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