Animals must maintain blood glucose levels within rather narrow limits to ensure proper functioning of the nervous system. The liver plays a major role in this process.

The amounts of glucose available to the blood vary with the nutritional status. In response to dietary glucose, homeostatic mechanisms come into play to promote uptake of glucose into cells and its use by tissues, thereby lowering glucose in the blood. When glucose levels fall, several hours after a meal, other mechanisms promote both glucose release, from intracellular glycogen stores, and gluconeogenesis, so that the normal level is maintained. Some of the homeostatic mechanisms are regulated hormonally.

The most important hormone promoting glucose uptake and use is insulin, whereas both glucagon and epinephrine act conversely, to increase blood glucose levels. The major effects of these agents are summarized in Table 23.2.

Figure 23.2 illustrates the interplay between insulin and glucagon.