The Digestive
System
Chapter 23 - Digestive Processes
Digestive Processes in Mouth,
Pharynx, Esophagus
• Mouth ingests, begins mechanical digestion,
initiates swallowing, and starts chemical
digestion of polysaccharides (starches and
glycogen)into smaller fragments.
• If you chew a piece of bread for a few
minutes, it will begin to taste sweet as sugars
are released.
• Essentially no absorption occurs.
Digestive Processes in Mouth,
Pharynx, Esophagus
• Mastication (chewing)  mechanical digestion
– Cheeks and closed lips hold food between teeth,
the tongue mixes food with saliva to soften it, and
the teeth cut and grind solid foods into smaller
pieces
Digestive Processes in Mouth,
Pharynx, Esophagus - Swallowing
• Deglutition (Swallowing)
• Involves over 22 muscles!
• Buccal phase  occurs in mouth and is
voluntary. Initiated by the tongue
Digestive Processes in Mouth,
Pharynx, Esophagus
• Pharynx and esophagus merely serve as
conduits to pass food from the mouth to the
stomach.
• Only function in food propulsion,
accomplished by the role they play in
swallowing.
Digestive Processes in Mouth,
Pharynx, Esophagus - Swallowing
• Pharyngeal-esophageal phase  involuntary,
activated when food reaches receptors in the
posterior pharynx
• Once food enters the pharynx, respiration is
momentarily stopped and all routes except
that to the digestive tract are blocked off:
– The tongue blocks off the mouth
– Soft palate rises to close off the nasopharynx
– Epiglottis covers the glottis
Digestive Processes in Mouth,
Pharynx, Esophagus - Swallowing
• Solid foods travel to the stomach in about 2-8
seconds.
• If we try to talk or inhale while swallowing,
the protective mechanisms may be shortcircuited and food may enter respiratory
passageways.
Digestive Processes in Stomach
• Serves as a holding area for ingested food.
• Degrades food both physically and chemically.
• Then delivers chyme, into the small intestine.
Digestive Processes in Stomach
• Protein digestion is started in the stomach 
only type of enzymatic digestion that occurs.
• Proteins are denatured by the HCl  proteins
are unfolded which makes them more
accessible to enzymes.
Digestive Processes in Small Intestine
• Food reaching the small intestine is
unrecognizable, but it is far from being
chemically digested.
• Carbohydrates and proteins are partially
degraded, but virtually no fat digestion has
occurred.
Digestive Processes in Small Intestine
• Chyme spends 3-6 hours in the small intestine.
During this time absorption of water and
virtually all nutrients occur.
•Optimal activity also depends
on a slow, measured delivery
of chyme from the stomach.
Gives time for the
modification of the pH and
the mixing with the digestive
juices so that digestion can be
continued.
Digestive Processes in Small Intestine
• Anything that inhibits liver or pancreas
function or delivery of their juices to the small
intestine severely hinders our ability to digest
food and absorb nutrients.
Digestive Processes in Small Intestine
• Segmentation is the most common form of
movement (and mixing) in the small intestine
 looks like the intestinal contents are being
massaged  the chyme is simply moved back
and forth in the lumen a few centimeters at a
time.
Digestive Processes in Large Intestine
• What is finally delivered to the large intestine
contains few nutrients, but it still has 12-24
hours more to go!
•Some digestion of the
residue by bacteria, no
further food breakdown
of food occurs in the
large intestine.
Digestive Processes in Large Intestine
• Harvests vitamins made by the bacteria and
reclaims most of the remaining water and
some electrolytes (sodium and chloride).
• Absorption is not the major function of this
organ. Function is the propulsive activities
that force the fecal material towards the anus
and then eliminate it from the body
(defecation).
Digestive Processes in Large Intestine
• Large intestine is important for
our comfort, but not essential
for life. If the colon is removed,
the terminal ileum is brought
out to the abdominal wall in a
procedure called ileostomy, and
food residues are eliminated
from there into a sac attached
to the abdominal wall.