Download Physio Chap 65 - Digestion and Absorption in the GI Tract [10-26-13].

Physio Chap 65 - Digestion and Absorption in the GI Tract
Friday, February 8, 2013
4:27 PM
Digestion and Absorption in the GI Tract
 Digestion by Hydrolysis
o Carbs, Fats, and Proteins are all digested by hydrolysis, meaning that they are broken apart
and a water molecule is added.
 The only difference is the type of enzyme used to do this.
 Digestion of Carbs
o Three main sources of carbs in the human diet: sucrose, lactose, and starches.
o Cellulose is also a carb but the human body can't break it down.
o The mouth and Stomach Digest Carbs
 Process begins in the mouth with saliva, which contains ptyalin (α-amylase), secreted
mainly by the parotid glands.
 Breaks starch into maltose and other polymers of glucose.
 Ptyalin is nonactive in the stomach due to the acidic pH of 4.0, but can work in the
fundus and body until gastric juice hits it.
o Pancreatic Amylase is almost identical to salivary amylase but is several times as powerful.
 15-30 minutes after carbs hit the small intestine, they are digested into maltose and
other small glucose polymers.
o Intestinal Epithelial Cells contain lactase, sucrase, maltase, and α-dextrinase which break
down disaccharides lactose, sucrose, and maltose into monosaccharides.
 These enzymes found in intestinal microvilli brush border.
 Lactose splits into galactose and glucose.
 Sucrose splits into fructose and glucose.
 Maltose splits into glucose molecules.
 These are all water soluble and are absorbed immediately into the portal blood.
 Digestion of Proteins
o Pepsin is the most important peptic enzyme of the stomach and is most active at a pH of 2-3
and inactive above pH of 5.
 Digests collagen, the major constituent of intercellular connective tissue of meats.
 So if you want meat, you gotta have Pepsin!
 Only initiates the process. Opens the door. Picks the lock. Flirts.
 Hydrochloric acid is released at pH of 0.8 but after mixing with the other secretions
and stomach contents the pH is about 2-3. Perfecto para Pepsin!
o Digestion by Pancreatic Secretions
 Most occurs in the upper small intestine by trypsin, chymotrypsin,
carboxypolypeptide, and proelastase.
 Trypsin and Chymotrypsin split protein molecules into smaller peptides.
 Carboxypolypeptide then cleaves individual amino acids off
 Proelastase, activated to elastase, then digests elastin fibers that partially hold
the meat together.
o Digestion of Peptides by Enterocytes of Small Instestinal Villi
 Enterocyte Brush Border Microvilli have multiple peptidases that come out to play.
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Aminopolypeptidase and Dipeptidases split remaining larger polypeptides into
di- and tripeptides which are easily transported to the interior of the
enterocyte.
Other peptidases wait inside to digest further.
More than 99% of the final protein digestive products are individual amino
acids.
Digestion of Fats
o Most abundant are triglycerides but this group is accepting of phospholipids, cholesterol
and cholesterol esters.
o Lingual Lipase digests a little bit in the stomach.
o Emulsification by Bile Acids and Lecithin and then Digestion by Pancreatic and Enteric
Lipase
 First step is to break the fat into little pieces so water soluble enzymes can get access Emulsification.
 This begins in the stomach with mixing of the fats with stomach contents.
 Most occurs in the duodenum by bile.
 Bile contains bile salts and lecithin that act to break up and surround fat
molecules.
 Work as detergents.
 This increases the available surface area of the fats so that Lipases, especially
Pancreatic, can attach and digest the fats.
o End Products
 Pancreatic Lipase splits the triglycerides into free fatty acids (FFAs) and 2monoglycerides.
 When Bile Salts are high enough in concentration they form micelles, which surround
the digestive products with the polar groups of bile salts facing the hyrdophillic
atmosphere.
 These micelles transport FFAs and 2-monoglycerides to the brush border so they
can be absorbed.
 Cholesterol esters and Phospholipids are broken down by cholesterol ester hydrolase
and phospholipase A2 and then are carried off by micelles to be absorbed.
Basic Principles of GI Absorption
o All but 1.5 liters of 8-9 liters is absorbed by the intestines each day.
o Absorption of Small Intestinal Mucosa Villi
 It's all about SURFACE AREA!
 In the mucosa there are many folds called valvulae conniventes (Folds of Kerckring)
that serve to increase SA.
 ON the epithelial surface of these folds you find villi which increase SA 10-fold.
 ON each villi there are epithelial cells containing a brush border of
microvilli to increase SA another 20-fold. Get it? Lots of Surface Area to
absorb.
 Inside these microvilli there are arteries, veins, and central lacteals to
catch everything.
 Actin filaments contract rhythmically to keep the microvilli moving into
new intestinal fluid.
Absorption in the SI
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Bottom line - the SI can absorb a whole lot more than it normally does on a daily basis.
There were a bunch of numbers but the capacity was about 5-10 times the normal
absorption.
Absorption of Water
o Transported by diffusion using the laws of osmosis. Water follows the solute which helps to
dilute the incoming stomach contents and then is reabsorbed as the nutrients are absorbed.
Absorption of Ions
o Sodium
 Less than .5% of intestinal sodium is lost in the feces each day.
 Basic Mechanism has two parts
1. Intestinal epithelial cells use active transport to kick sodium out of the cell and
into the interstitial space.
2. This active transport causes a steep electrochemical gradient inside the cell so
sodium slides down that gradient by facilitated diffusion with glucose INTO the
epithelial cells
 This mechanism also pulls chloride and water with it by electro gradients and by
osmosis.
 Water follows salt not into the cells but into the paracellular spaces as the
sodium is transported out of the cell into the interstitial spaces.
o Aldosterone
 When a person is dehydrated, aldosterone is secreted.
 Aldosterone increases enzyme and transport mechanisms for all aspects of sodium
resorption.
 It also acts on the large intestine to not allow any fecal sodium so that the water
will follow.
o Chloride in the Duodenum and Jejunum
 Sodium absorption causes electronegativity in the chyme which makes a nice gradient
for the chloride to follow. It does so rapidly by diffusion.
o Bicarbonate
 One method of sodium transport exchanges a proton (H+) for a sodium (Na+). This H+
combines with secreted bicarb in the intestine to become H2CO3 and then H2O and
CO2.
 The CO2 is then absorbed and converted back to bicarb.
 Secretion of Bicarb
 Exchange of bicarb for chloride helps to decrease the acidity of the chyme.
 Fun Facts
 Immature epithelial cells secrete sodium chloride and water to provide
water flow for absorption.
 This secretion is then reabsorbed by mature epithelial cells.
 Cholera and other bacteria stimulate so much secretion that reabsorption
can't keep up. This leads to diarrhea.
o Other Ions
 Calcium:
 Parathyroid hormone and vitamin D greatly enhance calcium absorption.
 Vit D is activated by PTH so it has double effects.
 Iron
 Potassium, magnesium, phosphate, and others
 Monovalent ions are absorbed easily.
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 Bivalent ions are only absorbed in small amounts.
Absorption of Nutrients
o Carbs
 Essentially all are absorbed as monosaccharides, the most abundant is glucose.
 Virtually all are absorbed by active transport.
 Glucose
 Absorbed by Sodium/Glucose Co-Transport
 Intestinal epithelial cells use active transport to kick sodium out of the cell and
into the interstitial space.
 This active transport causes a steep electrochemical gradient inside the cell so
sodium slides down that gradient by facilitated diffusion with glucose INTO the
epithelial cells
 Once inside the cell, other facilitated diffusion gets the glucose into the blood.
 The first step of sodium transport is what drives the absorption of glucose.
 Fructose and Galactose
 Galactose is transported the same as glucose.
 Fructose goes by facilitated diffusion through the intestinal epithelium but not
with sodium. It is then phosphorylated to glucose.
o Proteins
 Sodium co-transport provides the energy for absorption of di- and tripeptides along
with amino acids.
 This is all called Secondary Active Transport.
o Fats
 Micelles ferry the FFAs and monoglycerides to the brush border where they are
absorbed and then packaged into chylomicrons.
Absorption in Large Intestines: Formation of Feces
o Most absorption happens in the proximal one half of the colon (mostly water).
o The other half stores the feces.
o The first half absorbs sodium and chloride while secreting bicarb (in exchange for chloride).
 This causes an osmotic gradient for water to follow.
 Tight junctions are stronger or tighter here to prevent backflow of ions.
o Large intestine absorbs a max of 5-8 liters of fluid and electrolytes each day.
 Toxins can disrupt this and cause diarrhea.
o Brown color of poop is from derivatives of bilirubin.