Digestive System II: Digestive Activities Digestive Activities in the Mouth, Swallowing Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion Contributions of the Liver in Digestion Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids Absorption in the Large Intestine and Feces Formation
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Digestive System II: Digestive Activities Digestive Activities in the Mouth, Swallowing Digestive Activities in the Stomach Gastric juice Hormones.
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Digestive System II: Digestive Activities
Digestive Activities in the Mouth, Swallowing
Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and enzyme secretion
Contributions of the Liver in Digestion
Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids
Absorption in the Large Intestine and Feces Formation
Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenum
Regulation of Stomach Activity
Digestive System II: Digestive Activities
Digestive Activities in the Mouth, Swallowing
Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and enzyme secretion
Contributions of the Liver in Digestion
Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids
Absorption in the Large Intestine and Feces Formation
Liver Largest gland in the body
Connected to the gall bladder via the common hepatic duct
Bile Production in the Liver
• Composition
o Bile salts, bile pigment (broken down hemoglobin, cholesterol, phospholipids
Bile is introduced into the duodenum in the presence of fatty food to cause emulsification
Emulsification solubilizes lipids so that they can come in contact with water-soluble lipases and be broken down into fatty acids and glycerol.
Role of the Liver in Metabolism
Produces blood proteins (albumin, clotting proteins) and lipoproteins Degrades hormones
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Figure 23.25a, b
(a) (b)Lobule Central vein Connectivetissue septum
The Liver Processes/Detoxifies Absorbed Food in Lobules
Digestive System II: Digestive Activities
Digestive Activities in the Mouth, Swallowing
Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and enzyme secretion
Contributions of the Liver in Digestion
Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids
Absorption in the Large Intestine and Feces Formation
Digestion in the Small Intestine Enzymes from the brush border
• Break double sugars into simple sugars (carbohydrases)
• Complete some protein digestion (proteases)
Pancreatic enzymes play the major digestive function; usually zymogens
• Help complete digestion of starch (pancreatic amylase)
• Carry out about half of all protein digestion (trypsin, etc.)
• Responsible for fat digestion (lipase)
• Digest nucleic acids (nucleases)
• Bicarbonate ion neutralizes acidic chyme, raises pH to 8
Chemical Digestion : Pancreas
Figure 14.6
Produces a wide spectrum of digestive enzymes that break down all categories of food
Acinar cells produce enzymes that are secreted into the duodenum
Alkaline fluid introduced with enzymes neutralizes acidic chyme
Stimulation of the Release of Pancreatic Juice Vagus nerve: secretion of enzymes,
weak release of bile
Local hormones from duodenal mucosa
• Secretin (enzyme release, bile production, gastrin & stomach activities)
• Cholecystokinin (enzyme and bicarbonate release, bile release)
Liver +
Digestive System II: Digestive Activities
Digestive Activities in the Mouth, Swallowing
Digestive Activities in the Stomach
• Gastric juice
• Hormones that stimulate stomach acid and enzyme secretion
Contributions of the Liver in Digestion
Digestion in the Small Intestine
• Pancreatic juice
• Hormonal stimulation of pancreas
• Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids
Absorption in the Large Intestine and Feces Formation
Other Activities in the Small Intestine Water absorption
Nutrient absorption by mucosal cells via active or passive transport
Lipids are absorbed by diffusion into lacteals
Substances are transported to the liver by the hepatic portal vein or lymph (lacteal)
Figure 23.32 (1 of 4)
Carbohydrate digestion
• 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.
Starch and disaccharides
Oligosaccharidesand disaccharides
Lactose Maltose Sucrose
Glucose Fructose
Salivaryamylase
Mouth
Pancreaticamylase
Brush borderenzymes in small intestine(dextrinase, gluco-amylase, lactase, maltase, and sucrase)
Smallintestine
Smallintestine
Foodstuff
Galactose
Path of absorptionEnzyme(s)and source
Site ofaction
Summary of Carbohydrate Digestion
(monosaccharides)
Figure 23.32 (2 of 4)
Protein digestion
• Amino acids are absorbed by cotransport with sodium ions.• Some dipeptides and tripeptides are absorbed via cotransport with H+
and hydrolyzed to amino acids within the cells.
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• 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.
Micelles made up of fatty acids, monoglycerides,and bile salts
1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.
2 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.
3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.
4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.
Lipid Digestion and Fatty Acid/Glycerol Import
Figure 23.32 (3 of 4)
Fat digestion
Small intestine
Small intestine
Foodstuff
Unemulsifiedfats
Emulsification by the detergent action of bile salts ductedin from the liver
Pancreatic lipases
Monoglyceridesand fatty acids
Glyceroland
fatty acids
Path of absorptionEnzyme(s)and source
Site ofaction
• 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, and 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.
Summary of Lipid Digestion
Figure 23.32 (4 of 4)
Nucleic acid digestion
• 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.