Download VIII. Digestion

Digestion: Chapter Goals
After studying this chapter, students should be able to . . .
1. describe the functions of the digestive system and lists its structures and
regions.
2. explain how one-way transport is accomplished in the digestive tract.
3. describe the layers of the gastrointestinal tract and the function(s) of each.
4. describe the structure of the gastric mucosa, list the secretions of the
mucosa and their functions, and identify the cells that produce each of these
secretions.
5. explain the roles of HCl and pepsin in digestion and explain why the stomach
does not normally digest itself.
6. describe the structure and function of the villi and microvilli in the small
intestine.
7. describe the location and functions of the brush border enzymes of the
intestine.
8. explain the electrical activity that occurs in the intestine and describe the
nature of peristalsis and segmentation.
Digestion: Chapter Goals
9. explain how the large intestine absorbs fluid and electrolytes.
10. describe the flow of blood in the liver and explain how the liver
modifies the chemical composition of the blood.
11. describe the composition and functions of bile.
12. explain the exocrine structures of the pancreas and describe
the composition and functions of pancreatic juice.
13. explain how gastric secretion is regulated during the cephalic,
gastric, and intestinal phases.
14. explain how pancreatic juice and bile secretion is regulated by
nerves and hormones.
Digestion: Chapter Goals
15. discuss the nature and action of the different gastrointestinal
hormones.
16. describe the enzymes involved in the digestion of
carbohydrates, lipids, and proteins and explain how
monosaccharides and amino acids are absorbed.
17. describe the roles of bile and pancreatic lipase in fat digestion
and trace the pathways and structures involved in the
absorption of lipids.
Digestion
•
•
•
•
Processes
Anatomy
Movements
Digestive Processes
Processes
1. Ingestion - taking food into the body
2. Mastication - chewing
3. Digestion - chemical breakdown of molecules,
too large to be absorbed, in to smaller
molecules that can be absorbed. Typically
involves a "hydrolysis" reaction.
4. Absorption - Uptake of digestion products
from the intestinal lumen into the bloodstream.
Processes (cont’d)
5. Metabolism - Usage of absorbed products
a. Anabolism - Synthesis of larger molecules
from smaller building blocks
c. Catabolism - Further degradation of small
molecules, usually for energy production.
6. Egestion - elimination of undigested waste
products.
Ingestion/Mastication
Old 16-5
Digestion/Hydrolysis
16-2
Anatomy
• Gross (Macroscopic)
• Histology (Microscopic)
See Table 16-2
16-3
16-22
Movements
•
•
•
•
•
1. Peristalsis
2. Rhythmic segmentation
3. Pendular movement
4. Tonus changes
5. Villar Movement
Peristalsis
16-7
Rhythmic Segmentation
16-20
Pendular Movement
16-9a
16-9b
Movements
1. Peristalsis
2. Rhythmic segmentation
3. Pendular movement
4. Tonus changes
5. Villar Movement
Digestive Processes
•
•
•
•
•
•
Mouth
Pharynx and Esophagus
Stomach
Pancreas and Gall Bladder
Small Intestine
Large Intestine
Mouth
• Anatomy
• Secretions
Anatomy
Old 16-5
Secretions
• Saliva
– Amylase (Starch --> Maltose)
– Mucus
– Lysozyme (Antibacterial)
• Mucus
Table 16-2
Pharynx and Esophagus
16-6a
16-6b
Stomach
• Anatomy
• Hormonal Control
• Secretions
Stomach Anatomy
Stomach
16-8
Hormonal Control of Digestion
TRIGGER: Amino acids from partially digested
proteins; vagus stimulation due to
distension
HORMONE: Gastrin
FROM:
Antral mucosa
ACTS ON: Fundic parietal cells; chief cells
TO:
Secrete HCl; pepsinogen
Table 18.6
16-14
Secretions
• Gastric Juice
– HCl (Activates Pepsinogen, antibacterial, softens cellulose)
– Pepsinogen (activated to Pepsin): proteins --> peptides, amino
acids
– Mucus
Table 16-2
Table 16-4
16-10
16-11
Stomach Protection/Ulcers
16-12
Old 16-17
Pancreas
• Anatomy
• Hormonal Control
• Secretions
Anatomy
16-13
Hormonal Control
16-14
Secretions
• Pancreatic Digestive Enzymes
– Trypsinogen (Activated by enterogastrone to trypsin): Proteins
--> peptides
– Chymotrypsinogen (activated by Trypsin): Proteins -->
peptides
– Procarboxypeptidase (activated by trypsin): Peptides -->
amino acids
– Amylase: Starch --> Maltose
– Lipase: Fats --> fatty acids + glycerol and monoglycerides
• Pancreatic aqueous NaHCO3
Liver and Gall Bladder
• Anatomy
• Hormonal Control
• Secretions
Anatomy
16-13
Hepatic Circulation
16-15
Hormonal Control
• Cholecystokinin --> gall bladder emptying
Secretions
• Bile
– Bile Salts: Emulsifies fats
– Alkaline secretions
– Bilirubin
Circulation of Bile Salts
16-17
Function of Bile Salts
16-19
Small Intestine
• Anatomy
• Hormonal Control
• Secretions
Anatomy
16-21
Secretion
• Succus entericus
– Mucus
– Salt
• Enzymes are not secreted but act within the cells
– Disaccharidases: Disaccharides ---> monosaccharides
– Aminopeptidases: Peptides --> amino acids
16-25
16-26
16-27
16-28
Large Intestine
• Anatomy
• Function
16-30
Functions
• Water Absorption
• Bacterial Action - Produce minor nutrients
(e.g. vitamin K)
– a. Fermentation - action on carbohydrates
– b. Putrification - action on proteins
Chapter Summary
Introduction to the Digestive System
I. The digestion of food molecules involves the hydrolysis of these molecules into their
subunits.
A. The digestion of food occurs in the lumen of the GI tract and is catalyzed by
specific enzymes.
B. The digestion products are absorbed through the intestinal mucosa and enter the
blood or lymph.
II. The layers (tunics) of the GI tract are, from the inside outward, mucosa, submucosa,
muscularis, and serosa.
A. The mucosa consists of a simple columnar epithelium, a layer of connective
tissue called the lamina propria, and a thin layer of smooth muscle called the
muscularis mucosa.
B. The submucosa is composed of connective tissue; the muscularis consists of
layers of smooth muscles; and the serosa is connective tissue covered by the
visceral peritoneum.
C. The submucosa contains the submucosal plexus, and the muscularis contains the
myenteric plexus of autonomic nerves.
Chapter Summary
Esophagus and Stomach
I. Peristaltic waves of contraction push food through the lower esophageal sphincter
into the stomach.
II. The stomach consists of a cardia, fundus, body, and pyloris (antrum). The pylorus
with the pyloric sphincter.
A. The lining of the stomach is thrown into folds, or rugae, and the mucosal surface
forms gastric pits that lead into gastric glands.
B. The parietal cells of the gastric glands secrete HCl; the chief cells secrete
pepsinogen.
C. In the acidic environment of gastric juice, pepsinogen is converted into the
active protein- digesting enzyme called pepsin.
D. Some digestion of protein occurs in the stomach, but the most important
function of the stomach is the secretion of intrinsic factor, which is needed for
the absorption of vitamin B12 in the intestine.
Chapter Summary
Small Intestine
I. The small intestine is divided into the duodenum, jejunum, and ileum. The common
bile duct and pancreatic duct empty into the duodenum.
II. Fingerlike extensions of mucosa called villi project into the lumen, and at the bases
of the villi the mucosa forms narrow pouches called the crypts of Lieberkuhn.
A. New epithelial cells are formed in the crypts.
B. The membrane of intestinal epithelial cells is folded to form microvilli; this is
called the brush border of the mucosa and serves to increase surface area.
III. Digestive enzymes, called brush border enzymes, are located in the membranes of
the microvilli.
IV. The small intestine exhibits two major types of movements, peristalsis and
segmentation.
Chapter Summary
Large Intestine
I. The large intestine is divided into the cecum, colon, rectum, and anal canal.
A. The appendix is attached to the inferior medial margin of the cecum.
B. The colon consists of ascending, transverse, descending, and sigmoid portions.
C. Bulges in the walls of the large intestine are called haustra.
II. The large intestine absorbs water and electrolytes.
A. Although most of the water that enters the GI tract is absorbed in the small
intestine, 1 to 1.5 L/day pass to the large intestine each day. The large intestine
absorbs about 90% of this amount.
B. Na+ is actively absorbed and water follows passively, in a manner analogous to
the reabsorption of NaCl and water in the renal tubules.
III. Defecation occurs when the anal sphincters relax and contraction of other muscles
raises the rectal pressure
Chapter Summary
Liver, Gallbladder, and Pancreas
I. The liver, the largest internal organ, is composed of functional units called lobules.
A. Liver lobules consists of plates of hepatic cells separated by capillary sinusoids.
B. Blood flows from the periphery of each lobule, where the hepatic artery and portal vein
empty, through the sinusoids and out the central vein.
C. Bile flows within the hepatocyte plates, in canaliculi, to the bile ducts.
D. Substances excreted in the bile can be returned to the liver in the hepatic portal blood;
this is called an enterohepatic circulation.
E. Bile consists of a pigment called bilirubin, bile salts, cholesterol, and other molecules.
F. The liver detoxifies the blood by excreting substances in the bile, by phagocytosis, and by
chemical inactivation.
G. The liver modifies the plasma concentrations of proteins, glucose, triglycerides, and
ketone bodies.
Chapter Summary
Liver, Gallbladder, and Pancreas
II. The gallbladder stores and concentrates the bile. It releases bile through the cystic
duct and common bile duct to the duodenum.
III. The pancreas is both an exocrine and endocrine gland.
A. The endocrine portion, known as the islets of Langerhans, secretes the hormones insulin
and glucagon.
B. The exocrine acini of the pancreas produce pancreatic juice, which contains various
digestive enzymes and bicarbonate.
Chapter Summary
Neural and Endocrine Regulation of the Digestive System
I. The regulation of gastric function occurs in three phases.
A. In the cephalic phase, the activity of higher brain centers, acting via the vagus
nerve, stimulates gastric juice secretion.
B. In the gastric phase, the secretion of HCl and pepsin is controlled by the gastric
contents and by the hormone gastrin, secreted by the gastric mucosa.
C. In the intestinal phase, the activity of the stomach is inhibited by neural reflexes
and hormonal secretion from the duodenum.
II. Intestinal function is regulated, in part, by local short reflexes.
A. The submucosal and myenteric plexuses contain autonomic motor neurons,
sensory neurons, and interneurons.
B. Short reflexes include the gastroileal reflex, ileogastric reflex, and intestinointestinal reflexes.
Chapter Summary
Neural and Endocrine Regulation of the Digestive System
III. The secretion of the hormones secretin and cholecystokinin (CCK) regulates
pancreatic juice and bile secretion.
A. Secretin secretion is stimulated by the arrival of acidic chyme into the
duodenum.
B. CCK secretion is stimulated by the presence of fat in the chyme arriving in the
duodenum.
C. Contraction of the gallbladder occurs in response to a neural reflex and to the
secretion of CCK by the duodenum.
IV. Gastrointestinal hormones may be needed for the maintenance of the GI tract and
accessory digestive organs.
Chapter Summary
Digestion and Absorption of Carbohydrates, Lipids, and Proteins
I. The digestion of starch begins in the mouth through the action of salivary amylase.
A. Pancreatic amylase digests starch into disaccharides and short-chain
oligosaccharides.
B. Complete digestion into monosaccharides is accomplished by brush border
enzymes.
II. Protein digestion begins in the stomach through the action of pepsin.
A. Pancreatic juice contains the protein-digesting enzymes trypsin, chymotrypsin,
among others.
B. The brush border contains digestive enzymes that help to complete the digestion
of proteins into amino acids.
C. Amino acids, like monosaccharides, are absorbed and secreted into capillary
blood entering the portal vein.
Chapter Summary
Digestion and Absorption of Carbohydrates, Lipids, and Proteins
III. Lipids are digested in the small intestine after being emulsified by bile salts.
A. Free fatty acids and monoglycerides enter particles called micelles, formed in
large part by bile salts, and they are absorbed in this form or as free molecules.
B. Once inside the mucosal epithelial cells, these subunits are used to resynthesize
triglycerides.
C. Triglycerides in the epithelial cells, together with proteins, form chylomicrons,
which are secreted into the central lacteals of the villi.
D. Chylomicrons are transported by lymph to the thoracic duct and there enter the
blood.