digestive system ii: digestive activities
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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 - PowerPoint PPT PresentationTRANSCRIPT
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
Salivary Glands Extrinsic salivary glands (parotid,
submandibular, and sublingual)
Intrinsic (buccal) salivary glands are scattered in the oral mucosa
Secretion (saliva)
• Cleanses the mouth
• Moistens and dissolves food chemicals
• Aids in bolus formation
• Contains enzymes that begin the breakdown of starch
Composition
• 97–99.5% water, slightly acidic solution containing:
• Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3
–
• Salivary amylase and lingual lipase; mucin;
• Metabolic wastes—urea and uric acid; lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms
Control of Digestive Activity and Mouth Mostly controlled by reflexes via
the parasympathetic division
Enteric Nervous System
• (ENS) Intrinsic nerve supply of the alimentary canal
o Sympathetic impulses inhibit secretion and motility
o Parasympathetic impulses stimulate
Chemical and mechanical receptors are located in organ walls that trigger reflexes
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
Food Breakdown in the Stomach
Gastric juice is regulated by neural and hormonal factors
Presence of food or falling pH causes the release of gastrin
Gastrin causes stomach glands (chief cells) to produce protein-digesting enzymes (pepsinogen)
Hydrocholoric acid from parietal cells makes the stomach contents very acidic, kills bacteria
Heartburn from Acid Reflux
Acid reflux animation online
Ulcer formation movie online
Chief cellpepsinogen
Digestion and Absorption in the Stomach Protein digestion enzymes
(proteases)
Stomach proteins are exported into the stomach as zymogens (extra long, inactive polypeptide chains)
• Pepsin – an active protein digesting enzyme (converted from pepsinogen through action of HCl and active pepsin)
• Rennin (chymosin)– works on digesting milk casein protein in infants
The only absorption that occurs in the stomach is of alcohol and aspirin
Conversion of pepsinogen to pepsin
The action of chymosin on casein
Figure 23.17
Presence of lowpH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty
Distension;presence offatty, acidic,partiallydigested foodin theduodenum
Briefeffect
Intestinal(enteric)gastrinreleaseto blood
Entero-gastricreflex
Release of intestinalhormones (secretin,cholecystokinin, vasoactiveintestinal peptide)
Localreflexes
Vagalnucleiin medulla
Pyloricsphincter
Stimulate
Inhibit
1
1
2
Stomachsecretoryactivity
Sight and thoughtof food
Stomachdistensionactivatesstretchreceptors
Stimulation oftaste and smellreceptors
Food chemicals(especially peptides and caffeine) and rising pHactivate chemoreceptors
Loss ofappetite,depression
Emotionalupset
Lack ofstimulatoryimpulses toparasym-patheticcenter
Cerebralcortex
Cerebral cortexConditioned reflex
Vagovagalreflexes
Localreflexes
Medulla
G cells
Hypothalamusand medullaoblongata
Vagusnerve
Vagusnerve
Gastrinreleaseto blood
Gastrinsecretiondeclines
G cells
Overridesparasym-patheticcontrols
Sympatheticnervoussystemactivation
1
11
1
2
2
2
Stimulatory events Inhibitory events
Cephalicphase
Gastricphase
Intestinalphase
Excessiveacidity (pH <2) in stomach
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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•
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•
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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.
+
• 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.
Smallintestine
Smallintestine
Stomach
Foodstuff
Protein
Large polypeptides
Pepsin(stomach glands)in presence of HCl
Small polypeptides,small peptides
Pancreaticenzymes (trypsin, chymotrypsin,carboxypeptidase)
Amino acids(some dipeptidesand tripeptides)
Brush border enzymes(aminopeptidase,carboxypeptidase,and dipeptidase)
Path of absorptionEnzyme(s)and source
Site ofaction
Summary of Protein Digestion
Figure 23.34
Epithelialcells ofsmallintestine
Fat dropletscoated withbile salts
Fat globule
Lacteal
Bile salts
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.
Smallintestine
Smallintestine
Foodstuff
Nucleic acids
Pancreatic ribo-nuclease and deoxyribonuclease
Brush borderenzymes(nucleosidasesand phosphatases)
Nucleotides
Path of absorptionEnzyme(s)and source
Site ofaction
Summary of Nucleic Acid Digestion
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
Processes in the Large Intestine No digestive enzymes are
produced
Resident bacteria digest remaining nutrients
• Produce some vitamin K and B
• Release methane and hydrogen sulfide gases
Water and vitamins K and B are absorbed
Remaining materials are eliminated via feces
CH4 H2S
Vitamin KVitamin B
Propulsion in the Large Intestine (Colon) Sluggish peristalsis
Mass movements
• Slow, powerful movements
• (Three to four times per day)
Lack of fiber and weakening of colon walls cause diverticula
Presence of feces in the rectum causes defecation reflex
• Internal anal sphincter is relaxed
• Defecation occurs with relaxation of the voluntary external anal sphincter
Neural control of the rectum and anal sphicters
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