Functions of the GI Tract
Motility:
ƒ
Chapter 18
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Movement of of food through the GI tract.
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Ingestion:
„
Mastication:
„
Deglutition:
„
Peristalsis:
„
The Digestive System
„
„
„
Functions of the GI Tract
„
„
„
„
Exocrine:
„
HCl, H20, HC03-, bile, lipase, pepsin, amylase, trypsin,
elastase, and histamine are secreted into the lumen of the
GI tract.
Stomach and small intestine secrete hormones to help
regulate the GI system.
„
„
GI tract
divided into:
„
„
„
Alimentary
canal.
Accessory
digestive
organs.
(continued)
Breakdown of food particles into subunits
(chemical structure change).
Process of the passage of digestion
(chemical subunits) into the blood or
lymph.
Storage and elimination:
„
Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and
somatostatin.
Digestive System (GI)
„
Rhythmic wave-like contractions that move food
through GI tract.
Absorption:
„
Endocrine:
„
Swallowing the food.
Digestion:
„
Includes both exocrine and endocrine
secretions.
„
Chewing the food and mixing it with saliva.
Functions of the GI Tract
(continued)
Secretion:
„
Taking food into the mouth.
Temporary storage and elimination of
indigestible food.
Layers of GI Tract
„
Composed of 4 tunics:
„
„
Mucosa.
Submucosa.
„
„
Muscularis.
Serosa.
GI tract is 30’
long and
extends from
mouth to
anus.
1
Esophagus
From Mouth to Stomach
„
Mastication (chewing):
„
Enzyme that can catalyze the partial digestion of starch.
„
Deglutition (swallowing):
„
„
„
„
„
Oral phase is voluntary.
Pharyngeal and esophageal phases are involuntary.
„
„
Cannot be stopped.
Larynx is raised.
Epiglottis covers the entrance to respiratory tract.
„
Most distensible part of GI tract.
Functions of the stomach:
„ Stores food.
„ Initiates digestion of proteins.
„ Kills bacteria.
„ Moves food (chyme) into intestine.
Gastric Glands
„
Secrete gastric juice:
„
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Goblet cells: mucus.
Parietal cells: HCl and intrinsic factor (B12).
Chief cells: pepsinogen.
G cells: gastrin.
Several others…
Rate of 2-4 cm/sec.
Stomach
„
Insert 18.4a
Circular smooth muscle
contract behind, relaxes in
front of the bolus.
Followed by longitudinal
contraction (shortening) of
smooth muscle.
„
Stomach
„
Produced by a series of
localized reflexes in response
to distention of wall by bolus.
Wave-like muscular
contractions:
Begins as a voluntary activity.
Involves 3 phases:
„
„
Peristalsis:
„
Mixes food with saliva which contains salivary
amylase.
„
„
„
Contractions
of the
stomach
churn chyme.
„
„
(continued)
Insert fig. 18.5
Mix chyme
with gastric
secretions.
Push food
into
intestine.
Digestion and Absorption in the
Stomach
„
„
„
Proteins partially digested by pepsin.
Carbohydrate digestion by salivary
amylase is soon inactivated by acidity.
Alcohol and aspirin are the only
commonly ingested substances
absorbed.
2
Protective Mechanisms of
Stomach
„
„
„
„
Parietal and chief cells impermeable to
HCl.
Alkaline mucus contains HC03-.
Tight junctions between adjacent
epithelial cells.
Rapid rate of cell division (entire
epithelium replaced in 3 days).
Small Intestine
„
„
„
Each villus is a fold
in the mucosa.
Covered with
columnar epithelial
cells interspersed
with goblet cells.
Lamina propria
contain lymphocytes,
capillaries, and
central lacteal.
Intestinal Contractions and
Motility
Absorption in Small Intestine
„
„
Majority of absorption (90%) takes
place here
Duodenum and jejunum:
„
„
„
„
2 major types of
contractions occur in the
small intestine:
„
„
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Slow movement.
Pressure at the pyloric end
of small intestine is greater
than at the distal end.
Segmentation:
„
Bile salts, vitamin B12, electrolytes, and
H20.
„
Major contractile activity of
the small intestine.
Contraction of circular
smooth muscle.
„
Large Intestine
„
„
Outer surface bulges outward to form haustra.
Little absorptive function.
„
„
„
„
Mix chyme.
Defecation
„
„
Absorbs H20, electrolytes, several vitamin B complexes,
vitamin K, and folic acid.
Intestinal microbiota (bacteria) produce significant
amounts of folic acid and vitamin K.
Bacteria ferment indigestible molecules to produce
short-chain fatty acids.
Insert fig. 18.14
Peristalsis:
„
Carbohydrates, amino acids, lipids, iron,
and Ca2+.
Ileum:
Insert fig. 18.12
„
Waste material passes to the rectum.
Occurs when rectal pressure rises and
external anal sphincter relaxes.
Defecation reflex:
„
Does not contain villi.
Longitudinal rectal muscles contract to
increase rectal pressure.
„
„
Relaxation of internal anal sphincter.
Excretion is aided by contractions of
abdominal and pelvic skeletal muscles.
„
Push feces from the rectum.
3
Major Categories of Liver Function
Accessory Organs of Digestion
Bile Production and Secretion
„
„
The liver produces and secretes 250–1500 ml of
bile/day.
Bile pigment (bilirubin) is produced in spleen, bone
marrow, and liver.
„
„
Derivative of the heme groups (without iron) from
hemoglobin.
Gallbladder
„
„
„
Converted by bacteria in intestine to urobilinogen.
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Urobilogen is absorbed by intestine and enters the hepatic
vein.
„
Recycled, or filtered by kidneys and excreted in urine.
Pancreas
Pancreatic Juice
„
„
Exocrine:
„
Acini:
„
„
Sac-like organ attached to the liver.
Stores and concentrates bile.
Contraction of the gallbladder ejects
bile into the common bile duct into
duodenum.
Contains H20, HC03- and digestive enzymes.
Insert fig. 18.26
Secrete
pancreatic
juice.
Endocrine:
„
Islets of
Langerhans:
„
Secrete
insulin and
glucagon.
4
Regulation of Gastric Function
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„
Cephalic Phase
„
„
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Stimulated by sight, smell, and taste of food.
Activation of vagus:
„ Stimulates chief cells to secrete pepsinogen.
„ Directly stimulates G cells to secrete gastrin.
„ Directly stimulates ECL cells to secrete
histamine.
„ Indirectly stimulates parietal cells to secrete
HCl.
„
Gastric motility and secretion are automatic.
Extrinsic control of gastric function is divided
into 3 phases:
„
Cephalic phase.
Gastric phase.
Intestinal phase.
Continues into the 1st 30 min. of a meal.
„
Gastric Phase
„
„
Intestinal Phase
Arrival of food in stomach stimulates the gastric
phase.
Gastric secretion stimulated by:
„ Distension.
„ Chemical nature of chyme (amino acids and short
polypeptides).
„ Stimulates G cells to secrete gastrin.
„ Stimulates chief cells to secrete pepsinogen.
„ Stimulates ECL cells to secrete histamine.
„ Histamine stimulates secretin of HCl.
„
„
Inhibits gastric activity when chyme enters
the small intestine.
Arrival of chyme increases osmolality and
distension.
„
Activates sensory neurons of vagus and produces
an inhibitory neural reflex:
„
„
„
Hormone secretion:
„
Inhibit gastric activity:
„
Intestinal Reflexes
„
Gastroileal reflex:
„
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Ileogastric reflex:
„
„
Increased gastric activity causes increased motility
of ileum and movement of chyme through
ileocecal sphincter.
Distension of ileum, decreases gastric motility.
Intestino-intestinal reflex:
„
Overdistension in 1 segment, causes relaxation
throughout the rest of intestine.
Inhibits gastric motility and secretion.
In the presence of fat, enterogasterone inhibits gastric
motility and secretion.
Somatostatin, CCK, and GLP-1.
Secretion of Pancreatic Juice
„
„
„
Secretion of pancreatic juice and bile is stimulated by:
Secretin:
„ Occurs in response to duodenal pH < 4.5.
„ Stimulates production of HC03 by pancreas.
„ Stimulates the liver to secrete HC03 into the bile.
CCK:
„ Occurs in response to fat and protein content of
chyme in duodenum.
„ Stimulates the production of pancreatic enzymes.
„ Enhances secretin.
„ Stimulates contraction of the sphincter of Oddi.
5