CHOLESTEROL
Cholesterol is a a lipid found in the cell membranes
of all body tissues, known as a sterol [steroid and
alcohol].
Most cholesterol is synthesized by the body from the
food we eat, and transported in the blood plasma to
the tissues of the body.
Cholesterol is found in higher concentrations in those
tissues which either synthesize it more readily, or
have densely-packed membranes. This includes the liver,
spinal cord, brain, and arterial plaques.
Cholesterol is insoluble in blood, but is transported
in the circulatory system bound to a water-soluble lipoprotein.
When levels of cholesterol build up in the body beyond
healthy levels, they stick to these membranes and artery
walls interfering with normal body transport functions.
This reasults in cardiovascular disease.
Purpose Of Cholesterol
Cholesterol is required in the membrane of cells for
normal cellular function. The main functions of Cholesterol
are:
- Build and maintain cell membranes
- Regulate membrane fluidity over a wider range of
temperatures
- Aid in the manufacture of bile stored in the gallbladder
and helps digest fats),
- Metabolism of fat soluble vitamins, including vitamins
A, D, E and K.
- The major precursor for the synthesis of vitamin
D and of the various steroid hormones (which include
cortisol and aldosterone in the adrenal glands, and
the sex hormones progesterone, the various estrogens,
testosterone, and derivatives).
- Some research suggests that cholesterol may also
act as an antioxidant.
- Cholesterol has also been implicated in cell signalling
processes, where it forms lipid rafts in the plasma
membrane.
- Reduces the permeability of the plasma membrane
to hydrogen ions (protons) and sodium ions
How Cholesterol Is Transported
Cholesterol is minimally soluble in water; it cannot
dissolve and travel in the water-based bloodstream.
Instead, it is transported in the bloodstream by water-soluble
lipoproteins. These carry both cholesterol and triglycerides
internally. The apolipoproteins forming the surface
of the given lipoprotein particle determine from what
cells cholesterol will be removed and to where it will
be supplied.
Lipoproteins
There are two main forms of lipoproteins:
- Low density lipoproteins (LDL),
which carry cholesterol from the liver to the cells.
- High density lipoproteins (HDL),
which return the extra cholesterol to the liver.
The largest lipoproteins, which primarily transport
fats from the intestinal mucosa to the liver, are called
chylomicrons. They carry mostly fats in the form of
triglycerides and cholesterol. In the liver, chylomicron
particles release triglycerides and some cholesterol.
LDL - The Bad Choleseterol
The liver converts unburned food metabolites into very
low density lipoproteins (VLDL) and secretes them into
plasma. Here, they are converted to low-density lipoprotein
(LDL) particles and non-esterified fatty acids, which
can affect other body cells.
In healthy individuals, the relatively few LDL particles
are large. In contrast, large numbers of small dense
LDL (sdLDL) particles are strongly associated with the
presence of disease within the arteries.
For this reason, LDL is referred to as "bad cholesterol".
HDL - Good Cholesterol
High-density lipoprotein (HDL) particles transport
cholesterol back to the liver for excretion. They vary
considerably in their effectiveness in doing so.
Having high volume of large HDL particles correlates
with better health; hence it is commonly called "good
cholesterol".
Having small amounts of large HDL particles is independently
associated with disease within the arteries.
Total Blood Cholesterol Tests
Cholesterol tests measure 'blood lipids'; all the fatty
substances in the blood, including HDL cholesterol,
LDL cholesterol and triglycerides.
Also see: Cholesterol
Tests
Related Reading
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