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Chapter 23

Chapter 23

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 Intestineand CH 23 – Anatomy of the Large Intestine for a review of digestive system structure

The Digestive System

G.R. Pitts, Ph.D., J.R. Schiller, Ph.D. and

James F. Thompson, Ph.D.

Digestive processes
Digestive Processes

  • Ingestion

  • Movement of food

  • Digestion

    • Mechanical digestion

    • Chemical digestion

  • Absorption

  • Defecation

General structure
General Structure

  • Digestive organs divided into 2 main groups

    • GI (alimentary) tract

    • Accessory structures

      • cheeks, teeth, tongue, salivary glands

      • liver, gallbladder, pancreas

Salivary glands
Salivary Glands

  • 3 pairs salivary glands

    • Parotid glands

    • Submandibular glands

    • Sublingual glands

Salivary glands1
Salivary Glands

  • Composition 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 glands2
Salivary Glands

  • Functions of Saliva

    • Water dissolves food for taste and digestion

    • Mucous moistens and lubricates food

    • Mucous lubricates oral surfaces for smooth actions in swallowing and speech

    • Cl- ions activate amylase

    • HCO3- and PO4- ions buffer bacterial acids

    • IgA, lysozymes, cyanide, defensins: protect against microorganisms

Salivary glands3
Salivary Glands

  • Secretion of Saliva - 1-1.5 L l day

    • Primarily under nervous control

      • Parasympathetic (ANS)  normal salivary secretions

        • saliva swallowed

        • most reabsorbed

      • Sympathetic (ANS)  reduced flow (dry mouth)

    • Food (mechanically, chemically) stimulates salivation

      • behavioral – memories from cortex

      • starts digestion

      • continues after ingestion is complete

      • irritating foods or nausea

Physiology of digestion in mouth
Physiology of Digestion in Mouth

  • Mechanical digestion

    • Chewing = mastication

    • Food 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 deglutition swallowing
Physiology of Deglutition (Swallowing)

  • Moving bolus from mouth to stomach

    • Three phases

    • Facilitated by saliva, mucous secretions

    • Involves mouth, pharynx, esophagus

  • Buccal phase

    • Voluntary

    • Moves bolus to oropharynx

Physiology of deglutition
Physiology of Deglutition

  • Pharyngeal phase

    • Involuntary

    • Receptors in oropharynx stimulate medulla and pons to:

      • Block mouth with tongue

      • Block nasopharynx with soft palate

      • Raise larynx to seal epiglottis, blocking airways

      • Relax upper esophageal sphincter

    • Bolus is moved through pharynx into esophagus

Physiology of deglutition1
Physiology of Deglutition

  • Esophageal stage

    • Upper esophageal sphincter closes

    • Gastroesopaheal sphincter opens

    • Esophagus controls involuntary peristaltic movement

    • Epiglottis reopens

    • Bolus moves from esophagus to stomach


  • Peristalsis

    • 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


  • Physiology

    • 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


  • Physiology of digestion - Mechanical digestion

    • peristaltic movement (mixing waves) back and forth between body and pylorus

    • 3 muscle layers: longitudinal, circular, and oblique

    • chyme


  • Physiology of digestion - Chemical digestion

    • parietal cells secrete intrinsic factor for B12 absorption

    • parietal cells 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

    • chief cells secrete pepsinogen (inactive precursor)

      • activated to pepsin by HCl acid and by other pepsins

      • only an effective protease at acid pH

      • cleaves proteins into smaller peptides

Stomach mucosa
Stomach: Mucosa

  • Gastric 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 mucosa1
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 mucosa2
Stomach: Mucosa

  • Gastric 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
Stomach: Regulation of Secretion and Motility

  • Regulated by combination of neuronal and hormonal factors

  • 3 phases

    • Cephalic

    • Gastric

    • Intestinal

Stomach regulation of secretion and motility1
Stomach: Regulation of Secretion and Motility

  • Cephalic phase

    • Stimuli

      • sight

      • smell

      • taste

      • thoughts/memories

    • Effect

      • Parasympathetic impulses increase gastric secretion

Stomach regulation of secretion and motility2
Stomach: Regulation of Secretion and Motility

  • 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 motility3
Stomach: Regulation of Secretion and Motility

  • Gastric phase (cont.)

    • Gastrin

      • Inhibited at pH < 2

      • Gastrin transported in the blood to the gastric glands

      • Greatly stimulates HCl secretion

      • Stimulates histamine secretion

      • Slightly stimulates pepsinogen secretion

      • Contracts lower esophageal sphincter

      • Increases gastric motility

      • Relaxes pyloric sphincter



  • 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


Stomach regulation of secretion and motility4
Stomach: Regulation of Secretion and Motility

  • 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 motility5
Stomach: Regulation of Secretion and Motility

  • Intestinal phase (cont.)

    • Inhibitory

      • Enterogastric reflex: stretch receptors, chemoreceptors trigger 3 reflexes that

        • Inhibit vagoval reflex

        • Inhibit myenteric reflex

        • 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)

          • Secretin

          • Vasoactive intestinal peptide (VIP)

        • Hormones inhibit gastric secretion

Stomach regulation of gastric emptying
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




  • 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 activity, creates proper alkaline pH for enzymes acting in the intestine

    • Enzymes include:

      • pancreatic amylase

      • trypsinogen, chymotrypsinogen, procarboxypeptidase (inactive zymogens)

      • pancreatic lipase

      • ribonuclease and deoxyribonuclease

Regulation of pancreatic secretion
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 supply
Liver: Blood Supply

  • Two 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


Central veins return blood to the systemic circulation via the hepatic vein and inferior vena cava

Note the portal triads and fenestrated capillary sinusoids

hepatic portal venous blood and arterial blood mix in the sinusoids

Liver bile secretion
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)


  • 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 digestive secretion, part excretory product

      • bile salts help in emulsification of ingested fats

      • bilirubin and other bile pigments are wastes from lipid catabolism

Liver bile synthesis
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”


  • Physiology of the liver – processes vital to life

    • Carbohydrate metabolism – regulates blood glucose levels

      • glycogenesis (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


  • Physiology 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 bladder
Gall Bladder

  • Pear-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



Summary digestive hormones enteroendocrines
Summary: Digestive Hormonesenteroendocrines

  • Gastrin

  • Gastric Inhibitory Peptide

  • Secretin

  • Cholecystokinin

  • (There are others.)

Small intestine segmentation
Small Intestine: Segmentation

  • primary 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 peristalsis
Small Intestine: Peristalsis

  • as 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
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
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 digestion1
Small Intestine: Chemical Digestion

  • Brush border enzymes

    • Enteropeptidase (enterokinase) converts trypsinogen to trypsin

      • Trypsin activates other zymogens

    • Various other brush border enzymes complete digestion of protein and carbohydrate molecules

Small intestine chemical digestion2
Small Intestine: Chemical Digestion

  • Complete digestion is a function of bile, pancreatic secretions and intestinal secretions

    • Although produced by different organs, they all function in the small intestine

  • Prior to small intestine, only limited activity

    • mouth – salivary amylase

    • stomach

      • pepsin

      • lingual lipase

Small intestine chemical digestion3
Small Intestine: Chemical Digestion

  • Chemical digestion in the small intestine:

    • Carbohydrate digestion

      • pancreatic amylase digests starches

      • disaccharidases 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 deoxyribonuclease

      • brush border enzymes digest nucleotides

Small intestine absorption
Small Intestine: Absorption

  • About 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
Small Intestine: Nutrient Absorption

  • Electrolytes (minerals)

    • Na+: Primary active transport

    • K+: facilitated diffusion

    • Fe: Active transport

    • Ca2+: Active transport, vitamin D is a cofactor

  • Vitamins

    • Water-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 absorption1
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
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
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 digestion
Large Intestine: Digestion

  • Mechanical digestion

    • Chyme passage regulated by ileocecal sphincter

      • valve generally closed - slow passage

      • following a meal  gastroileal reflex: ileal motility increases, sphincter relaxes, chyme moves to the cecum

      • when the cecum is full, the sphincter contracts

    • Colon movements start when chyme passes sphincter

      • haustral churning

        • haustra relaxed, distended until full

        • then contract, squeeze contents into next haustrum

      • peristalsis is slow

      • mass 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 digestion1
Large Intestine: Digestion

  • Chemical 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 intestine1
Large Intestine

  • Absorption and feces formation

    • Chyme

      • after 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 intestine2
Large Intestine

  • Physiology of defecation

    • Mass peristalsis

      • pushes fecal matter into rectum

      • distension stimulates stretch

        receptors initiating reflex for


    • Parasympathetic ANS

      stimulated by stretch receptors

      • stimulates contraction of rectum

      • shortens and increases pressure in


      • parasympathetic stimulation relaxes internal sphincter

    • Conscious stimulation relaxes external sphincter

      • feces expelled

Large intestine3
Large Intestine

  • Pathology of the intestines

    • diarrhea/dysentery

    • enteritis

    • appendicitis

    • constipation/fecal impaction

    • diverticulosis

    • Crohn’s disease/ulcerative colitis

    • polyps/colon cancer