The Digestion and Absorption of Food
539
The chylomicrons released from the epithelial cells pass
into lacteals—lymphatic capillaries in the intestinal villi—
rather than into the blood capillaries. The chylomicrons cannot
enter the blood capillaries because the basement membrane (an
extracellular glycoprotein layer) at the outer surface of the cap-
illary provides a barrier to the diffusion of large chylomicrons.
In contrast, the lacteals do not have basement membranes and
have large, slit pores between their endothelial cells that allow
the chylomicrons to pass into the lymph. The lymph from the
small intestine, as from everywhere else in the body, eventually
empties into systemic veins via the thoracic duct. In Chapter
16 we describe how the lipids in the circulating blood chylomi-
crons are made available to the cells of the body.
Figure 15–12
summarizes the pathway fat takes in mov-
ing from the intestinal lumen into the lymphatic system.
Vitamins
The fat-soluble vitamins—A, D, E, and K—follow the pathway
for fat absorption described in the previous section. They are
solubilized in micelles; thus, any interference with the secre-
tion of bile or the action of bile salts in the intestine decreases
the absorption of the fat-soluble vitamins (
malabsorption
).
Malabsorption syndromes can lead to defi ciency of fat-soluble
vitamins. For example,
nontropical sprue
—a loss of intestinal
surface area due to a sensitivity to the wheat protein
gluten
can lead to vitamin D malabsorption, which ultimately results in
a decrease in calcium absorption in the GI tract (Chapter 11).
With one exception, water-soluble vitamins are absorbed
by diffusion or mediated transport. The exception, vitamin
B
12
(cyanocobalamin), is a very large, charged molecule. To
Micelle
Micelle
reformation
Micelle
breakdown
Fatty acids
Monoglyceride
Triglyceride
Bile salt
Emulsion droplet
Lipase
Diffusion
Intestinal
epithelial
cells
Figure 15–11
The products of fat digestion by lipase are held in solution in the
micellar state, combined with bile salts and phospholipids. For
simplicity, the phospholipids and colipase (see text) are not shown,
and the size of the micelle is greatly exaggerated. Note that micelles
and free fatty acids are in equilibrium so that as fatty acids are
absorbed, more can be released from the micelles.
Figure 15–12
Summary of fat absorption across the epithelial cells of the small
intestine.
Lumen of
small
intestine
Epithelial
cell
Lacteal
Diffusion
Free molecules of fatty acids
and monoglycerides
Micelles
Bile salts
Pancreatic lipase
Bile salts
Phospholipids
Emulsion
droplets
Fatty acids and
monoglycerides
Triglyceride synthetic
enzymes in endoplasmic
reticulum
Droplets of triglyceride
enclosed by membrane
from the endoplasmic
reticulum
Chylomicron
Fat droplet
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