1. Chapter 23C Digestive System Slides by Barbara Heard and W. Rose.
HESC310
12/5/2018
Chapter 23C
Digestive System
Slides by Barbara Heard and W. Rose.
figures from Marieb & Hoehn 9th ed.
Portions copyright Pearson Education
Axial Skeleton

2. Digestive System Introduction/Overview Functional Anatomy
HESC310
12/5/2018
Digestive System
Introduction/Overview
Functional Anatomy
Physiology of Digestion & Absorption
Chemical Digestion
Absorption
Axial Skeleton

3. Digestion Digestion Enzymes Hydrolysis
Catabolic; macromolecules  monomers small enough for absorption
Enzymes
Intrinsic and accessory gland enzymes break down food
Hydrolysis
Water is added to break bonds
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4. Digestion of Carbohydrates
Only monosaccharides can be absorbed
Monosaccharides absorbed as ingested
Glucose, fructose, galactose
Digestive enzymes
Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)
Break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch
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5. Digestion of Carbohydrates
Starch digestion
Salivary amylase (saliva)  oligosaccharides at pH 6.75 – 7.00
Pancreatic amylase (small intestine)  breaks down any that escaped salivary amylase  oligosaccharides
Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase)  oligosaccharides  monosaccharides
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6. Starch and disaccharides
Carbohydrate digestion
Foodstuff
Enzyme(s)
Site of action
Path of absorption
Starch and disaccharides
Glucose and galactose are
absorbed via cotransport with
sodium ions.
Fructose passes via facilitated
diffusion.
All monosaccharides leave the
epithelial cells via facilitated
diffusion, enter the capillary blood
in the villi, and are transported to
the liver via the hepatic portal vein.
Salivary amylase
Mouth
Pancreatic
amylase
Small
intestine
Oligosaccharides
and disaccharides
Brush border
enzymes in
small intestine
(dextrinase, gluco-
amylase, lactase,
maltase, and sucrase)
Small
intestine
Lactose
Maltose
Sucrose
Galactose
Glucose
Fructose
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7. Digestion of Proteins
Source is dietary, digestive enzymes, mucosal cells; digested to amino acid monomers
Begins with pepsin in stomach at pH 1.5 – 2.5
Inactive in high pH of duodenum
Pancreatic proteases
Trypsin, chymotrypsin, and carboxypeptidase
Brush border enzymes
Aminopeptidases, carboxypeptidases, and dipeptidases
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8. Protein digestion and absorption in the small intestine.
Lumen
of intestine
Amino acids of protein fragments
Brush border enzymes
Pancreatic
proteases
Apical membrane (microvilli)
Na+
1. Pancreatic proteases & brush border proteases break down proteins to AAs.
Absorptive
epithelial
cell
Protein digestion and absorption in the small intestine.
Na+
2. Amino acids absorbed by active transport.
Amino
acid
carrier
3. Amino acids leave by facilitated diffusion, enter capillaries.
Capillary
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9. Protein digestion Foodstuff Enzyme(s) and source Site of action
Path of absorption
Proteins
Amino acids are absorbed via
cotransport with sodium ions.
Some dipeptides and tripeptides
are absorbed via cotransport with
H+ and hydrolyzed to amino acids
within the cells.
Infrequently, transcytosis of small
peptides occurs.
Amino acids leave the epithelial
cells by facilitated diffusion, enter
the capillary blood in the villi, and
are transported to the liver via the
hepatic portal vein.
Stomach
Pepsin
(stomach glands)
in presence
of HCl
Large polypeptides
Pancreatic
enzymes (trypsin,
chymotrypsin,
carboxypeptidase)
Small
intestine
Small polypeptides,
small peptides
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Small
intestine
Amino acids
(some dipeptides
and tripeptides)
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10. Pre-treatment—emulsification by bile salts Enzymes—pancreatic lipases
Digestion of Lipids
Pre-treatment—emulsification by bile salts
Does not break bonds
Enzymes—pancreatic lipases
 Fatty acids and monoglycerides
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11. Emulsification, digestion, and absorption of fats.
Fat globule
Emulsification, digestion, and absorption of fats.
Bile salts
Bile salts in duodenum emulsify large fat globules.
Pancreatic lipase yields free fatty acids and monoglycerides. These associate with bile salts to form micelles.
Fat droplets
coated with
bile salts
Micelles made up of fatty acids,
monoglycerides, and bile salts
Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipids & proteins to form chylomicrons.
Chylomicrons are extruded from epithelial cells by exocytosis. Chylomicrons enter lacteals and are carried away from the intestine in lymph (chyle).
Epithelial
cells of
small
intestine
Lacteal
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12. Fat digestion Foodstuff Enzyme(s Site of action Path of absorption
Unemulsified triglycerides
Fatty acids and monoglycerides
enter the intestinal cells via
diffusion.
Fatty acids and monoglycerides are
recombined to form triglycerides
and then combined with other lipids
and proteins within the cells. The
resulting chylomicrons are
extruded by exocytosis.
The chylomicrons enter the lacteals
of the villi and are transported to
the systemic circulation via the
lymph in the thoracic duct.
Some short-chain fatty acids are
absorbed, move into the capillary
blood in the villi by diffusion, and
are transported to the liver via the
hepatic portal vein.
Lingual lipase
Mouth
Gastric lipase
Stomach
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Small
intestine
Pancreatic
lipases
Small
intestine
Monoglycerides (or diglycerides
with gastric lipase) and fatty acids
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13. Digestion of Nucleic Acids
Enzymes
Pancreatic ribonuclease and deoxyribonuclease  nucleotide monomers
Brush border enzyme nucleosidases and phosphatases  free bases, pentose sugars, phosphate ions
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14. Nucleic acid digestion
Foodstuff
Enzyme(s)
Site of action
Path of absorption
Nucleic acids
Pancreatic ribo-
nuclease and
deoxyribonuclease
Small
intestine
Units enter intestinal cells by active
transport via membrane carriers.
Units are absorbed into capillary
blood in the villi and transported to
the liver via the hepatic portal vein.
Brush border
enzymes
(nucleosidases
and phosphatases)
Small
intestine
Pentose sugars,
N-containing bases,
phosphate ions
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15. ~ All food; 80% electrolytes; most water absorbed in small intestine
Absorption
~ All food; 80% electrolytes; most water absorbed in small intestine
Most prior to ileum
Ileum reclaims bile salts
Most absorbed by active transport  blood
Exception - lipids
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16. Absorption of Carbohydrates
Glucose and galactose
Secondary active transport (cotransport) with Na+  epithelial cells
Move out of epithelial cells by facilitated diffusion  capillary beds in villi
Fructose
Facilitated diffusion to enter and exit cells
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17. Absorption of Carbohydrates
Glucose and galactose
Secondary active transport (cotransport) with Na+  epithelial cells
Move out of epithelial cells by facilitated diffusion  capillary beds in villi
Fructose
Facilitated diffusion to enter and exit cells
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18. Amino acids transported by several types of carriers
Absorption of Protein
Amino acids transported by several types of carriers
Most coupled to active transport of Na+
Dipeptides and tripeptides actively absorbed by H+-dependent cotransport; digested to amino acids within epithelial cells
Enter capillary blood by diffusion
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19. Homeostatic Imbalance
Whole proteins not usually absorbed
Can be taken up by endocytosis/exocytosis
Most common in newborns  food allergies
Usually disappear with mucosa maturation
Allows IgA antibodies in breast milk to reach infant's bloodstream  passive immunity
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20. Absorption of Lipids Absorption of monoglycerides and fatty acids
Cluster with bile salts and lecithin to form micelles
Released by micelles to diffuse into epithelial cells
Combined with lecithin, phospholipids, cholesterol, & coated with proteins to form chylomicrons
Enter lacteals; transported to systemic circulation
Hydrolyzed to free fatty acids and glycerol by lipoprotein lipase of capillary endothelium
Cells can use for energy or stored fat
Absorption of short chain fatty acids
Diffuse into portal blood for distribution
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21. Absorption of Nucleic Acids
Active transport across epithelium  bloodstream
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22. Absorption of Vitamins
In small intestine
Fat-soluble vitamins (A, D, E, and K) carried by micelles; diffuse into absorptive cells
Water-soluble vitamins (vitamin C and B vitamins) absorbed by diffusion or by passive or active transporters.
Vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis
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23. Absorption of Vitamins
In large intestine
Vitamin K and B vitamins from bacterial metabolism are absorbed
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24. Absorption of Electrolytes
Most ions actively along length of small intestine
Iron and calcium are absorbed in duodenum
Na+ coupled with active absorption of glucose and amino acids
Cl– transported actively
K+ diffuses in response to osmotic gradients; lost if poor water absorption
Usually amount in intestine is amount absorbed
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25. Absorption of Electrolytes
Iron and calcium absorption related to need
Ionic iron stored in mucosal cells with ferritin
When needed, transported in blood by transferrin
Ca2+ absorption regulated by vitamin D and parathyroid hormone (PTH)
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26. 9 L water, most from GI tract secretions, enter small intestine
Absorption of Water
9 L water, most from GI tract secretions, enter small intestine
95% absorbed in the small intestine by osmosis
Most of rest absorbed in large intestine
Net osmosis occurs if concentration gradient established by active transport of solutes
Water uptake coupled with solute uptake
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27. Malabsorption of Nutrients
Causes
Anything that interferes with delivery of bile or pancreatic juice
Damaged intestinal mucosa (e.g., bacterial infection; some antibiotics)
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28. Malabsorption of Nutrients
Gluten-sensitive enteropathy (celiac disease)
Immune reaction to gluten
Gluten causes immune cell damage to intestinal villi and brush border
Treated by eliminating gluten from diet (all grains but rice and corn)
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29. Examples of Anatomic Disorders of GI System
Cleft palate and cleft lip
Tracheoesophageal fistula
Opening between esophagus and trachea
Anovaginal fistula
Can occur during vaginal childbirth; rare in developed countries where C sections are used; reatbale with surgery; “modern day lepers”
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30. Genetic Disorder of GI System
Cystic fibrosis
Mutation in “CFTR” gene which codes for a chloride membrane channel protein
Abnormal chloride transport causes thick mucus
Mucus can block pancreatic duct and airways
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31. Stomach and colon cancers rarely have early signs or symptoms
Metastasized colon cancers frequently cause secondary liver cancer
Prevention
Regular dental and medical examination
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