T h e s m a ll i n t e s t i n e a b s o r b s n u t r i e n t s •
F ig u r e 1 4 . 8
The absorptive cells on the villi of the small intestine absorb various molecules through
simple or facilitated diffusion as well as active transport. Depending on the nature of the
absorbed substance, it passes from the absorptive cell either into the blood and hepatic
portal circulation (amino acids, sugars) or into the lacteal and lymphatic system (lipids).
Absorbed substances must then pass from the intesti-
nal epithelial cells in the villi into the circulation. Amino
acids, sugars, and short-chain fatty acids pass from the
epithelial cells into the blood by diffusion. In the blood,
these substances travel through the hepatic portal circu-
lation to the liver for processing and then subsequently
into the general circulation. Fats are handled differently.
Inside the absorptive cells, various lipids (for example,
long-chain fatty acids, monoglycerides, cholesterol, tri-
glycerides)
are
packaged
into protein-coated
spheres
called chylomicrons. Chylomicrons exit the absorptive
cells by exocytosis into lymph vessels within villi called
lacteals. Chylomicrons travel through the lymphatic sys-
tem and into the general circulation to adipose tissue and
the liver. There, the chylomicrons are removed, and lipids
are stored for future use.
Let's Journey Through the Digestive System 411
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