Chapter
14 Digestion and Energy Metabolism
I.
Overview
1.
Digestion is a process to
break down large particles and high-molecular-weight substances into small
molecules.
2.
The epithelial cells of
digestion that line the gastrointestinal tract (or GI tract) are composed of
many cell types: absorptive cells that take up nutrients, glands that secrete
chemicals (mucus, acid, ions, and enzymes), muscles that control the GI tract
shape and motility, and nerves that regulate GI tract function.
3.
Egestion is the expulsion of
undigested food (feces) from the digestive tract.
II.
Digestion
1.
The digestion system contains
the GI tract (mouth, pharynx, larynx, esophagus, stomach, small intestine, large
intestine, and anus) and accessary organs (salivary glands, liver, gallbladder,
and pancreas).
2.
The early embryonic gut is
derived from endoderm, and divided into 3 regions: foregut, midgut, and hindgut.
(1)
The foregut endoderm gives
rise to the esophagus, stomach, anterior region of the duodenum of the small
intestine, pancreas, and liver.
(2)
The midgut endoderm develops
into the posterior part of the duodenum, the remainder of the small intestine
(jejunum and ileum) and most of the large intestine.
(3)
The hindgut endoderm develops
into the remainder of the colon and rectum.
3.
The surface of the gut has a
complex topography that serves to maximize surface area.
(1)
The human small intestine's
total surface area is about 300 m2, the area of a tennis court.
(2)
Villi (singular: villus) are
fingerlike projections on the surface of small intestine.
The surface of each enterocyte of a
villus contains many microvilli.
4.
Specialized compartments
increase the efficiency of digestion.
(1)
Birds and bony fish possess
ceca (singular: cecum) that branch from the GI tract and contain bacteria that
help in digestion. The crop is an outpouching of the esophagus that enables a
bird to store partially digested food.
(2)
Ruminants (cows, deer,
giraffe, and sheep) possess a modified digestric stomach that allow vegetation
to be more effectively digested.
1)
The modified stomach is
composed of 4 chambers divided into 2 functional groups.
2)
Vegetation passes through the
esophagus into the first pair of compartments: the rumen and reticulum. These
two regions house the fermentative bacteria that digest cellulose. Ruminants can
regurgitate food from the rumen back to the mouth, where it can chew the
partially degraded material again.
3)
The second pair of
compartments is the omasum and abomasum. The abomasum serves as the glandular
stomach, secreting digestive enzymes.
5.
Salivary glands secrete water
and digestive enzymes.
(1)
Saliva secreted from salivary
glands can dissolve food molecules, including mucus to moisten food particles
and amylase to partially digest polysaccharides.
(2)
At least 2 types of cells can
be found in salivary glands: mucus-secreting cells (mucous cells) and serous cells to secreted
the degradative enzymes.
(3)
A dog has 4 pairs of salivary
glands: parotid glands, orbital glands, sublingual glands, and mandibular
glands.
(4)
An average human may secrete
more than 1 liter of water in saliva every day.
6.
The stomach secretes acid and
mucus.
(1)
Stomach can store, dissolve,
and partially digest food, and regulate the rate of movement.
(2)
The surface of the stomach is
composed of columnar epithelial cells linked together via tight junctions.
(3)
There are 4 main types of
cells cover the surface of the stomach (gastric pits in mucosa):
1)
Mucous (neck) cells, also known
as goblet cells, are found near the gastric pit opening and secrete an acid type
of mucus.
2)
Parietal cells in the middle
of the pit secrete acid, mainly HCl (to dissolve food and kill bacteria).
3)
Chief cells are found near the
base of the pit and secrete digestive enzymes, primarily the pepsinogen (an
inactive proenzyme of pepsin).
4)
Enteroendocrine cells secrete
several hormones into the blood in response to stomach contents, such as
gastrin.
(4)
There are 4 layers of the
stomach:
1)
Mucosa (mucous, parietal,
chief, and enteroendocrine cells)
2)
Submucosa (connective tissues
with blood and lymphatic vessels, and nerve cells)
3)
Muscularis (oblique, circular,
and longitudinal smooth muscles, and nerve cells)
4)
Serosa (connective tissues)
7.
Most nutrients are hydrolyzed
and absorbed in the small intestine.
(1)
There are 4 major layers of
the intestine:
1)
Mucosa (enterocytes are the
absorptive cells with microvilli, mucus-secreting goblet cells, enteroendocrine
cells, Paneth cells which secrete antimicrobial molecules, and stem cells that
divide and differentiate to other cell types)
2)
Submucosa (connective tissues
with blood and lymphatic vessels, and nerve cells)
3)
Circular smooth muscles
4)
Longitudinal smooth muscles
(2)
The crypt of Lieberkühn
is a pit at the base of intestinal villi (mucosa).
(3)
The center of each villus is
occupied both by lacteal (absorb fat) and a capillary network (absorb others).
(4)
Duodenal glands in submucosa
secrete basic mucus to neutralize the acid arriving from the stomach.
(5)
The smooth muscles control the
movement of food along the GI tract.
(6)
Large intestine can absorb
salt and water.
8.
Liver contains both endocrine
and exocrine systems.
(1)
In the exocrine part, liver
cells (hepatocytes) produce bile which is a complex solution containing
bicarbonate ions, cholesterol, phospholipids, bile pigments, and bile salts.
(2)
Only 2 types of molecules in
bile play a role in digestion: phospholipids and bile salts. Both are
amphipathic molecules with nonpolar regions that bind to fats and polar regions
that interact with water.
(3)
Phospholipids, such as
lecithin, help in the uptake of lipids.
(4)
Bile salts help emulsify fats
in the duodenum.
9.
The gallbladder stores and
concentrates bile, and then empties into the duodenum via the bile duct.
10.
Pancreas contains both
endocrine and exocrine systems.
(1)
In the exocrine part, pancreas
secretes numerous digestive enzymes into the duodenum.
(2)
Proteases are produced in the
form of inactive proenzymes. This reduces the risk that pancreas will digest
itself. For example, trypsin is secreted as the inactive precursor trypsinogen.
When trypsinogen enters the intestinal lumen, enterokinase converts trypsinogen
to the active trypsin. Trypsin in turn activates two other pancreatic enzymes,
(procarboxypeptidase into) carboxypeptidase and (chymotrypsinogen into)
chymotrypsin.
11.
In conclusion, most nutrients
are degraded and absorbed in the duodenum (and jejunum).