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Digestion in the small intestine (31)
• Virtually all nutrient absorption takes
place in the small intestine
• Chime is released slowly into the
• Mixing is required because chime is
hypertonic and has low pH
• Required substances are supplied by
the liver
Motility in the small intestine (31)
The most common motion of the small
intestine is segmentation
Moves contents steadily toward the
ileocecal valve
After nutrients have been absorbed:
Peristalsis begins with each wave starting
distal to the previous
Meal remnants, bacteria, mucosal cells, and
debris are moved into the large intestine
The ileocecal valve is a sphincter muscle
situated at the junction of the small
intestine (ileum) and the large intestine.
Subdivisions of the large intestine (32)
rectum and the
anal canal
Vermiform appendix – now thought to
house friendly bacteria
Regions of the colon (33)
– ascending colon
– hepatic flexure,
– transverse colon,
– splenic flexure,
– descending colon
– sigmoid colon
Large intestine
Why don’t we poop when we toot? (34)
• 3 valves of the rectum stop feces from
being passed with gas
Anal sphincters – voluntary & involuntary
• The internal anal sphincter is composed
of smooth muscle (involuntary)
• The external anal sphincter is composed
of skeletal muscle (voluntary)
Internal – smooth muscle
External – skeletal muscle
Origin & significance of bacterial flora in
the large intestine (36) Friendly!
• Bacteria surviving the small intestine
that enter the cecum
• Bacteria entering from the anus
• These bacteria colonize the colon,
ferment indigestible carbs, release
irritating acids and gases (flatus),
synthesize B complex vitamins and
vitamin K
Function of the large intestine (37)
Other than digestion of enteric
bacteria, no further digestion takes
Vitamins, water, and electrolytes are
Propulsion of fecal material toward the
anus is the MAJOR function
You Tube video here
General protocol for defecation (38)
• Distension of rectal walls caused by
– Stimulates contraction of the rectal walls
– Relaxes the internal sphincter
• Voluntary signals stimulate relaxation of
the external anal sphincter and
defecation occurs
How nutrients are digested & absorbed
• Enzymes released by the pancreas become
active in the duodenum for protein digestion
• Multiple enzymes are needed to break down
proteins into amino acids
• Fats are emulsified by bile released from
storage in gall bladder
• Carbs are initially broken down in the mouth
by saliva
• Virtually all nutrient absorption takes place in
the small intestine
Electrolyte absorption (40)
• Electrolytes are reclaimed in the large
intestine – along with any remaining
water and vitamins
• concentration gradient allows diffusion
back into mucosal cells
Water absorption (41)
• 95% of water is absorbed in the small
intestines by osmosis
• water uptake is coupled with solute
uptake and as water moves into
mucosal cells substances follow along
their concentration gradients