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The Human Gastrointestinal (GI) Tract
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Ingestion
Food placed in the mouth is
ground into finer particles by the teeth,
moistened and lubricated by saliva (secreted by three
pairs ofsalivary glands) small amounts of starch are digested by the amylase
present in saliva
the resulting bolus of food is swallowed into the
esophagus and
carried by peristalsis to the stomach.
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The stomach
a J-shaped area of thegastrointestinal (GI) tract thatsits in the upper left side ofthe abdomen.
It is the most dilated area ofthe tract and has severalregions - the fundus (theexpanded part of thestomach), the cardia, thebody, and the funnel-shapedantrum.
Food enters the stomach
through the cardiac orgastroesophageal sphincterand leaves it via the pyloricsphincter.
approximately 25cm long
and can expand to hold up to
4L of food and drink,
although its empty volume is
only 50ml collapses in on itself when it
is empty and forms folds or
rugae.
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Absorption in the stomach
As the contents of the stomach become
thoroughly liquefied, they pass into the
duodenum, the first segment
Two ducts enter the duodenum:
one draining the gall bladder and hence the
liver
the other draining the exocrine portion of
the pancreas.
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FUNCTIONS OF THE STOMACH
Food reservoir
Absorption
Mucus secretion.
Gastric juice secretion
The wall of the stomach is lined with millions ofgastric glands, which together secrete
400800 ml of gastric juice at each meal. Several kinds of cells are found in the gastricglands
parietal cells
chief cells
-It stops the proliferation of bacteria in the stomach;
- It inactivates salivary amylase, mixed with the food in the mouth;
- It curdles milk to prepare it for digestion;
- It tenderises proteins (by denaturing them);- It converts the pepsinogen produced by chief cells into pepsin, which starts to digest
protein;
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FUNCTIONS OF THE STOMACH
Churning food
- Food that enters the stomach is mixed with and diluted by the
gastric secretions into a thick soup-like substance called chyme. The
chyme is churned by waves of peristalsis. Each wave lasts about half a
minute and 'flows' from the top of the stomach to the bottom;
Production of intrinsic factor. The parietal cells in the stomach also produce intrinsic factor, which is
essential for the absorption of vitamin B12 from the ileum in the small
intestine. Vitamin B12 is necessary for the healthy functioning of nerve
fibres in the body, for the formation of myelin sheaths on the nerves in the
spinal cord and for the formation of red blood cells.
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The liver
The liver secretes bile. Between meals it accumulates in
the gall bladder. When food, especially when it contains
fat, enters the duodenum, the release of the hormone
cholecystokinin (CCK) stimulates the gall bladder to
contract and discharge its bile into the duodenum.
Bile contains ;
bile acids
bile pigments
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The pancreasThe pancreas consists of clusters if endocrine cells (
the islets ofLangerhans) and exocrine cells whosesecretions drain into the duodenum.
Pancreatic fluid contains:
sodium bicarbonate (NaHCO3). This neutralizesthe acidity of the fluid arriving from the stomach
raising its pH to about 8.
pancreatic amylase. This enzyme hydrolyzes
starch into a mixture ofmaltose and glucose.
pancreatic lipase. The enzyme hydrolyzes
ingested fats into a mixture offatty acids and
monoglycerides. Its action is enhanced by the
detergent effect of bile.
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The secretion of pancreatic fluid is
controlled by two hormones:
secretin, which mainly affects the release of
sodium bicarbonate, and
cholecystokinin (CCK), which stimulates
the release of the digestive enzymes.
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The Small Intestine
Digestion within the small
intestine produces a
mixture of disaccharides,
peptides, fatty acids, and
monoglycerides. The finaldigestion and absorption
of these substances occurs
in the villi, which line the
inner surface of the smallintestine.
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Incorporated in the plasma membrane of the
microvilli are a number of enzymes that complete
digestion:aminopeptidases attack the amino terminal (N-
terminal) of peptides producing amino acids.
disaccharidasesThese enzymes convertdisaccharides into their monosaccharide subunits.
omaltase hydrolyzes maltose into glucose.
osucrase hydrolyzes sucrose (common table sugar)into glucose and fructose.
olactase hydrolyzes lactose (milk sugar) into glucose
and galactose.
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Fructose simply diffuses into the villi, but both
glucose and galactose are absorbed by
active transport.
fatty acids and monoglycerides. These become
resynthesized into fats as they enter the cells of the
villus. The resulting small droplets of fat are thendischarged by exocytosis into the lymph vessels,
called lacteals, draining the villi.
Humans with a rare genetic inability to form
microvilli die of starvation.
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The large intestine (colon)
The large intestine receives the liquid residue after digestion and absorption arecomplete. This residue consists mostly of water as well as materials (e.g. cellulose)that were not digested. While the contents of the small intestine are normally sterile,the colon contains an enormous population of microorganisms.
Reabsorption of water is the chief function of the large intestine. Thelarge amounts of water secreted into the stomach and small intestine
by the various digestive glands must be reclaimed to avoiddehydration. If the large intestine becomes irritated, it may dischargeits contents before water reabsorption is complete causing diarrhea.On the other hand, if the colon retains its contents too long, the fecalmatter becomes dried out and compressed into hard masses causing
constipation.
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