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Chapter 23C
Digestive System
Slides by Barbara Heard and W. Rose.
figures from Marieb & Hoehn 9th ed.
Portions copyright Pearson Education
Digestive SystemIntroduction/Overview
Functional Anatomy
Physiology of Digestion & AbsorptionChemical Digestion
Absorption
© 2013 Pearson Education, Inc.
Digestion
• Digestion– Catabolic; macromolecules monomers
small enough for absorption
• Enzymes– Intrinsic and accessory gland enzymes break
down food
• Hydrolysis– Water is added to break bonds
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (1 of 4)
Foodstuff Enzyme(s) and source Site of action Path of absorption
Starch and disaccharides
Oligosaccharidesand disaccharides
Carbohydratedigestion
Lactose Maltose Sucrose
Galactose Glucose Fructose
Salivary amylase
Pancreatic amylase
Brush border enzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)
Mouth
Small intestine
Small intestine
• 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.
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Figure 23.33 Protein digestion and absorption in the small intestine. Slide 1Lumen of intestine
Pancreaticproteases
Amino acids of protein fragments
Brush border enzymes
Na+
Absorptiveepithelialcell
Apical membrane (microvilli)
Aminoacidcarrier
Capillary
Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.
The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side.
The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary viaintercellular clefts.
Na+
1
2
3
© 2013 Pearson Education, Inc.
Figure 23.33 Protein digestion and absorption in the small intestine. Slide 2Lumen of intestine
Pancreaticproteases
Amino acids of protein fragments
Brush border enzymes
Na+
Absorptiveepithelialcell
Apical membrane (microvilli)
Capillary
Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.
Na+
1
© 2013 Pearson Education, Inc.
Figure 23.33 Protein digestion and absorption in the small intestine. Slide 3Lumen of intestine
Pancreaticproteases
Amino acids of protein fragments
Brush border enzymes
Na+
Absorptiveepithelialcell
Apical membrane (microvilli)
Aminoacidcarrier
Capillary
Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.
The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side.
Na+
1
2
© 2013 Pearson Education, Inc.
Figure 23.33 Protein digestion and absorption in the small intestine. Slide 4Lumen of intestine
Pancreaticproteases
Amino acids of protein fragments
Brush border enzymes
Na+
Absorptiveepithelialcell
Apical membrane (microvilli)
Aminoacidcarrier
Capillary
Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxy- peptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase)of mucosal cells.
The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side.
The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary viaintercellular clefts.
Na+
1
2
3
© 2013 Pearson Education, Inc.
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (2 of 4)
Proteindigestion
Proteins
Large polypeptides
Small polypeptides,small peptides
Amino acids(some dipeptidesand tripeptides)
Pepsin (stomach glands)in presence of HCl
Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)
Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)
Stomach
Small intestine
Small intestine
• 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.
Foodstuff Enzyme(s) and source Site of action Path of absorption
© 2013 Pearson Education, Inc.
Digestion of Lipids
• Pre-treatment—emulsification by bile salts– Does not break bonds
• Enzymes—pancreatic lipases Fatty acids and monoglycerides
© 2013 Pearson Education, Inc.
Fat globule
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets).
Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.
Micelles made up of fatty acids,monoglycerides, and bile salts
Bile salts
Fat dropletscoated withbile salts
Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons.
Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph.
Lacteal
Epithelialcells ofsmallintestine
1
2
3
4
Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 1
© 2013 Pearson Education, Inc.
Fat globule
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets).
Bile salts
Fat dropletscoated withbile salts
1
Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 1
© 2013 Pearson Education, Inc.
Fat globule
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets).
Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.
Micelles made up of fatty acids,monoglycerides, and bile salts
Bile salts
Fat dropletscoated withbile salts
1
2
Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 3
© 2013 Pearson Education, Inc.
Fat globule
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets).
Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.
Micelles made up of fatty acids,monoglycerides, and bile salts
Bile salts
Fat dropletscoated withbile salts
Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons.
Lacteal
Epithelialcells ofsmallintestine
1
2
3
Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 4
© 2013 Pearson Education, Inc.
Fat globule
Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets).
Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.
Micelles made up of fatty acids,monoglycerides, and bile salts
Bile salts
Fat dropletscoated withbile salts
Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons.
Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph.
Lacteal
Epithelialcells ofsmallintestine
1
2
3
4
Figure 23.34 Emulsification, digestion, and absorption of fats. Slide 5
© 2013 Pearson Education, Inc.
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (3 of 4)
Fat digestion
Unemulsified triglycerides
Lingual lipase
Gastric lipase
Emulsification by the detergent action of bile salts ductedin from the liver
Pancreatic lipases
Monoglycerides (or diglycerideswith gastric lipase) and fatty acids
Mouth
Stomach
Small intestine
Small intestine
• 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.
Foodstuff Enzyme(s) and source Site of action Path of absorption
© 2013 Pearson Education, Inc.
Digestion of Nucleic Acids
• Enzymes– Pancreatic ribonuclease and
deoxyribonuclease nucleotide monomers– Brush border enzyme nucleosidases and
phosphatases free bases, pentose sugars, phosphate ions
© 2013 Pearson Education, Inc.
Figure 23.32 Flowchart of digestion and absorption of foodstuffs. (4 of 4)
Nucleic aciddigestion
Nucleic acids
Pentose sugars, N-containing bases,
phosphate ions
Pancreatic ribo-nuclease and deoxyribonuclease
Brush borderenzymes(nucleosidasesand phosphatases)
Small intestine
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.
Foodstuff Enzyme(s) and source Site of action Path of absorption
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Absorption of Nucleic Acids
• Absorption– Active transport across epithelium
bloodstream
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Absorption of Vitamins
• In large intestine– Vitamin K and B vitamins from bacterial
metabolism are absorbed
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Malabsorption of Nutrients
• Causes– Anything that interferes with delivery of bile or
pancreatic juice – Damaged intestinal mucosa (e.g., bacterial
infection; some antibiotics)
© 2013 Pearson Education, Inc.
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)
© 2013 Pearson Education, Inc.
Developmental Aspects
• Oral membrane mouth opening
• Cloacal membrane anus
• By week 5 alimentary canal continuous tube from mouth to anus
• Shortly after, accessory organs bud from mucosa
© 2013 Pearson Education, Inc.
Figure 23.36 Embryonic development of the digestive system.
Brain
Oral membrane
Heart
Yolk sac
Cloacalmembrane
Body stalk
Endoderm
Proctodeum
Hindgut
Spinal cord
Midgut
Site of liverdevelopment
Foregut
Stomodeum
Lung bud
Liver Stomach
Bileduct
Cystic duct
Gall-bladder
Ventral pancreatic bud
Dorsalpancreaticbud
Duodenum
© 2013 Pearson Education, Inc.
Homeostatic Imbalance
• Cleft palate and cleft lip
• Tracheoesophageal fistula– Opening between esophagus and trachea
• Cystic fibrosis– Genetic disease thick mucus can block
pancreatic duct
© 2013 Pearson Education, Inc.
Developmental Aspects
• Fetal nutrition via placenta, but GI tract stimulated to mature by amniotic fluid swallowed in utero
• Newborn's rooting reflex helps infant find nipple; sucking reflex aids in swallowing
• Newborns double birth weight in six months; adult diet by 2 years
• Cholecystitis, ulcers – problems of middle age
© 2013 Pearson Education, Inc.
Developmental Aspects
• During old age – GI tract activity declines, less digestive juice,
absorption less efficient, peristalsis slows less frequent bowel movements
– Taste/smell less acute; periodontal disease often develops
– Diverticulosis, fecal incontinence, and cancer of GI tract fairly common
© 2013 Pearson Education, Inc.
Cancer
• Stomach and colon cancers rarely have early signs or symptoms
• Metastasized colon cancers frequently cause secondary liver cancer
• Prevention– Regular dental and medical examination