In negative feedback regulation, the initial stimulus is reduced by the response it provokes. The response eliminates the initial stimulus and the pathway is halted. Negative feedback is demonstrated in the regulation of red blood cell production or erythropoiesis. The kidneys monitor oxygen levels in the blood. When oxygen levels are too low, the kidneys produce and release a hormone called erythropoietin (EPO). EPO stimulates red bone marrow to produce red blood cells. As blood oxygen levels return to normal, the kidneys slow the release of EPO resulting in decreased erythropoiesis.
The early steps in the biosynthesis of steroids of both plants and animals are the same, except that in plants lanosterol is replaced by the related compound cycloartenol, which contains a three-membered ring (C9, C10, C19) in lieu of the nuclear double bond of lanosterol. The side chains of the phytosterols, such as stigmasterol, and of the sterol ergosterol of yeasts and other fungi contain extra carbon atoms that are incorporated in reactions involving S -adenosylmethionine, which donates methyl groups in numerous biological processes. Although most plant tissues contain only traces of cholesterol, this sterol is the biogenetic precursor of such important plant steroids as the sapogenins , glycosides , and alkaloids . Because pregnane derivatives are intermediates in some of these transformations, plants and animals appear to have important features of steroid metabolism in common.