Chapter 23 The 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
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Chapter 23 The 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.
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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
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
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