Lipids in the Blood:
Lipids ingested as food are digested in the small intestine
where bile salts are used to emulsify them and pancreatic lipase
hydrolyzes lipids into fatty acids, glycerol, soaps, or mono-
and diglycerides. There is still some dispute about the lipid
form that passes through the intestinal wall -- whether as fatty
acids or as glycerides. In either case, triglycerides are found
in the lymph system and the blood.
|Quiz: According to properties studied
earlier, are lipids generally soluble or insoluble in the aqueous
portion of blood? Explain.
Since lipids are not soluble in blood, they are transported
as lipoproteins after reaction with water-soluble proteins in
the blood. Fatty acids are generally transported in this form
There is always a relatively constant supply of lipids in
the blood, although of course, the concentration increases immediately
following a meal. Lipids in the blood are absorbed by liver cells
to provide energy for cellular functions. The liver is responsible
for providing the proper concentrations of lipids in the blood.
Some lipids are utilized by brain cells to synthesize brain and
Excess lipids in the blood are eventually converted into adipose
tissue. If lipid levels in the blood become too low, the body
synthesizes lipids from other foods, such as carbohydrates, or
removes lipids from storage. The body also excretes some lipids
in the form of fats, soaps, or fatty acids as a normal component
Abnormally high levels of triglycerides and cholesterol are
thought to be involved in hardening of the arteries. Lipids may
be deposited on the walls of arteries as a partial consequence
of their insolubility in the blood.
The summary of lipid functions is given in the graphic on