The document discusses digestion and absorption in the gastrointestinal tract. It describes: - The mechanical and chemical breakdown of food that occurs in the mouth, stomach, and small and large intestines. This includes the roles of enzymes like pepsin, trypsin, amylase, and lipases. - How nutrients are absorbed in the small intestine, including the roles of micelles in fat absorption and iron-binding proteins in iron transport. - Causes of malabsorption like problems delivering bile or pancreatic juices or damage to the intestinal lining.

1. Digestion

2. Digestive processes occurring in
the mouth, pharynx, and
esophagus
Mastication is the mechanical breakdown of
food (chewing).
Deglutition is the transport of food from the
mouth to the stomach (swallowing).

3. The buccal phase of swallowing occurs in
the mouth, and involves the coordinated
movements of the tongue and palate to
force food into the oropharynx.

4. The pharyngeal-esophageal phase of
swallowing occurs after the buccal phase,
and includes blockage of the mouth by the
tongue, rising of the soft palate and the
epiglottis.

6. Digestive processes occurring in
the stomach
Pepsin is the primary enzyme secreted by
the stomach.
Intrinsic factor is required for intestinal
absorption of vitamin B12, which stimulates
the production of erythrocytes.

7. Phases of gastric secretion
The cephalic (reflex) phase
The gastric phase
The intestinal phase

10. The enterogastric reflex inhibits the vagal
nuclei in the medulla, inhibits local reflexes,
and activates sympathetic fibers that cause
the pyloric sphincter to tighten and prevent
further food entry into the small intestine.

11. Gastric motility and emptying
Receptive relaxation occurs in response to
food entry, and causes the smooth muscles
of the stomach to relax.
Adaptive relaxation also occurs in response
to food entry, but is initiated by stretch
receptors in the wall of the stomach and
involves nitric oxide.

12. Plasticity is the intrinsic ability of visceral
smooth muscle to exhibit the stress-
relaxation response, which is to stretch
without increasing tension.
The basic electrical rhythm refers to the rate
of the stomach’s peristaltic waves, which is
approximately three/minute, and is induced
by spontaneous pacemaker cells.

15. Emesis (vomiting) is stomach emptying
through the mouth due to stretching, or the
presence of irritants.
The emetic center of the medulla initiates
the contraction of skeletal muscles of the
abdominal wall and diaphragm.

16. Hormone Site of Production Stimulus Target Organs Activity
Gastrin Stomach mucosa Food Stomach Causes gastric glands to
increase secretory activity
Stimulates gastric emptying
Small intestine Stimulates contraction of
intestinal muscle
Ileocecal valve Relaxes ileocecal valve
Large intestine Stimulates mass movements
Serotonin Stomach mucosa Food Stomach Causes contraction of stomach
muscle
Histamine Stomach mucosa Food Stomach Activates parietal cells to
release HCL
Somatostatin Stomach mucosa & Food Stomach Inhibits gastric secretion,
Duodenal mucosa motility, and emptying
Pancreas Inhibits secretion
Small intestine Inhibits GI blood flow
Gallbladder Inhibits contractin & bile release

17. Hormone Site of Production Stimulus Target Organs Activity
Secretin Duodenal mucosa Acidic Stomach Inhibits gastric gland secretion
chyme and motility
Pancreas Increases output of pancreatic
juice rich in bicarbonate ions
Liver/pancreas Increases bile output
Cholecystokinin Duodenal mucosa Fatty Liver Augments secretin's actions
chyme Pancreas Increases output of enzyme
rich pancreatic juice
Gallbladder Stimulates organ to contract
and expel stored bile
Hepatopancreatic Relaxes to allow entry of bile
sphincter and pancreatic juice into
duodenum

18. Intestinal juice: Composition and
control
Intestinal juice has a pH similar to that of
blood, is isotonic with blood plasma, and is
composed of water, and mucus, but is
enzyme poor.

19. Regulation of bile release into
the small intestine
Gallstones can obstruct the flow of bile
from the gall bladder, and form as a result
of the crystallization of cholesterol due to
either too much cholesterol or too few bile
salts.

22. Digestive processes occurring in
the small intestine
The migrating mobility complex is the
pattern of peristaltic activity initiated in the
duodenum, whose waves sweep along the
intestine 10-70 cm before dying out.
Enhanced activity of the stomach initiates
the gastroileal reflex, which is a long reflex
that enhances the force of segmentation in
the ileum.

23. Digestive processes occurring in
the large intestine
Haustral contractions are slow segmenting
movements that occur every 30 minutes,
stimulated by distension of its muscles.
Mass movements are powerful contractile
waves that move over large areas of the
colon three or four times daily, and force
the contents toward the rectum.

24. The gastrocolic reflex is the colon’s
equivalent to the gastroileal reflex in the
small intestine, and is responsible for
initiating mass movements.
Diverticulitis is a result of lack of bulk in
the colon, causing small herniations of the
colon called diverticula which become
inflamed.

25. Defecation reflex causes emptying of the
rectum due to stretching of the rectal wall as
a result of mass movements.
Diarrhea is watery stools, which results
from any condition that rushes food residue
through the large intestine before that organ
has had sufficient time to absorb the
remaining water.

26. When food remains in the colon for
extended periods, too much water is
absorbed and the stool becomes hard and
difficult to pass, resulting in a condition
called constipation.

27. Chemical digestion

28. Mechanism of chemical
digestion: Enzymatic hydrolysis
Chemical digestion is a catabolic process in
which large food molecules are broken
down to monomers small enough to be
absorbed by the GI tract lining.
The enzymatic breakdown of any type of
food molecule is called hydrolysis, due to
the addition of a water molecule to each
molecular bond broken.

30. Chemical digestion of specific
food groups
The monomers of carbohydrates are called
monosaccharides.
Salivary amylase splits starch into smaller
fragments of two to eight linked
monosaccharides.

31. Carbohydrates that escape being broken
down by salivary amylase are acted on by
pancreatic amylase.
Dextrinase and glucoamylase are brush
border enzymes that act on polysaccharides
composed of three simple sugars.

32. Maltase, sucrase, and lactase are brush
border enzymes that hydrolyze maltose,
sucrose, and lactose respectively into their
constituent monosaccharides.

33. Pepsin begins protein digestion in the
stomach, when it is converted in an acidic
environment, from pepsinogen.
Rennin is a stomach enzyme that coagulates
milk protein, but is not produced in adults.

35. Trypsin and chymotrypsin are pancreatic
enzymes secreted into the duodenum that
cleave protein fragments into smaller
peptides.

36. Carboxypeptidase, secreted by both the
intestinal brush border and the pancreas,
splits off one amino acid at a time from the
end of the polypeptide chain with the
carboxyl group.
Aminopeptidase and dipeptidase are brush
border enzymes that liberate the final amino
acid products.

38. Lipases are fat-digesting enzymes secreted
by the pancreas, that hydrolyze triglyceride
molecules, yielding free fatty acids and
monoglycerides.

40. Nucleic acids are hydrolyzed into their
nucleotide monomers by pancreatic
nucleases.
Nucleotides are broken down by
nucleosidases and phosphatases, which are
intestinal brush-border enzymes.

41. Absorption

42. Absorption of specific nutrients
Micelles are collections of fatty elements
clustered together with bile salts in such a
way that the polar ends of the molecules
face the water and the nonpolar portions
from the micelle core.

44. Chylomicrons are composed of triglycerides
combined with small amounts of
phospholipids and a cholesterol, and coated
with proteins to form water-soluble
lipoprotein droplets.
The triglycerides of chylomicrons are
hydrolyzed to free fatty acids and glycerol
by lipoprotein lipase in the bloodstream.

45. Iron binds with a protein called ferritin after
it is transported into mucosal cells, which is
like a storehouse for iron.
Iron binds with a protein called transferrin
in the blood, which transport it in the
circulation.

46. Calcium absorption is regulated by the
active form of vitamin D, which acts as a
cofactor to facilitate calcium absorption.

47. Malabsorption of nutrients
Malabsorption can result from anything that
interferes with the delivery of bile or
pancreatic juice to the small intestine, as
well as factors that damage the intestinal
mucosa.