Page 291 - Biology_F5

Biology for Advanced Level Secondary Schools

Pepsinogen: It is produced by the chief type is the large intestine, which among
cells of the stomach wall. It is a precursor other functions, is mainly important in re-
or inactive form of pepsin. Pepsinogen absorption of water from the food residues
is activated by hydrochloric acid in the before they are egested.
stomach to form pepsin; an enzyme
responsible for the breaking down of The small intestine
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polypeptides into peptides. This part of the intestine has smaller
diameter compared to that of the large
Prorennin: This is a precursor of rennin, intestine and for this reason it is called a
the enzyme that catalyses the conversion
of soluble milk protein into an insoluble small intestine. The diameter of the small
milk protein. In other words, rennin is intestine of human being is approximately
an important enzyme in coagulation or 2.5 cm while the large intestine has a
curdling of milk in the stomach. The diameter of about 7.6 cm, which is about
coagulated milk is semi solid and it can three times wider than the small intestine.
be retained in the stomach for a relatively The small intestine is the longest of all
long time for proper digestion. This is very parts of the alimentary canal. It has the
important in lactating young mammals. length of about 9 metres in a living person.
In the stomach, mechanical digestion also The small intestine muscles are tight in
occurs due to continuous contractions the living body which makes it slightly
of the stomach wall, which enhances shorter, unlike in a dead person where it
breaking down of food mechanically. becomes somewhat longer (about twice as
The combined action of mechanical and much longer as its normal size in a living
chemical digestion produces a creamy person). This is because death makes the
paste, an acid chyme. The formation of small intestine to lose its muscle tone and
this chyme stimulates the receptors on the stretches as it becomes loose. Unlike the
stomach wall, which in turn stimulates large intestine, the small intestine has
the pyloric sphincter muscles to relax and numerous folds and projections called
allow food to pass into the duodenum. villi on its lining, which are important in
Digestion of food in the stomach may take increasing the surface area for digestion
on average about 2 to 4 hours. and absorption of food. The surface area
of the small intestine of a human is about
2
The intestines 200 m which is surprisingly closer to
The intestines are contained within the 100 times the surface area of our body
abdominal part of the body and they skin. Schematically, the small intestine is
constitute the longest and massive part of normally surrounded by the large intestine
the alimentary canal. The word intestine in three sides.
comes from a Latin word intestinum,
meaning 'gut or internal'. The intestines are The small intestine is not straight rather
of two types, namely the small intestine a coiled tube, which literally has three
which forms a vital part in the digestion major distinct regions, namely duodenum,
and absorption of foods. The second jejunum, and ileum. The sectioning of
the small intestine is based on its internal

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