Chapter 23
The Digestive System
897
23
then binds to specific mucosal receptor sites in the terminal il-
eum, which trigger its active uptake by endocytosis.
Electrolyte Absorption
Absorbed electrolytes come from both ingested foods and gas-
trointestinal secretions. Most ions are actively absorbed along
the entire length of the small intestine. But absorption of iron
and calcium is largely limited to the duodenum.
As we mentioned earlier, absorption of sodium ions in the
small intestine is coupled to active absorption of glucose and
amino acids. For the most part, anions passively follow the
electrical potential established by sodium transport. In other
words, Na
1
is actively pumped out of the epithelial cells by a
Na
1
-K
1
pump a±er entering those cells. Chloride ions are also
transported actively, and in the terminus of the small intestine
HCO
3
2
is actively secreted into the lumen in exchange for Cl
2
.
Potassium ions move across the intestinal mucosa passively
by facilitated diffusion (or via leaky tight junctions) in response
to changing osmotic gradients. As water is absorbed from the
lumen, rising potassium levels in chyme create a concentration
Nucleic Acid Absorption
Special carriers in the epithelium of the villi actively transport
the breakdown products of nucleic acid digestion—pentose
sugars, nitrogenous bases, and phosphate ions—across the epi-
thelium. Tese then enter the blood.
Vitamin Absorption
Te small intestine absorbs dietary vitamins, and the large intes-
tine absorbs some of the K and B vitamins made by its enteric
bacterial “guests.” As we already noted, fat-soluble vitamins (A,
D, E, and K) dissolve in dietary fats, become incorporated into
the micelles, and move across the villus epithelium passively (by
diffusion). It follows that gulping pills containing fat-soluble vi-
tamins without simultaneously eating some fat-containing food
results in little or no absorption of these vitamins.
Most water-soluble vitamins (B vitamins and vitamin C)
are absorbed easily by diffusion or via specific active or passive
transporters. Te exception is vitamin B
12
, which is a very large,
charged molecule.
Intrinsic factor
, produced by the stomach,
binds to vitamin B
12
. Te vitamin B
12
–intrinsic factor complex
Capillary
Na
+
-glucose
symport
transporter
Facilitated
diffusion
transporter
Glucose
Fructose
Galactose
Brush border of
intestinal cell
Na
+
Na
+
Na
+
Na
+
K
+
2
As Na
+
moves across the
membrane through a
membrane cotransporter
protein (in this case SGLT), it
drives glucose against its
concentration gradient into
the cells.
1
The Na
+
-K
+
pump
stores energy that drives
glucose (and galactose)
uptake by creating a
steep concentration
gradient for Na
+
entry
into intestinal cells.
3
Fructose enters the
cell by facilitated
diffusion.
4
All three
monosaccharides exit across
the basolateral membrane
via facilitated diffusion on
the GLUT2 sugar transporter.
GLUT2
Na
+
-K
+
pump
ATP
Figure 23.35
Absorption of monosaccharides.
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