Plasma components. Lipids
The concentration of lipids in plasma varies, particularly
in relation to meals, but ordinarily does not exceed
one gram per 100 millilitres. The largest fraction consists
of phospholipids, complex molecules containing phosphoric
acid and a nitrogen base in addition to fatty acids
and glycerol. Triglycerides, or simple fats, are molecules
composed only of fatty acids and glycerol. Free fatty
acids, lower in concentration than triglycerides, are
responsible for a much larger transport of fat. Other
lipids include cholesterol, a major fraction of the
total plasma lipids. These substances exist in plasma
combined with proteins of several types as lipoproteins.
The largest lipid particles in the blood are known as
chylomicrons and consist largely of triglycerides; after
absorption from the intestine they pass through lymphatic
channels and enter the bloodstream through the thoracic
lymph duct. The other plasma lipids are derived from
food or enter the plasma from tissue sites.
