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Digestion Part II
Human Digestion
Mouth- teeth used to mechanically break down food
Tongue- moves food around, monitors texture and chemistry via
chemoreceptors (taste buds)
Taste of food stimulate saliva
Saliva glands- produce a substance known as saliva.
Saliva-95% H2O, ions, lubricating mucus, and amylase (breaks
up starch)
Amylase enzyme works well in
slightly alkaline solutions
After food is chewed sufficiently, it
is called a bolus and is swallowed
down the pharynx which joins the
nasal cavity
Pharynxanterior larynx- when raised closes
trachea
-posterior laryngopharynx
When swallowing occurs the pharynx closes off the trachea to
push food down the esophagus
Esophagus
-upper is under voluntary control
nervous system
-lower is under involuntary control
nervous system
There are two layers of muscle
around the esophagus. One is
longitudinal and the other is
circular. The two will squeeze
together to form a peristaltic wave.
A number of these waves are used
to move the bolus down to the stomach.
The digestive tract has three layers, which can be seen throughout
the entire gastrointestinal tract.
1. Muscularis- outer layer
A. serosa outer fiber
layer
B. circular and longitudinal muscle (smooth,
involuntary muscle)
2. Submucosa- connective
tissue, blood vessels, nerves,
lymph vessels and glands
3. Mucosa- inner layer
epithelium connective tissue
and lymph nodes
The bolus moves down the esophagus via
peristalsis. The bolus enter the stomach via cardiac
sphincter which is muscle that opens and closes the
stomach.
Stomach has the standard three layers with an
additional layer of diagonal muscle in the
muscularis; allows for peristalsis. The upper
part of the stomach has pits with
leading to glands. Parietal cells
produce HCl acid. Mucus cells
that produce mucus to protect
the lining of the stomach. Chief
cells secrete pepsinogen
(precursor of pepsin). These
produce gastric juices. Pepsin is
an enzyme that starts to
breakdown proteins.
Hydrochloric acid is used
to destroy bacteria found
on food, breakdown other
materials like bone,
eggshell and other things.
The pH of the
hydrochloric acid is
around 2. The Cl- comes
from salt and the H+
comes from water.
Once the bolus enters the
stomach it distends; this
distention causes increase
motility and a release of a
hormone gastrin.
The release of gastrin
stimulates the release of HCl
and pepsinogen (precursor
of pepsin); pepsinogen is made into an active enzyme pepsin by HCl
and other pepsin
Enzymes secreted by the stomach
Pepsin- hydrolyzes protein to polypeptides
Rennin- soluble milk protein- milk curd
Gastric lipase- neutral fats- F.A. and glycerol and does contribute to
lipid digestion (not all )
Peristalsis moves the bolus back and
forth digesting it to chyme.
Very acidic 1.5-2 pH; coating of
mucus prevents acid from breaking
down the tissue of the stomach.
Finally the acidic chyme passes into
the pyloric sphincter into the
duodenum or small intestines.
The small intestine (4.5 meters long/ 20 ft.) is kept in place by a
membrane called mesentery. The first 25 cm is called the
duodenum. The rest of the small intestine is divided up into the
jejunum and the ileum. A major portion of digestion of
carbohydrate, protein, lipid and nucleic acid digestion occurs there.
Absorption of nutrients occurs also occurs there.
Acidic chyme passes into the
duodenum which causes the
secretion of two hormones
(CCK and secretin). Both of
these hormones causes stomach
to stop peristalsis and HCl
production.
Cholecystokinin (CCK) is
induced by the presence of fat
It causes the release of
pancreatic enzymes and the
contraction of the gall bladder. Secretin is induced by the acid in the
chyme. It causes the pancreas to release sodium bicarbonate to
neutralize the acid in the chyme.
The small intestine produces digestive enzymes. The pancreas also
produces digestive enzymes that are put into the small intestine.
Pancreas-is an accessory organ that secretes a number of enzymes
through the pancreatic duct. It also secretes sodium bicarbonate to
neutralize acid.
Enzymes secreted by the pancreas
1. trypsin- breaks protein into smaller polypeptide chains.
2. Chymotrypsin- breaks proteins
into smaller polypeptide chains
difference is where the enzymes
cut the chain. Note-both are
secreted a zymogens (pre-cursor
enzymes) and then once in the
small intestine they become
activated.
3. Pancreatic lipase- Breaks down
fats into fatty acids and glycerol
4. Pancreatic amylase- Breaks
starch down to maltose
5. Nucleases- breaks down nucleic
acids into nucleotides
The pancreas also produces
hormones insulin and glucagon.
These regulate glucose levels in
the blood
Liver- secrets bile which is stored in gallbladder. Then the bile moves
from the gallbladder to the small intestines.
Bile emulsifies fats so that lipase can break fats down.
Small intestine- has glands which also secretes digestive enzymes.
1. Dipeptidases- dipeptides-> single amino acids
2. Carboxyl peptidase removes the end amino acid off of protein
chain at the carboxyl end.
3. Amino peptidase removes the end amino acid off of a protein at
the amine end
4. Maltase- maltose-> 2glucose
5. Lactase- lactose-> glucose and galactose
6. Sucrase- sucrose-> glucose and fructose
The small intestine is like a “wrinkled-terry cloth towel” The large
folds increase surface area. Each finger-like projection (like the ones
found on the towel) is termed a villus. The villi are lined with
epithelial cells. The epithelial membrane facing the lumen of the
small intestine is also folded. These folds are called microvilli. All the
folding increases the surface area. A villus inside has structure
called a lacteal which is connected to a lymph vessel.
The lacteal is surrounded by a capillary bed. The venule takes blood
from the intestines to the hepatic portal vein, which takes the blood
to the liver. The epithelial cells absorb nutrients by different
methods.
Fatty acids and monoglycerides are absorbed by passive diffusion in
the epithelial cell. Then are reassembled into triglycerides. These
Triglycerides combines with phospholipids, cholesterol and proteins to form a
chylomicron, which is transported to the
lacteal and not the capillary beds. The
lacteal puts the fat into the lymphatic
system, which will dump the fats into a
major vein near the heart.
All the other nutrients move from the
epithelial cell to the capillary bed. How
the other nutrients move into the
epithelial cell from the lumen of the small
intestines, varies. Fructose is transported
by facilitated diffusion and then
transferred to the capillary bed. Glucose,
amino acids and vitamins are
transported by active transport.
Large Intestine- six feet long and its purpose
is H2O and mineral reabsorption allowing
fecal matter to become a semi-solid to go out
cecum, Parts in humans 1. ascending, 2.
transverse, 3. descending 4.rectum- anal
canal- anus When food leaves the small
intestine it passes into the large intestine or
colon.
The main job of the colon is to remove water
from the undigested material to produce
feces. Note that a major part of fecal matter
is dead bacteria that live in the digestive tract
and not undigested food matter. The greater
the plant matter in the diet, the less compact
the fecal matter. THIS is a GOOD thing.