Abstract: A dynamical-systems model of plasma glucose concentration, and
its regulation by insulin and glucagon, is described, as pertains to types 1 and 2 diabetes. The hyperglycemic case is seen to bedependent only on insulin concentration, while the hypoglycemic case requires consideration of both insulin and glucagon concentrations.  The role of healthy $\alpha$ cells in maintaining proper levels of glucose and the hormones is also highlighted.