This is because feedback mechanisms are the organisms’ way of knowing what their body needs, when it needs it, and how much.
Two opposite hormones regulate the concentration of glucose in the blood (vital for cellular respiration and more). They both act in simple endocrine pathways that are regulated by negative feedback. Insulin triggers the uptake of glucose from the blood when the glucose is above the normal range. Glucagon triggers the release of glucose from energy stores when the glucose dips below the normal level. Diabetes mellitus is caused by the lack of insulin. This causes the blood glucose levels to rise, but cells are unable to take them up, so fat becomes the main substrate for cellular respiration. This causes dire effects in the blood.