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18. 1 Introduction




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Answers

1. Brunner's glands are only found in the duodenum. Villi
are only present in the small intestine. The serosa is the
outermost layer of the GI tract wall.

a. False;

b. False;
с True;

d. True;

e. False.

2. Swallowing is a complex reflex, the first part of which is
under voluntary control. The bolus is moved along the
esophagus by a wave of peristaltic contraction. The saliva
contains an a-amylase. The pH of saliva rises as its
secretion accelerates due to an increased bicarbonate
concentration.

a. False;

b. False;
с True;

d. True;

e. True.

3. Intrinsic factor is secreted by the parietal cells. CCK
inhibits gastric secretion as part of the enterogastric
reflex. Gastric secretion begins before food reaches the
stomach (the cephalic phase of secretion). The intestinal
phase contributes little to overall gastric secretion.

a. False;

b. True;
с False;

d. True;

e. False.

4. During vomiting acid is lost from the stomach. As a
result, metabolic alkalosis is seen, with a rise in plasma
pH and a fall in bicarbonate.

a. True;

b. True;

432 18 The gut and nutrition

с True;

d. False;

e. True.

5. Pancreatic acinar cells store proteolytic enzymes as inac­
tive precursors (trypsinogens) to avoid self-digestion.
CCK stimulates pancreatic secretion. Pancreatic juice
contains the enzymes required for protein digestion. In
their absence protein utilization by the body will be
impaired.

a. False;

b. False;
с True;

d. True;

e. True.

6. Bile is concentrated many times in the gallbladder.
Chylomicrons are found in the lacteals after a fatty meal.
Micelles are the aggregations of bile salts and fats formed
in the small intestine. They form a stable suspension that
permits the digestion of fats by pancreatic lipases.

a. True;

b. True;
с False;

d. True;

e. False.

7. Bile salts are formed in the liver. Bile pigments are the
breakdown products of hemoglobin. Reabsorption of bile
salts stimulates further bile secretion via the entero-
hepatic circulation. Fat-soluble vitamins are absorbed
along with fats and therefore depend upon bile salts in
the same way. Bile salts are amphipathic, i.e. they possess
both hydrophobic and hydrophilic regions.

a. False;

b. True;
с True;

d. True;

e. False.

8. The enzymes of the small intestine are associated with
the brush border of the epithelial cells. Virtually all the
digestion products of ingested carbohydrate are absorbed
in the small intestine.

a. True;

b. False;
с False;

d. True;

e. True;

f. True.

9. Intrinsic factor, secreted by the parietal cells, is needed
for the absorption of vitamin B12. Following gastrectomy,
large amounts of hypertonic chyme enter the duodenum.
Water enters the gut lumen down the osmotic gradient,

leaving the plasma more concentrated. This is known as 'dumping'. Parasympathetic activity stimulates intestinal motility. Fatty chyme leaves the stomach more slowly than chyme that contains little fat.

a. True;

b. False;
с False;

d. True;

e. False.

10. Vitamin К is required for normal blood clotting.
Bleeding disorders are associated with deficiency. Vit­
amin A deficiency causes night blindness. Vitamin B12
deficiency leads to pernicious anemia. Plant foods do not
contain all the essential amino acids.

a. False;

b. False;
с False ;

d. True;

e. True.

11. Dietary fiber increases the transit time through the colon.
Vitamin К synthesis by the intestinal bacteria is import­
ant for the maintenance of normal hemostasis.

a. True;

b. True;
с False;

d. True;

e. True.

^ Data handling

1. An experiment was cattied out in which the concentration of gastrin in the plasma of a human volunteer was measured every 15 min for a period of 2 h. At time zero (0 min) the subject ate a protein-rich meal. The data are tabulated below.



Time (min)

Plasma gastrin concentration




(pg ml-1)

-60

75

^45

60

-30

60

-15

70

0

80

15

100

30

130

45

160

60

180

75

110

90

80

105

60

120

40

^ Answers to quantitative problems: 433

a. Plot the data on linear graph paper.

b. Discuss the shape of the graph in relation to the regula­
tion of gastric secretion.

с How might the shape of the graph differ if the subject had eaten a meal rich in fats rather than protein?

2. The pH of the gastric contents of a subject were measured following a meal (time zero).



Time (hours)

pH

0

1.9

0.5

1.0

1.0

3.1

1.5

2.2

2.0

1.9

2.5

1.6

3.0

1.5

3-5

1.5

4.0

1.5

a. Plot a graph to illustrate the changes in pH with time.

b. What causes the initial rise in pH?

с Why does the pH fall between 0.5 and 2.5 h?

d. What mechanism is responsible for the stable value of
intragastric pH after 2.5 h?

e. What function does the low intragastric pH serve?

Answers to quantitative problems:

  1. (b) The initial rise in plasma gastrin levels is the result of
    local reflexes in response to distention of the stomach wall.
    Subsequently, protein digestion products (mainly peptides)
    act directly on the G-cells to stimulate their secretion. The
    subsequent fall in gastrin levels is the result of inhibition
    of gastrin secretion by the low pH of the gastric con­
    tents, (c) As fats inhibit gastric emptying the secretion of
    gastrin will be more prolonged but the plasma levels will
    probably not rise so high.

  2. (b) The initial rise in intragastric pH is due to buffering
    of the gastric contents by the ingested food, (c) the sub­
    sequent fall in pH reflects the secretion of gastric acid by
    the parietal (oxyntic) cells in response to local and vagal
    reflexes. It is also the the result of increased acid secretion
    in response to gastrin secretion by the G-cells. (d) the sub­
    sequent fall in intragastric pH inhibits the secretion of
    gastrin and limits the secretion of acid by the parietal
    cells, (e) Low intragastric pH denatures dietary protein and
    renders it susceptible to attack by pepsin. It kills bacterial
    and other micro-organisms present in the stomach and
    solubilizes iron and calcium salts.
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