Why do we need a homeostatic system to regulate blood glucose concentration? 

It is essential that the blood glucose concentration is maintained within the normal range. Blood glucose concentration cannot be allowed to fall to low, because the neurones in the brain are relying on glucose to produce the energy they require to function. In previous questions you have looked at the idea that the cell membrane potential depends on the pumping action of the Na-K ATPase carrier in the cell membrane. Nerve function depends on maintenance of the cell membrane potential, so you can imagine that the pump is very active in neurones, and that therefore neurones have high energy requirements. They also have high oxygen requirements because the high levels of metabolic activity, and that is one of the reasons why the brain is the first organ to experience irreversible damage when the circulation fails. A low blood glucose concentration is known as a state of hypoglycaemia.

An elevated blood glucose concentration (hyperglycaemia) also creates problems. The higher than normal concentration of glucose molecules in the blood causes an osmotic effect that shifts water in the body. Free water with electrolytes and glucose is lost via urinary excretion producing causing moderate to severe dehydration. Hyperglycemia can be due to reduced insulin secretion, decreased glucose utilization, or increased glucose production. The excretion of excessive amounts of glucose in the urine is known as glycosuria. The abnormally high number of glucose molecules in the urine has an osmotic effect that reduces water reabsorption in the kidneys. Glucose is filtered out of the blood in the filtration area of the kidney (the glomerulus) but is subsequently reabsorbed in the tubule system. People with diabetes mellitus develop like a serious because the reabsorbed if capacity of the tubules is insufficient to cope with the large number of glucose molecules passing through the tubules. As a result urine production is increased (this is known as polyuria). The excessive water loss results in dehydration that triggers thirst, so increased water intake (polydipsia) is a common symptom of diabetes mellitus.