Download Digestion and Absorption in the Gastrointestinal Tract

Digestion and Absorption
in the Gastrointestinal
Tract
By Sylvan Clark and Angela Posh
Content
Overview of GI tract
Discuss the process by which the carbohydrates, fats and
proteins are digested by hydrolysis
Absorption of carbohydrates, fats and proteins
The mechanisms by which the digestive end products are
absorbed
Overview of GI Tract
Digestion by Hydrolysis
Hydrolysis- process by which specific enzymes in the
digestive juices of the gastrointestinal tract return the
hydrogen and hydroxyl ions from water to the
polysaccharides, thereby separating the monosaccharide's
from each other
Reverse process is condensation.
R”-R’ + H2O ---digestive enzyme--> R”OH +R’H
Three types of substances to be digested:
Fats, proteins and carbohydrates
Digestion of Carbohydrates
Carbohydrates- Sucrose, Lactose, and Starches
Mouth:
Chewed and mixed with saliva
Enzyme ptyalin(an α-amylase) hydrolyzes starch into disaccharide maltose and
other small polymers of glucose.
Small Intestine:
Pancreatic α-amylase
Carbohydrates converted into maltose and small glucose polymers before passed
beyond the duodenum or upper jejunum
Enterocytes lining the villi of small intestine contain 4 enzymes
Lactase (lactose), Sucrase, (sucrose), maltase, (maltose), and α dextrinase.
Disaccharides and small glucose polymers hydrolyzed into monosaccharaides by
intestinal epithelial enzymes.
Made into (water soluble) glucose and absorbed in the blood portal
Free glucose and stored glucose (liver)
Digestion of Proteins
Pepsin(initiator of protein digestion)- more active based on pH
levels in the stomach.
Most protein digestion conducted by proteolysis enzymes from the
pancreases in the duodenum and jejunum.
Trypsin, chymotrypsin, carboxypolypeptidase, and proelastase
Breaks down proteins into amino acids. Figure 65-2
Last protein digestive stage is achieved in the intestinal lumen by
enterocytes that line the villi of the small intestine (mainly
duodenum and Jejunum) . Peptidases split remaining polypeptides
into amino acids and then the amino acids. More peptidases are
located in the cytosol of enterocytes where remaining polypeptides
are split into amino acids and thus are able to pass through
enterocyte and into the blood.
Digestion of fats
Mouth/Stomach: lingual lipase breaks down less than 10% neutral fats.
Essentially all fat break down occurs in the small intestine
Step one is break down fat globules into smaller sizes by physical break down
of fat globules(emulsification)
Begins in stomach by agitation of the fat globules.
Most emulsification occurs in duodenum under influence of bile
Bile salts and lecithin secreted from the liver
Enables smaller water soluble enzymes to work on cell surface and decrease size
of fat globules—makes fat globules readily fragment able by agitation with the
water in the small bowel.
Triglycerides digested by pancreatic lipase
End products of fat digestion are free fatty acids
Bile salts form micelles that “ferry” fats for absorption.
Gastrointestinal Absorption
Basic Principles
Total fluid absorbed each day by the intestines = ingested fluid
(about 1.5 liters) + gastrointestinal secretions (about 7 liters)
~8-9 liters total
All but 1.5 liters of this is absorbed in the small intestine
1.5 liters pass through the ileocecal valve into the colon each day
Stomach has poor absorption area because it lacks villus type
of absorptive membrane and tight junctions
Only alcohol and some drugs like aspirin can be absorbed in
small quantities
Small intestine
Absorptive surface of the small intestinal lined with valvulae
conniventes (folds of Kerckring)
Increase surface area of the absorptive mucosa 3 fold
Duodenum and jejunum ~8mm into the lumen
Ileocecal valve contain small villi ~1mm
Increase total absorptive area another 10 fold
Cont’
Brush Border ~1mm in length
Protruding into the intestinal chyme
In combination with Kerckring, the villli, and the microvilli
increases total absorptive area of the mucosa ~1000fold
Absorption In Small Intestine
Absorption of several hundred grams of carbohydrates each
day
100g or more of fat, 50-100g amino acids, 50-100g ions and 78L of water
Absorptive capacity of the normal smaller intestine is greater
Water by Osmosis
Entirely by diffusion
Make the chyme isosmotic with the plasma
Absorption of Ions
Sodium actively transported through interstitial membrane
Important role in absorbing sugars & amino acids
Mechanisms:
Active transport
Co-transport (secondary active transport)
1) sodium glucose co-transporter
2) sodium amino acid co-transporter
3)sodium hydrogen exchange
Osmotic movement of H2O
Creates fluid flow into and through the paracelluar spaces
into circulating blood of the villus
Occurs by large osmotic gradient by elevated concentration
of ions
Tight junctions between the apical borders of the epithelial cells
Most of it occurs through the cells themselves
Aldosterone
Greatly enhances Sodium
When dehydrated—aldosterone secretion increased
Effect of aldosterone is important in the colon
Allows virtually no loss of sodium chloride in feces and little
water loss
Thus this is the same as aldosterone in the renal tubles
Bicarbonate Ions
Reabsorbed from the upper small intestine
Due to large amounts of bicarbonate ions secreted into the
duodenum in both pancreatic secretion and bile
Readily absorbed into the blood and expired through the lungs
“active absorption of bicarbonate ions”.
Same mechanisms that occurs in the tubules of the kidneys
Secretion of bicarbonate ions
Occurs in the epithelial cells on the surface of the villi in the ileum.
Special capability of secreting bicarbonate ions in exchange for
absorption of chloride
Neutralizes acid products formed by bacteria in the large intestine
Active Absorption of Calcium,
Iron, Potassium, Magnesium
and Phosphates
Calcium readily absorbed into the blood
Ions are actively absorbed from small intestine
Potassium, Magnesium, Phosphates absorbed in intestinal
mucosa
Absorption of Nutrients
All Carbohydrates are absorbed in form of monosaccharide
Glucose most abundantly absorbed ~80%
~20% galactose & fructose
Absorption of Fats
broken down into monoglycerides and fatty free acids
Dissolved by bile micelles and carried to surfaces of microvilli
of intestinal cell brush boarder
Then taken to the microvilli and diffused out of micelles and into
the interior epithetical cells
It is then taken up by smooth ER and flows up through the lyph
duct and into circulating blood
Direct absorption of fatty acids
Directly into portal blood rather than being converted into
triglycerides and absorbed by way of lymphatic.
Large Intestine: Formation of
Feces
Essentially all ions absorbed
~1-5 mEq Na and Cl lost in Feces
Most absorption occurs in proximal left half
Distal colon—feces storage until excretion
Feces= ¾ water, ¼ solid mater