Download Chapter 24: The Digestive System

Chapter 22: The Digestive System
Objectives:
1. List and define briefly the major processes occurring during digestive system activity.
2. Describe the tissue composition and the general function of each of the four layers of the
alimentary tube.
3. Describe the anatomy and basic function of each organ and accessory organ of the alimentary
canal.
4. Describe the composition and functions of saliva, and explain how salivation is regulated.
5. Describe the mechanisms of chewing and swallowing.
6. Identify structural modifications of the wall of the stomach and small intestine that enhance the
digestive process in these regions.
7. Describe the composition of gastric juice, name the cell types responsible for secreting its
various components, and indicate the importance of each component in stomach activity.
8. Explain how gastric secretion and motility in the stomach are regulated.
9. Describe the function of local hormones produced by the small intestine.
10. State the roles of bile and of pancreatic juice in digestion.
11. Describe how entry of pancreatic juice and bile into the small intestine is regulated.
12. List the major functions of the large intestine, and describe the regulation of defecation.
13. List the enzymes involved in chemical digestion; name the foodstuffs on which they act and
then end products of protein, fat, carbohydrate, and nucleic acid digestion.
14. Describe the process of absorption of digested foodstuff that occurs in the small intestine.
Processes of the digestive system See figure 22.2
1.
2.
3.
4.
5.
6.
Ingestion: taking food into digestive tract
Propulsion: moving food through – swallowing and peristalsis
Mechanical digestion: chewing, churning, segmentation See figure 22.3
Chemical digestion
Absorption
Defecation
Microscopic anatomy of alimentary canal See figure 22.6
1. Mucosa or mucous membrane: secretion, absorption, protection, made of simple columnar
epithelium
2. Submucosa: dense connective tissue w/ blood and lymphatic vessels, lymph nodules, and nerves
3. Muscularis: inner circular layer and outer longitudinal layer of smooth muscle
4. Serosa: visceral peritoneum of areolar CT covered w/ simple squamous epithelium
Anatomy of the stomach
1. Layers See figure 22.14
a. Serosa: outer layer
b. Muscularis layer has 3 layers of smooth muscle: circular, longitudinal, and oblique to churn
stomach for mechanical digestion
c. Submucosa
d. Mucosa: made of simple columnar epithelium of goblet cells which secrete alkaline mucus. It is
dotted with gastric pits which lead to gastric glands that produce gastric juice (mucus, gastrin)
See figure 22.15
2. Other secretory cells See figure 22. 15
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Mucous neck cells produce acidic mucus
Parietal cells make HCl (pH 1.5-3.5)and intrinsic factor for absorption of vitamin B12 in sm.
intestine
Chief cells make pepsinogen (positive feedback)
Enteroendocrine cells make gastrin, and other hormones
3. Mucosal barrier: made of bicarbonate rich mucus, tight junctions of epithelial cells, cell membranes
of glandular cells impermeable to HCl, surface epithelium renewed every 3-6 days.
a. inflammation: gastritis occurs when mucosal barrier is breached. Gastric ulcers occur due to
hypersecretion of HCl, hyposecretion of mucus, and presence of Helicobacter pylori.
4. Other information: A bolus enters the stomach and leaves as acid chyme. Protein digestion w/
pepsin occurs in stomach. In children, the stomach also secretes rennin for milk protein digestion.
Aspirin and alcohol can pass through stomach mucosa into blood.
Regulation of gastric secretion: 3 L daily See figure 22.16
1. Nervous: vagus and enteric nerves. Parasympathetic (through vagus) increases secretion,
sympathetic decreases secretion.
2. Hormonal: gastrin stimulates HCl and enzymes; small intestine hormones are gastrin antagonists
See table 22.1
3. Phases
a. cephalic: (conditioned reflex) before food enters. Aroma, taste, sight, or thought of food.
Signals go to hypothalamus, stimulates vagal nuclei that in turn signal stomach glands)
b. gastric: once food reaches stomach. Stimuli are distention, peptides, low acidity of stomach.
Peptides, caffeine and increasing pH activate gastrin secretion. Then more HCl is released.
c. intestinal: both excitatory and inhibitory. Excitatory: food goes into duodenum and gastrin is
released that stimulates gastric glands. Inhibitory: occurs as intestine distends and substances in chime
trigger enterogastric reflex
Enterogastric reflex: inhibits vagal nuclei, activates sympathetic fibers to tighten pyloric sphincter.
Gastric secretion decreases, hormones (secretin, CCK, VIP, GIP) all inhibit gastric secretion.
Microscopic Anatomy of Small Intestines: adapted for nutrient absorption See figure 22.21,
22.22
 Plicae circularis, villi, macrovilli, all increase surface area
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
Villi contain capillary bed and lacteal for absorption. They have a brush border of microvilli
w/brush border enzymes for final stages of digestion
Made of absorptive cells, goblet cells, lymphocytes, enteroendocrine cells
Microscopic Anatomy of Liver See figure 22.24
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Hepatocytes filter and process nutrient rich blood. Leaky capillaries (sinusoid) to allow blood
through. Macrophages (Kupffer cells) in liver help protect.
Hepatocytes produce bile, process nutrients (glucose), store fat soluble vitamins (ADE) and
detoxify blood.
Composition of bile:
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yellow green, alkaline w/ bile salts, pigments (bilirubin from RBC’s) cholesterol, phospholipids,
electrolytes.
Bile salts emulsify fats and also facilitate fat and cholesterol absorption. Bile salts are recycled in
the large intestine and the rest is eliminated in feces (give them their color). Fatty chyme
stimulates the release of bile and increases bile output.
Bile Release See figure 22.25
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Stored in gall bladder when not needed for digestion.
Gall bladder and sphincter of Oddi are stimulated when CCK (cholecystikinin) is released during the
intestinal phase.
Bile is released when acidic, fatty chime enters duodenum. CCK also stimulates secretion of
pancreatic juice.
Pancreas See figure 22.26

Produces pancreatic juice of water, enzymes, electrolytes. Clusters of acinar cells produce
enzymes. Epithelial cells release HCO 3- that neutralizes acid chime and provides optimal
environment for intestinal and pancreatic enzymes.
Regulation of pancreatic secretion See figure 22.28
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Secretin released from enteroendocrine cells of duodenum stimulates release of bicarbonate-rich
pancreatic juice to neutralize HCl.
CCK is released in response to entry of fats and protein into duodenum. Stimulates release of
enzyme-rich pancreatic juice.
Large intestine See figure 22.29, 22.31
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Responsible for the recovery of water and electrolytes from remaining materials. 90% of water has
already been absorbed, but considerable water and electrolytes remain (Na and Cl) and must be
recovered.
Forms and stores feces. The remaining material is dehydrated, mixed with bacteria and mucus and
formed into feces.
Microbes in the large intestine produce enzymes capable of digesting many molecules that are
indigestible to us, mainly cellulose. As they digest carbohydrates, intestinal gas develops. Vitamin
K is synthesized by colonic bacteria.
Chemical Digestion: food broken down into monomers by hydrolysis See figure 22.23
1. Carbohydrates: monosaccharides (glucose, fructose, galactose) produces by salivary amylase
(mouth), pancreatic amylase (pancreas into duodenum), and intestinal brush border enzymes
(duodenum).
2. Proteins: polypeptides are produed from protein by pepsin in stomach (in presence of acidic
pH). Trypsin, chymotrypsin, and carboxypeptidase from pancreas (activated in duodenum at alkaline or
neutral pH) produce small peptides. Brush border peptidases finish digestion into amino acids.
3. Lipids: small intestine is the sole site for digestion. Bile from liver emulsifies while lipases from
pancreas break it up into glycerol and fatty acids. See figure 22.35
4. Nucleic acids: pancreatic nucleases and brush border enzymes digest to nucleotide
components.
Absorption: 10 L enter but 1 L or less reaches the large intestine. All food, 80% of electrolytes, and
most water is absorbed in the small intestine. Most absorption is completed by the time it gets to the
ileum. Bile salts are absorbed by the ileum. Most nutrients are absorbed through mucosa by cotransport
and active transport. Bile salts and lecithin w/ lipid byproducts form micelles for absorption into the
lacteals.
Electrolytes: sodium is absorbed with glucose and amino acids. Chloride ions are absorbed by active
transport and exchanged with bicarbonate ions. Potassium is absorbed as water is absorbed. Anything
interfering with water absorption will affect potassium absorption. Also, as diarrhea occurs, it pulls
potassium into lumen of intestine. Calcium absorption is regulated by the active form of vitamin D.
Vitamins A,D,E, and K need to be dissolved in fat-containing food for absorption. Vitamins B and C are
water soluble, except Vit B12 needs intrinsic factor that is made by the stomach.