685
CHAPTER SEVENTEEN
Digestive System
In addition to absorbing the products of carbohydrate,
protein, and fat digestion, the intestinal villi absorb elec-
trolytes and water. Certain ions, such as those of sodium,
potassium, chloride, nitrate, and bicarbonate, are readily
absorbed; but others, including ions of calcium, magnesium,
and sulfate, are poorly absorbed.
Electrolytes are usually absorbed by active transport, and
water by osmosis. Thus, even though the intestinal contents
may be hypertonic to the epithelial cells at F rst, as nutrients
and electrolytes are absorbed, the contents become slightly
hypotonic to the cells. Then, water follows the nutrients and
electrolytes into the villi by osmosis.
Table 17.10
summa-
rizes the absorption process.
framework and secretes HDL molecules into the blood-
stream. As it circulates, the HDL picks up cholesterol from
peripheral tissues and returns the cholesterol to the liver.
Here, the cholesterol molecules enter liver cells by receptor-
mediated endocytosis. The liver disposes of the cholesterol it
obtains in this manner by secreting it into bile or by using it
to synthesize bile salts.
The intestine reabsorbs much of the cholesterol and
bile salts in bile, which are then transported back to the
liver, and the secretion-reabsorption cycle repeats. During
each cycle, some of the cholesterol and bile salts escape
reabsorption, reach the large intestine, and are eliminated
with the feces.
Lipase
C
17
H
35
C
H
C
HO
HO
HO
H
C
H
H
C
H
HO
C
C
17
H
35
COO
H
H
C
H
C
17
H
35
COO
3H
2
O
Lipase
++
Water
Fatty acids
Glycerol
Fat
C
H
C
17
H
35
COO
C
H
O
C
17
H
35
C
HO
C
O
C
17
H
35
C
HO
C
O
FIGURE 17.41
Fatty acids and glycerol result from fat digestion. Intestinal villi absorb them, and most are resynthesized into fat molecules before
they enter the lacteals.
Chylomicrons
leave epithelial cell
and enter lacteal
Fatty acids
Fatty acids
resulting from fat
digestion enter
epithelial cell
Lymph in lacteal
transports chylomicrons
away from intestine
Fatty acids are
used to synthesize
fats in endoplasmic
reticulum
Lacteal
Lumen of
intestine
To blood
Nucleus
Endoplasmic
reticulum
Epithelial
cell
Chylomicrons
Lymph
1
2
Fats collect in clusters
encased in protein to
form chylomicrons
3
4
5
FIGURE 17.42
Fat absorption takes place in several steps.
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