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Small Intestine
Most digestion and absorption occurs in small intestine
Small intestine is about 10 feet long.
Duodenum (about 5 feet)
Jejunum (about 2.5 feet)
Ileum (about 2.5 feet)
Small Intestinal Motility
Segmentation contractions (unique to small intestine:
Mostly mixing of contents and bringing contents into contact with
mucosal surface. Minor propulsion because frequency is greater
in proximal end and is reduced as we go further down the small
intestine.
Peristalsis in Small Intestine
Peristalsis is major propulsion mechanism in small intestine.
Peristaltic waves in small intestine typically travel only a few
inches, then end. But when there’s an irritant in the small
intestine, a single peristaltic wave can traverse the entire 10 foot
length in a few seconds. This tends to rapidly force the irritant
out of the small intestine. This is known as a peristaltic rush.
Surface Specializations in Small Intestine
Muscularis Mucosae in Small Intestine
Muscularis mucosae is just under the mucosa. When it contracts,
villus shortens slightly. This mixes the content that is in contact
with the villus, important for absorption and digestion. It also
forces fluid from the villus interstitial space into the lymphatics.
Ileocecal Sphincter and Valve
Sphincter at distal end of ileum is normally closed, preventing
reflux from colon. When peristaltic waves reach it, it relaxes and
allows flow from ileum to colon.
Ileocecal valve, normally closed, opens passively. It’s a one-way
valve, much like those in veins and in the heart.
Cells in small intestinal mucosa secrete 1 to 2 liters/day of water
into lumen osmotically, hydrolyzed food being the solutes that
create the osmotic gradient.
How much fluid enters the gut?
1. Everything we drink (about 1 liter/day)
2. Saliva (1 to 2 liters/day)
3. Gastric secretions (2 to 3 liters/day)
4. Pancreatic and hepatic secretions (2 to 3 liters/day)
5. Secretions of small intestine (1 to 2 liters/day)
Nearly none is absorbed in mouth, esophagus or stomach, so the
volume entering the small intestine is 7 to 11 liters/day (let’s use
10 liters/day as an approximation)
About 500 ml/day enters the large intestine. Therefore, small
intestine must absorb about 95% of the water that enters it.
Absorption in Small Intestine
For all practical purposes, all digested nutrients are absorbed in the
small intestine. Except for bile salts and vitamin B12, absorption of
everything except electrolytes is complete before the gut contents
reach the ileum.
Surface Specializations in Small Intestine
These greatly increase (500 to 1,000 fold) the surface area of
mucosa in small intestine.
Enzymes of Brush Border Membranes
Proteases digest proteins mostly to the level of dipeptides.
Dipeptidases in brush border convert them to amino acids.
Pancreatic amylase converts starch to disaccharides (chains of two
sugars). Disaccharidases in brush border membranes convert them
to simple sugars (monosaccharides).
Brush border membranes contain active transport systems that
absorb amino acids and peptides as they are formed. This maintains
very high concentration gradients across the membranes, which
osmotically absorb water. Sodium is also actively absorbed through
the brush border membranes, which drives most of the osmotic
absorption of water.
Active transport of sugars and amino acids depend on simultaneous
sodium transport.
Disaccharidases
Starch gets hydrolyzed to the disaccharide level (maltose) before
being hydrolyzed all the way to glucose.
We also ingest some glucose as well as some disaccharides:
Maltose (= glucose + glucose)
Sucrose (= glucose + fructose)
Lactose (= glucose + galactose)
These are all hydrolyzed by brush border enzymes (maltase,
sucrase and lactase), and the products are absorbed. Fructose is
absorbed by simple diffusion.
Some people have a lactase deficiency (used to be called milk
allergy), and can’t digest lactase. A dairy meal results in
undigested lactose in the gut, which osmotically causes diarrhea.
Dipeptidases
There are at least three categories of brush border dipeptidases:
one for neutral amino acids, one for acidic amino acids, one for
basic amino acids
A very small percentage of peptides up to four amino acids long
are actively absorbed as well.
Newborns absorb very large proteins (antibodies) from mother’s
milk by endocytosis. Protease inhibitors in mother’s milk prevent
digestion of antibodies.
Fat Digestion and Absorption
Lipases only act on surfaces of fat droplets, making detergent action
of bile salts very important. Triglycerides (most dietary fat) are
hydrolyzed to free fatty acids, glycerol, and monoglycerides, all are
absorbed through brush border membranes. Free fatty acids are also
detergents, so they promote additional fat digestion along with the
bile salts. Free fatty acids and monoglycerides aren’t very water
soluble, and saturate the solution in the lumen of the small intestine.
With bile salts, the undissolved fatty acids and monoglycerides
aggregate to form micelles (about 20 lipid molecules per micelle),
which become a reservoir for free fatty acids and monoglycerides –
as they are absorbed, more leave the micelles and dissolve. Micelles
also store vitamin D and other fat soluble compounds. Fat
absorption is usually complete by the proximal part of the ileum. Bile
salts are reabsorbed in the terminal ileum.
More on Absorption
Most things absorbed by brush border cells diffuse through their
basal borders into the capillaries. Fats are an exception. Fatty acids,
glycerol and monoglycerides absorbed by brush border cells are
resynthesized into triglycerides. A layer of lecithin (another kind of
lipid) and protein is added within the cells. The resulting particles
are chylomicrons.
Chylomicrons are released into the interstitial fluid by exocytosis,
but are too big to enter capillaries. Instead, they diffuse into the
lymphatics via the central lacteal.
Muscularis mucosae is important in forcing interstitial fluid into
central lacteal.
Colon (= large intestine)
Length = 4 feet. No enzymes or villi. About 500 ml enters every day,
about 100 ml is lost in feces. Therefore, reabsorbs about 400 ml,
about 80% of the volume that enters it. MUCH less than small
intestine (9,500 ml and 95%, respectively).
Some vitamins and nutrients produced by colon flora are absorbed,
but are of little importance. They also produce nitrogen, CO2 and
other gases from stuff that we can’t digest. Beans contain lots of
undigestible (for us) stuff that colon bacteria can’t get enough of.
Some of the gases produced by colon flora are absorbed through
colon wall, but an average of more than 1 liter/day isn’t (lots more
for your uncle or brother). That’s flatus.
Colon secretes some mucous fluid and fairly small amounts of an
alkaline fluid that neutralizes acid produced by colon flora.
Absorption in Colon
Mainly, active transport of sodium, passively followed by chloride,
creating osmotic gradient that absorbs water (about 400 ml/day).
This is almost entirely in proximal half of colon, called absorbing
colon for that reason. Distal half, which stores fecal material for
long periods, is called storage colon.
Motility in Colon
Longitudinal muscle in colon is arranged in three bands (taeniae
coli).
Circular muscle generates something similar to segmentation
contractions, referred to as haustral contraction. All of these are
for mixing contents.
Motility in Colon (continued)
Peristalsis is rare, if it occurs at all. Propulsion in colon is mostly
by mass movements – rings of contraction that extend in length.
Very effective because of the high resistance to backward flow.
Stimuli to Mass Movements
1. Local distension of colon
2. Gastrocolic reflex – food entering stomach elicits reflex (via
vagus nerve) that initiates mass movements. Useful in
housebreaking a puppy
Defecation
Storage colon stores contents for hours, until mass movement
propels it into rectum.
This stretches rectal wall, initiating defecation reflex, which includes
vagal afferents from CNS and via myenteric plexus. Motor response
includes mass movements in storage colon, contraction of rectal
muscle, relaxation of internal and external anal sphincters. Uh-oh!
Modulation of Defecation Reflex
Rectal muscles are contracting, anal sphincters are relaxing. This
spells trouble. Good news! External anal sphincter is skeletal
muscle, so we can voluntarily contract it. Better news! Contracting
the external anal sphincter inhibits the defecation reflex, usually for
several hours. Toilet training is essentially learning to contract the
external anal sphincter.
The neural connections making it possible don’t develop in humans
until after the second birthday, doesn’t develop in puppies until the
third month.
Many animals (most herbivores) don’t have an external anal
sphincter, and can’t be housebroken. Don’t try to housebreak a
horse. It’s a waste of your time and, sooner or later, it will annoy
the horse.
Forces in Defecation
Much comes from muscles of colon and rectum.
Augmented by “straining movements” – contracting abdominal
muscles with glottis closed. This causes very high abdominal and
thoracic pressures. The abdominal pressure contributes to forcing
stool out of the colon.
The straining can cause very high arterial pressure, but the
elevated thoracic pressure can close off the veins, bringing venous
return to zero and, consequently, reducing cardiac output to zero.
Many strokes and heart attacks occur during straining movements.