Download The Digestive System

The Digestive System
• Components the Digestive Tract.
• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
The Small Intestine
• What are the Regions
of the Small
Intestine?
• Duodenum
• Jejunum
• Ileum
Small Intestine
• Digestion & absorption
• Duodenum: (1st section) digestive juices, major
chemical digestion
• Jejunum (2nd): absorb nutrients
• Ileum (3rd): absorb Vit. B12, bile salts, remaining
nutrients
Digestive Glands
Secrete into SI (duodenum)
Pancreas: neutralize acidic
chyme (bicarbonate),
enzymes (carbs, proteins,
fats)
Bile salts: made in liver,
stored in gallbladder
Emulsify fats (make
smaller droplets)
Pancreas
• Exocrine:
• Acini:
• Secrete
pancreatic juice.
• Endocrine:
• Islets of Langerhans:
• Secrete insulin
and glucagon.
Insert fig. 18.26
The Pancreas
•The exocrine pancreas produces a mixture
of buffers and enzymes essential for normal
digestion.
Pancreatic Juice
• Contains H20, HC03- and digestive enzymes.
• The exocrine pancreas is organized much like the salivary
glands: It resembles a bunch of grapes, with each grape
corresponding to a single acinus.
• The acinus, which is the blind end of a branching duct
system, is lined with acinar cells that secrete the enzymatic
portion of the pancreatic secretion.
The ducts are lined with ductal cells.
Ductal epithelial cells extend into a special region of
centroacinar cells in the acinus.
The centroacinar and ductal cells secrete the aqueous HCO3-containing component of the pancreatic secretion.
Enzymatic component of
pancreatic secretion (acinar
cells).
Pancreatic amylase and lipases
are secreted as active enzymes.
Pancreatic proteases are secreted
in inactive forms and converted to
their active forms in the lumen of
the duodenum; for example, the
pancreas secretes trypsinogen,
which is converted in the
intestinal lumen to its active form,
trypsin.
Trypsin (when activated by enterokinase) triggers the
activation of other pancreatic enzymes.
Aqueous component of pancreatic
secretion (centroacinar and ductal
cells).
Pancreatic juice is an isotonic
solution containing Na+, Cl-, K+, and
HCO3- (in addition to the enzymes).
The Na+ and K+ concentrations are
the same as their concentrations in
plasma, but the Cl- and HCO3concentrations vary with pancreatic
flow rate.
Centroacinar and ductal cells produce
the initial aqueous secretion, which is
isotonic and contains Na+, K+, Cl-, and
HCO3-.
Aqueous component of pancreatic secretion
(centroacinar and ductal cells).
This initial secretion is then modified by transport
processes in the ductal epithelial cells as follows:
In the presence of carbonic anhydrase, CO2 and
H2O combine in the cells to form H2CO3. H2CO3
dissociates into H+ and HCO3-.
The HCO3- is secreted into pancreatic juice by the
Cl--HCO3- exchanger in the apical membrane.
The H+ is transported into the blood by the Na+H+ exchanger in the basolateral membrane.
The net result, or sum, of these transport
processes is net secretion of HCO3- into
pancreatic ductal juice and net absorption of H+;
absorption of H+ causes acidification of
pancreatic venous blood.
Like gastric secretion, pancreatic secretion is
divided into cephalic, gastric, and intestinal
phases.
• In the pancreas, the cephalic and gastric phases are less
important than the intestinal phase.
• Briefly, the cephalic phase is initiated by smell, taste, and
conditioning and is mediated by the vagus nerve.
• The cephalic phase produces mainly an enzymatic secretion.
• The gastric phase is initiated by distention of the stomach
and is also mediated by the vagus nerve.
• The gastric phase produces mainly an enzymatic secretion.
The intestinal phase is the most important
phase and accounts for approximately 80% of
the pancreatic secretion
• Acinar cells (enzymatic secretion).
• The pancreatic acinar cells have receptors for
CCK and muscarinic receptors for ACh.
• During the intestinal phase, CCK is the most
important stimulant for the enzymatic
secretion.
• The I cells are stimulated to secrete CCK by
the presence of amino acids, small peptides,
and fatty acids in the intestinal lumen.
• Of the amino acids stimulating CCK
secretion, phenylalanine, methionine, and
tryptophan are most potent.
• In addition, ACh stimulates enzyme
secretion and potentiates the action of CCK
by vagovagal reflexes.
• Ductal cells (aqueous secretion of Na+, HCO3-, and H2O).
• The pancreatic ductal cells have receptors for CCK, ACh,
and secretin.
• Secretin, which is secreted by the S cells of the
duodenum, is the major stimulant of the aqueous HCO3-rich secretion.
• Secretin is secreted in response to H+ in the lumen of the
intestine, which signals the arrival of acidic chyme from
the stomach.
• To ensure that pancreatic lipases will be active (since they
are inactivated at low pH), the acidic chyme requires
rapid neutralization by the HCO3--containing pancreatic
juice.
• The effects of secretin are potentiated by both CCK and
ACh.
The Liver
• What is the Overview of the
Liver?
• Largest visceral organ
• Four Lobes
• Over 200 known functions
•The liver is the body’s center for metabolic regulation.
•It produces bile that will be ejected by the gallbladder
into the duodenum under stimulation of CCK.
•Bile is essential for the efficient digestion of lipids; it
emulsifies fats so that individual lipid molecules can be
readily attacked by digestive enzymes.
• Liver largest internal organ.
• Hepatocytes form hepatic
plates that are 1–2 cells thick.
• Arranged into functional units
called lobules.
• Plates separated by sinusoids.
• More permeable than other
capillaries.
• Contains phagocytic Kupffer
cells.
• Secretes bile into bile canaliculi,
which are drained by bile ducts.
Hepatic Portal System
• Products of digestion that are
absorbed are delivered to the
liver.
• Capillaries drain into the
hepatic portal vein, which
carries blood to liver.
• ¾ blood is deoxygenated.
• Hepatic vein drains liver.
Enterohepatic Circulation
• Compounds that recirculate
between liver and intestine.
• Many compounds can be
absorbed through small
intestine and enter hepatic
portal blood.
• Variety of exogenous
compounds are secreted by
the liver into the bile ducts.
• Can excrete these compounds
into the intestine with the bile.
Insert fig. 18.22
Major Categories of Liver Function
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.
• Free bilirubin combines with glucuronic acid and forms conjugated
bilirubin.
• Secreted into bile.
• Converted by bacteria in intestine to urobilinogen.
• Urobilogen is absorbed by intestine and enters the hepatic vein.
• Recycled, or filtered by kidneys and excreted in urine.
Bile Production and Secretion
• Bile acids are derivatives of cholesterol.
• Major pathway of cholesterol
breakdown in the body.
• Principal bile acids are:
• Cholic acid.
• Chenodeoxycholic acid.
• Combine with glycine or taurine to
form bile salts.
• Bile salts aggregate as micelles.
• 95% of bile acids are absorbed by ileum.
(continued)
Gallbladder
• Sac-like organ attached to the inferior surface of the liver.
• Stores and concentrates bile.
• When gallbladder fills with bile, it expands.
• Contraction of the muscularis layer of the gallbladder,
ejects bile into the common bile duct into duodenum.
• When small intestine is empty, sphincter of Oddi closes.
• Bile is forced up to the cystic duct to gallbladder.
The Small Intestine
• What is the Intestinal Wall?
• Plicae have small
projections, villi
• Both increase surface area of
mucosa for absorption
Folds, villi and microvilli increase surface area for absorption
The Small Intestine
• The Intestinal Wall
• Each villus is a fold in the
mucosa.
• Covered with columnar
epithelial cells interspersed
with goblet cells.
• Epithelial cells at the tips of
villi are exfoliated and
replaced by mitosis in crypt
of Lieberkuhn.
• Lamina propria contain
lymphocytes, capillaries,
and central lacteal.
Absorption
Glucose and galactose are
absorbed by mechanisms
involving Na+-dependent
cotransport.
Fructose is absorbed by
facilitated diffusion.
Absorption of Lipids
• Free fatty acids,
monoglycerides, and
lysolecithin leave micelles and
enter into epithelial cells.
• Resynthesize triglycerides
and phospholipids within
cell.
• Combine with a protein
to form chylomicrons.
• Secreted into central lacteals.
Transport of Lipids
• In blood, lipoprotein lipase hydrolyzes triglycerides to free
fatty acids and glycerol for use in cells.
• Remnants containing cholesterol are taken to the liver.
• Form Very-low-density lipoprotein which take triglycerides
to cells.
• Once triglycerides are removed, Very-low-density
lipoprotein are converted to low-density lipoprotein.
• low-density lipoprotein transport cholesterol to organs
and blood vessels.
• High-density lipoprotein transport excess cholesterol back to
liver.
•Absorption of
Proteins
• Free amino acids absorbed by
cotransport with Na+.
• Dipeptides and tripeptides
transported by secondary
active transport using a H+
gradient to transport them into
the cytoplasm.
• Hydrolyzed into free amino
acids and then secreted into
the blood.
The Small Intestine
• What is Intestinal Secretion?
• Intestinal juice
• Moistens chyme
• Buffers stomach acid
• Dissolves digestive enzymes
• Dissolves products of digestion
The Small Intestine
• What are the Intestinal
Hormones?
• Gastrin
• Secretin
• Cholecystokinin (CCK)
Paracrine Regulators of the Intestine
• Serotonin (5-HT):
• Stimulates intrinsic afferents, which send impulses into intrinsic
nervous system; and activates motor neurons.
• Motilin:
• Stimulates contraction of the duodenum and stomach antrum.
• Guanylin:
• Activates guanylate cyclase, stimulating the production of cGMP.
• cGMP stimulates the intestinal cells to secrete Cl- and H20.
• Inhibits the absorption of Na+.
• Uroguanylin:
• May stimulate kidneys to secrete salt in urine.
Absorption in Small Intestine
• Duodenum and jejunum:
• Carbohydrates, amino acids, lipids, iron, and Ca2+.
• Ileum:
• Bile salts, vitamin B12, electrolytes, and H20.
Intestinal Enzymes
• Microvilli contain brush border enzymes that are
not secreted into the lumen.
• Brush border enzymes remain attached to the
cell membrane with their active sites exposed to
the chyme.
• Absorption requires both brush border enzymes
and pancreatic enzymes.
Intestinal Reflexes
• Intrinsic and extrinsic regulation controlled by intrinsic and
paracrine regulators.
• Gastroileal reflex:
• Increased gastric activity causes increased motility of ileum
and movement of chyme through ileocecal sphincter.
• Ileogastric reflex:
• Distension of ileum, decreases gastric motility.
• Intestino-intestinal reflex:
• Overdistension in 1 segment, causes relaxation throughout
the rest of intestine.
The Large Intestine
• What is the Overview of
the Large Intestine?
• Reabsorbs water and
compacts feces
• Absorbs vitamins made by
bacteria
• Stores feces before
defecation
• Consists of three parts
• Cecum
• Colon
• Rectum
Appendix = extension of cecum, role in immunity
• The Large Intestine
The Large Intestine
Figure 16-17(b)
Large Intestine
• Outer surface bulges outward to form haustra.
• Little absorptive function.
• Absorbs H20, electrolytes, several vitamin B complexes, vitamin K,
and folic acid.
• Intestinal microbiota produce significant amounts of folic acid and
vitamin K.
• Bacteria ferment indigestible molecules to produce short-chain fatty
acids.
• Does not contain villi.
• Secretes H20, via active transport of NaCl into intestinal
lumen.
• Guanylin stimulates secretion of Cl- and H20, and inhibits absorption
of Na+ (minor pathway).
• Membrane contains Na+/K+ pumps.
• Minor pathway.
Defecation
• Waste material passes to the rectum.
• Occurs when rectal pressure rises and external anal
sphincter relaxes.
• Defecation reflex:
• 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.
Aging and the Digestive System
• What are Age-Related Changes in the
Digestive System?
• Thinner, more fragile epithelium
• Reduced epithelial stem cell division
• Weaker peristaltic contraction
• Reduced smooth muscle tone
Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings