Some bacteria are more demanding in the nutrients they require for growth, a quality that results in their being called fastidious (or "picky"). One way to encourage the growth of medically-important fastidious bacteria is to grow them on a medium containing defibrinated blood (blood with clotting proteins removed). Blood agar is a bright red, opaque medium. The variety of complex nutrients found in blood supports the growth of most bacteria, fastidious and otherwise, that would be encountered by students. Such media are said to be complex (incapable of being chemically recreated) and enriched (containing an uncommonly rich array of nutrients).