Page 297 - Biology_F5

Biology for Advanced Level Secondary Schools

and raising pH of its contents. Stretching bile and mineral salts from gall bladder and
activates the short and long reflexes that pancreas respectively. CCK stimulates the
lead to the production of acetylcholine secretion of enzymes from pancreas and
secreted by the parasympathetic nerve contraction of gall bladder to release bile.
fibres. The acetylcholine and histamine Moreover, secretin and CCK suppress
from the gastric glands together with gastric secretion and motility in which a
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gastrin stimulate parietal cells to secrete decline in gastrin secretion and contraction
hydrochloric acid. The chief cells of pyloric sphincter will limit admission
secrete pepsinogen in response to gastrin of more chyme into the duodenum. This
secretion. Digestion of protein into gives the duodenum ample time to work
peptides and amino acids stimulate gastrin on the chyme it has received before
cells directly to secrete more gastrin receiving more. The enteroendocrine cells
to accelerate further protein digestion. also secrete gastric-inhibitory peptide,
Normally, peptides buffer the stomach also called gastrointestinal inhibitory
acidity; therefore, as they leave the peptide (GIP), which inhibits the secretion
stomach, acidity increases and as pH gets of gastric acid in the stomach. Also GIP
below 2, a negative feedback is triggered stimulates insulin secretion in preparation
to inhibit the parietal and gastrin cells. for processing nutrients that are about to
This process winds up the gastric phase be absorbed by the small intestine. Trypsin
as the need for pepsin and HCl declines. in the duodenum inhibits the release of
Furthermore, when fat-containing food enzymes via the inhibition of CCK.This
enters the stomach, the hormone called is a feedback control mechanism which
enterogastrone or gastric inhibitory limits the quantity of enzymes in the
peptide is released from the wall of the small intestine and may have a protective
stomach. This hormone decreases the flow function.
of gastric juice and reduces movement
of the muscles of the stomach (churning Exercise 5.5
motions) or gastric peristalsis.

Intestinal phase: This phase occurs 1. Describe the structure of squamous
when chyme arrives in the first region epithelial tissue of the digestive
of the small intestine called duodenum system.
and triggers gastric activity and nervous 2. Explain how glandular tissues of the
reflexes. Food material in the duodenum
stimulates both the alkaline and enzyme- alimentary canal are adapted to the
rich components of pancreatic juice. The functions they perform.
alkaline component of pancreatic juice is 3. Describe the functions of the
secreted in response to the presence of acid mammalian stomach.
in the duodenum. The acidified chyme in
the duodenum triggers the secretion of 4. Explain why surgical removal of the
secretin and cholecystokinin (CCK) or stomach in human is not necessarily
pancreozymin (PZ) from the duodenal fatal.
walls. Secretin causes the production of

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