2009 Cengage-Wadsworth Chapter 2 The Digestive System: Mechanism for Nourishing the Body.

2009 Cengage-Wadsworth

Chapter 2

The Digestive System:Mechanism for Nourishing the

Body


The Structures of the Digestive Tract & the Digestive Process

• Main structures: – Oral cavity– Esophagus– Stomach– Small & large

intestines

• Accessory organs: – Pancreas– Liver– Gallbladder


The Structures of the Digestive Tract & the Digestive Process

• Lumen has 4 main layers:– Mucosa

• Epithelium, lamina propria, muscularis mucosa

– Submucosa• Connective tissue, lymphoid tissue, submucosal

plexus

– Muscularis externa• Circular & longitudinal smooth muscle, myenteric

plexus

– Serosa or adventitia• Connective tissue, visceral peritoneum


The Oral Cavity

• Mouth & pharynx• Salivary glands• Enzymes in saliva


The Esophagus

• Moves bolus from oral cavity to stomach

• Swallowing– Voluntary, pharyngeal, esophageal

• Peristalsis• Gastroesophageal sphincter• Selected diseases & conditions of

the esophagus - GERD


The Stomach

• 4 main regions:– Cardia region– Fundus– Body– Antrum or distal pyloric region

• Grinds food & mixes with gastric juices to form chyme

• Strong peristalsis for gastric emptying


The Stomach

• Gastric glands:– Cardiac glands– Oxyntic glands– Pyloric glands

• Cells in glands:– Neck (mucus)

cells– Parietal (oxyntic)

cells– Chief (peptic or

zymogenic) cells– Enteroendocrine

cells


The Stomach

• Gastric juice– HCl

• Converts pepsinogen to pepsin• Denatures proteins• Releases nutrients from organic complexes• Acts as bacteriocide

– Enzymes - pepsin, amylase, lipase– Mucus– Intrinsic factor


The Stomach

• Regulation of gastric secretions– Hormones & peptides that inhibit

gastric secretions– Hormones & neuropeptides that

stimulate gastric secretions – Other factors– HCl release stimulated by gastrin,

acetylcholine, histamine


The Stomach

• Selected diseases & conditions of the stomach - peptic ulcers

• Regulation of gut motility & gastric emptying


The Small Intestine

• Structural aspects of the small intestine– Folds of Kerckring - large circular

folds of mucosa– Villi - finger-like projections– Microvilli - hair-like extensions of the

cells’ plasma membranes– Brush border– Crypts of Lieberkühn


The Small Intestine

• Regulation of intestinal secretions & motility

• Immune system protection of the gastrointestinal tract– Mucosa-associated lymphoid tissue

(MALT)– Gut-associated lymphoid tissue

(GALT)


The Accessory Organs

• The pancreas– Digestive enzymes produced by

acinar exocrine cells– Pancreatic juice contains:

• Bicarbonate• Electrolytes• Pancreatic digestive enzymes

– Regulation of pancreatic secretions– Pancreatic digestive enzymes



• The liver– Consists of lobes made of lobules– Portal circulation– Bile canaliculi



• The gallbladder– Bile synthesis– Selected conditions/diseases of the

gallbladder– Bile storage– The function of bile– The recirculation & excretion of bile– Bile circulation & hypercholesterolemia


The Digestive & Absorptive Processes

• Digestion– Breaking nutrients down– Occurs in lumen & at brush border

• Most digestion & absorption occur in small intestine

• Absorption may be by diffusion, facilitated diffusion, active transport, or pinocytosis/ endocytosis


The Digestive & Absorptive Processes

• Mechanism of absorption depends on:– Solubility (fat vs. water)– Concentration or electrical gradient– Size of molecule

• Unabsorbed materials continue to colon


The Colon or Large Intestine

• Contracts to mix materials• Proximal colonic epithelia absorb

Na, Cl, H2O

• Materials dehydrated• Intestinal bacteria populations of

interest


Intestinal Bacteria (Microflora), Pre- and Probiotics, and Disease

• Fermentation - breakdown of CHO & protein anaerobically

• Generate lactate & short-chain fatty acids, gases

• Probiotics - foods containing live bacterial cultures

• Prebiotics - food ingredients that promote bacterial growth


Intestinal Bacteria (Microflora), Pre- and Probiotics, and Disease

• Probiotics are thought to:– Enhance immunity– Prevent colonization by pathogens– Lower pH of the colon– Transform/promote excretion of toxic

substances– Enhance fecal bulk


Coordination & Regulation of the Digestive Process

• Neural regulation– Myenteric plexus - peristalsis, motility– Submucosal plexus - secretions, local

blood flow

• Regulatory peptides– Hormones– Paracrines


Regulatory Peptides

• Gastrin• Cholecystokinin

(CCK)• Secretin• Motilin• Glucose-

dependent insulinotropic peptide (GIP)

• Peptide YY• Enteroglucagon• Amyline


Regulatory Peptides

• Paracrine-acting substances– Somatostatin– Glucagon-like

peptides– Insulin-like growth

factors

• Neurocrine peptides– Vasoactive

intestinal polypeptide (VIP)

– Gastrin-releasing peptide (GRP)

– Neurotensin– Substance powder

(P)


Perspective 2

An Overview of Selected Digestive System Disorders

with Implications for Nourishing the Body


Selected Disorders

• Disorder 1: Gastroesophageal reflux cisease

• Disorder 2: Inflammatory bowel diseases

• Disorder 3: Celiac disease• Disorder 4: Chronic pancreatitis

2009 Cengage-Wadsworth Chapter 2 The Digestive System: Mechanism for Nourishing the Body.

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