A sterol (see also steroid) occurring widely in animal tissues and also in some plants and algae. It can exist as a free sterol or esterified with a long-chain fatty acid. Cholesterol is absorbed through the intestine or manufactured in the liver. It serves principally as a constituent of blood plasma lipoproteins and of the lipid-protein complexes that form plasma membranes. It is also important as a precursor of various steroids, especially the bile acids, sex hormones, and adrenocorticoid hormones. The derivative 7-dehydrocholesterol is converted to vitamin D₃ by the action of sunlight on skin. Cholesterol is transported in the bloodstream chiefly as lipoproteins: low-density lipoproteins (LDLs) deliver cholesterol to cells, while high-density lipoproteins (HDLs) retrieve excess cholesterol and return it to the liver. A high ratio of LDLs to HDLs increases the risk of atherosclerosis and consequent heart attack or stroke.