BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.2. Regulation of Blood Glucose Levels

Glucose is one of the most vital metabolites found in the Blood. Its concentration (level) must be strictly controlled, as Glucose serves as the primary substrate for tissue Respiration and must be continuously supplied to the Cells. Brain cells are particularly sensitive to glucose deficiency, as they are incapable of using any other metabolites as an energy source. A lack of glucose leads to unconsciousness. The normal blood glucose ("sugar") level is approximately 90 mg per 100 ml (90 mg%), though it can safely fluctuate for a human from 70 mg% on an empty Stomach to 150 mg% after a meal. The sources of blood glucose and its interrelationships with other metabolites will be discussed in Section 19.6.2.

The Regulation of Blood glucose levels is a prime example of a complex, endocrine-controlled homeostatic mechanism involving the coordinated secretion of at least six Hormones and two negative feedback loops. An increase in blood glucose levels (hyperglycemia) stimulates Insulin secretion (Section 17.6.6), while a drop in these levels (hypoglycemia) suppresses insulin release and triggers the secretion of Glucagon (Section 17.6.6) along with Other Hormones, such as adrenaline, which raise blood glucose levels. The General scheme of this regulatory system is shown in Fig. 19.4.

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Fig. 19.4. Regulation of blood glucose levels. ACTH — adrenocorticotropic hormone (corticotropin); TSH — thyroid-stimulating hormone.