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 esophagusEsophagus
-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 enters 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 upperpart 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
soupy consistency called 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 cause 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- break 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 fingerlike 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.