:-)
--
oozing on the muggy shore of the gulf coast
l...@fbtc.net
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Good-day articulators of CS,
DIGESTION,stomach
The enzyme pepsin is secreted in the inactive form known as
*pepsinogen*. It is activated by being mixed with hydrochloric acid.
Once it is activated, pepsin attacks proteins and breaks the long
polypeptide chain into smaller fragments, such as tripeptides or
dipeptides. It does not digest proteins completely, all the way down
into amino acids. The mucin in the gastric juice protects the stomach
lining from being digested by the pepsin. The mucin forms a protective
coating over the inside lining of the stomach and prevents contact
between the enzyme and the tissues. Otherwise we would digest ourselfs.
Up to this point very little of the food has been digested completely.
The molecules are still too large and complex to be absorbed into the
cells. For example, in the mouth some of the starch has been broken down
into maltose. But the maltose must still be broken down further into
glucose molecules. In the stomach, the action of pepsin causes protein
chains to break down into smaller fragments. But these fragments must be
broken down still further into amino acids. Very little has happened to
fats up to this point. Yet to be of use to the cells, they must be
broken down to fatty acids and glycerin. The bulk of digestion will take
place in the small intestine.
The food which enters the stomach is delayed there by the contraction
of the sphincter muscles at the cardiac and pyloric openings. The cavity
of the stomach is always the size of its contents, which means that when
there is no food in it , it is contracted, but when food enters, it
expands just enough to hold it. The food first taken will entirely fill
the stomach and succeeding food will be arranged in definite layers. The
first food lays next to the wall of the stomach and fills the pyloric
region,while succeeding portions are arranged regularly in the interior
(center) in concentric layers.
Within a few minutes after the entrance of food small contractions
start in the middle region of the stomach and run toward the pylorus
These contractions are regular and in the pyloric region become more
forcible as digestion progresses. As a result of these movements the
food in the pyloric portions is macerated, mixed with acid gastric
fluid, and reduced to a thin liquid mass called *chyme*. At certain
intervals the pyloric sphincter relaxes, and the wave of contraction
forces some of the chyme into the duodenum. The fundic end of the
stomach is less actively concerned with these movements but serves as a
reservoir for food which is under slight pressure, as the muscles are in
a state of continual contraction or tone. Because of lack of movement
and muscular tone in this part of the stomach, the food at the fundic
end may remain undisturbed for an hour or more and thus escape rapid
mixture with the gastric juice, which therefore penetrates slowly to the
interior of the mass. Hence salivary digestion may continue for a time
in the stomach. As the chyme is gradually forced into the duodeum, the
pressure of the fundus forces the food into the pyloric end. The time
required for stomach digestion depends upon the nature of the food
eaten. Liquids taken on an empty stomach pass the pylorus promptly.
Small meals may remain from one to four hours, but average meals stay in
the stomach from five to seven hours. The ejection of chyme through the
pylorus occurs at regular intervals and is dependent mainly upon the
strength of the peristalic wave and the nature of the chyme. Solid
particles forced against the pylorus tend to keep it closed, but a
finely divided chyme and its acidity, produce relaxation of the pyloric
sphincter. Passing into the small intestine the presence of chyme causes
a closing of the sphincter, which remains closed until the next
peristaltic wave opens it again. It is the acidity of the chyme in the
duodenum that brings about closure.
The secretion of gastric juice is constant. Even in a period of
fasting there is a small continuous secretion, but during eating and
throughout the period of digestion the rate of secretion is greatly
increased. In an ordinary meal the secretion first started is due to the
sensations of eating and the taste and odor of food, which stimulate the
sensory end organs in the mouth and nose. This *psychical* secretion
insures the beginning of gastric digestion and is supplemented by
chemical action arising in the stomach. Some foods, such as meat juices
and extracts, contain substances called *secretagogues* or hormones
which are supposed to act directly upon the nerves of the pyloric mucous
membrane and form a substance called *gastrin* or *gastric secretin*,
which is absorbed into the blood and carried to the gastric glands. This
substance stimulates the glands to secretion. When foods are eaten,
stimulating secretions occur in the following steps. first
cephalic,psychical or appetite secretion. Next the secretion due to
secretagogues in food, and then the secretion due to secretagogues in
the products of digestion, proteoses and peptones. Intestinal hormones
also affect gastric secretion. Fats inhibit gastric secretion by the
production of intestinal *chalones* (enterogastrone).
Bless you Bob Lee
--
oozing on the muggy shore of the gulf coast
l...@fbtc.net
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