The beaver pond analogy of blood glucose control.

The concentration of glucose is tightly regulated in the blood by a complicated set of physiological variables. To provide students with a means to more readily understand these complex mechanisms, the control of the water level in a beaver pond is presented as an analogy. A beaver must maintain a constant water level in the pond for the proper functioning of the lodge, just as blood glucose is maintained for, among other reasons, brain function. The beaver controls the water level by changing outflow over the dam and inflow from stream beds. Water flow over the dam is analogous to glucose leaving the blood for tissues, which is controlled by insulin. Inflow of water from streams is analogous to glucose absorption from the gastrointestinal tract and glucose release from the liver, the latter being controlled by glucagon and other counterregulatory hormones. The analogy is extended by considering the effects of exercise in normal and diabetic individuals on blood glucose levels.