blood high density lipoprotein cholesterol level Measurement of the amount of cholesterol, a eukaryotic sterol that in higher animals is the precursor of bile acids and steroid hormones and a key constituent of cell membranes, carried in high-density lipoprotein (HDL) molecules in a specified volume of blood, the fluid that circulates through the heart, arteries, capillaries and veins carrying nutrients and oxygen to the body tissues and metabolites away from them. HDL is the smallest of the major lipoprotein particles, complex molecules that consist of a protein membrane surrounding a core of lipids. The HDL class of lipoproteins, specifically the subtypes of HDL2 and HDL3, have densities between 1.063 and 1.210 g/ml. Values for density ranges of the lipoprotein classes are approximate. The classes may be defined differently by different researchers, in part because class definitions depend on the number of subclasses into which the lipoprotein fractions are divided. blood HDL level blood HDL-C blood lipoprotein cholesterol level