies ammonia (NH3) by synthesizing urea (1 CO2 + 2 NH3 = urea)
can convert AA's from one to another (transamination) synthesizes and secretes most plasma proteins
LiverPhysiology of the liver - processes vital to life
Storage – oil-soluble vitamins, iron, other nutrients and minerals
Phagocytosis Removal of dietary toxins, hormones, drugs
detoxify or store or secrete compounds into bile metabolize thyroid, steroid hormones
Synthesis of bile salts Excretion of bile - bilirubin Activation of Vitamin D (?)
Pathologies of the liver – hepatitis (viral, toxic), cirrhosis, cancer
Gall BladderPear-shaped
sac, 7-10 cm long
Physiology stores and
concentrates bile between meals
CCK stimulates bile release for fatty meals
when the small intestine is empty, the hepatopancreatic sphincter closes, forcing bile into the gallbladder for storage
Pathology:gallstones
Summary: Digestive Hormonesenteroendocrines
GastrinGastric Inhibitory Peptide
SecretinCholecystokinin
(There are others.)
Small Intestine: Segmentationprimary action of small
intestine when food is present
a form of mechanical digestion
a mixing activity
alternate contraction, relaxation of antagonistic smooth (circular and longitudinal) muscle segments in the intestine
controlled by the autonomic nervous system
Small Intestine: Peristalsisas absorption
continues, distension decreases and true peristalsis starts
a movement which propels chyme onward
these weak movements which occur only after most nutrients have been absorbed
Small Intestine: Motility and Secretion Intestinal secretions
1-2 L/day, pH 7.6 mostly water and mucus bicarbonate buffer neutralizes gastric acid provide enzymes for final chemical digestion
Regulation of intestinal secretion and motility stimulated by distension and acidic chyme local reflexes increase Ach release VIP stimulates production of intestinal secretions basal motility is controlled by autorhythmic
pacemakers local hormones and parasympathetic ANS reflexes
increase motility
Small Intestine: Chemical Digestion Intestinal secretions
1-2 L/day, pH 7.6 mostly water and
mucus along with pancreatic
secretions provide acid neutralization, final chemical digestion, and more water for absorption
Brush border enzymes brush border enzymes
complete digestion of protein and carbohydrate molecules
Small Intestine: Chemical Digestion
Brush border enzymes Enteropeptidase
(enterokinase) converts trypsinogen to trypsinTrypsin activates other
zymogens
Various other brush border enzymes complete digestion of protein and carbohydrate molecules
Small Intestine: Chemical Digestion
Complete digestion is a function of bile, pancreatic secretions and intestinal secretionsAlthough produced by different organs, they
all function in the small intestine
Prior to small intestine, only limited activitymouth – salivary amylase stomach
pepsin lingual lipase
Small Intestine: Chemical Digestion
Chemical digestion in the small intestine: Carbohydrate digestion
pancreatic amylase digests starchesdisaccharidases liberate monosaccharides
Protein digestion pancreatic proteases (trypsin, chymotrypsin,
carboxypeptidase)finished by brush border proteases in the lining epithelium
Lipid digestion bile salts for emulsification pancreatic lipase
Nucleic acid digestion pancreatic ribonuclease and deoxyribonucleasebrush border enzymes digest nucleotides
Small Intestine: AbsorptionAbout 90% of all
absorption takes place in small intestine
Nutrient absorption
Monosaccharides: facilitated diffusion and Na+-driven secondary active transport
Amino acids: primary and Na+-driven secondary active transport
Di- and tripeptides: H+-driven secondary active transport
Nutrients enter capillaries via diffusion, facilitated diffusion, or active transport
Nutrients are transported in the blood to the liver via the hepatic portal circulation
Small Intestine: Nutrient Absorption
Electrolytes (minerals)Na+: Primary active transportK+: facilitated diffusionFe: Active transportCa2+: Active transport, vitamin D is a cofactor
VitaminsWater-soluble vitamins (B complex & C)
absorbed by diffusion - B12 absorbed with intrinsic factor
Fat-soluble vitamins (A, D, E, K) included with other lipids in micelles/chylomicrons
Small Intestine: Nutrient Absorption Lipids (neutral fats,
cholesterol, phospholipids, etc.) are emulsified by bile salts, forming micelles Pancreatic Lipase breaks
triglycerides into 2 fatty acids and 1 monoglyceride
monoglycerides, fatty acids and other lipids diffuse into cells
SER re-synthesizes triglycerides
all lipids packaged into chylomicrons by Golgi apparatus
chylomicrons leave the cell by exocytosis and enter lacteals of the lymphatic system
Small Intestine: Water Absorption Total volume added to the
small intestine/day - 9.3 L ~2.3 L from ingestion ~7.0 L from secretions
Small intestine absorbs ~8.3 L /day passive absorption following
nutrient molecules osmosis
The rest of the water (~1.0L/day) passes to large intestine where most is reabsorbed (~0.9 L/day)
Large Intestine Functions
Completion of absorption, especially final absorption of H2O
Normal flora manufacture certain vitamins (B complex, K)
Formation and expulsion of feces
Anatomy 1.5 m L, 6.5 cm W Divided into 4 general
areas: cecum colon rectum anal canal
Large Intestine: DigestionMechanical digestion
Chyme passage regulated by ileocecal sphinctervalve generally closed - slow passage following a meal gastroileal reflex: ileal motility increases,
sphincter relaxes, chyme moves to the cecumwhen the cecum is full, the sphincter contracts
Colon movements start when chyme passes sphincterhaustral churning
haustra relaxed, distended until full then contract, squeeze contents into next haustrum
peristalsis is slowmass peristalsis (gastrocolic reflex)
during or immediately following a meal, 3-4 times day strong peristaltic waves from middle of transverse colon push contents into the rectum
Large Intestine: DigestionChemical digestion
much mucus but no enzymes are secreted
some digestion of chyme by bacteria in colon
final breakdown of substances, mostly carbohydrates
bacteria produce some vitamins, B complex and K
some bacterial metabolites are toxic, but the liver normally deals successfully with them
Large IntestineAbsorption and feces formation
Chymeafter 3-10 hours in the large intestine, chyme
becomes solidified (due to water reabsorption) into feces
large intestine absorbs water, electrolytes, some vitamins and any toxins
Feces water, inorganic salts, sloughed off intestinal
epithelial cells, bacteria, products of bacterial decomposition, undigested parts of food
most water is reabsorbed in small intestine, but the large intestine is also important in water reabsorption
Large IntestinePhysiology of defecation
Mass peristalsispushes fecal matter into rectumdistension stimulates stretch
receptors initiating reflex for defecation
Parasympathetic ANS stimulated by stretch receptors stimulates contraction of rectum shortens and increases pressure in
rectumparasympathetic stimulation relaxes internal sphincter
Conscious stimulation relaxes external sphincter feces expelled
Large IntestinePathology of the intestines
diarrhea/dysenteryenteritisappendicitisconstipation/fecal impactiondiverticulosisCrohn’s disease/ulcerative colitispolyps/colon cancer
End Chapter 23
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