What physiological reaction occurs when blood glucose levels fall?

When blood glucose levels decrease, the body requires a mechanism to increase glucose availability in the bloodstream. The pancreas responds to falling blood glucose levels by releasing the hormone glucagon. Glucagon acts primarily on the liver, signaling it to convert stored glycogen into glucose, which is then released into the bloodstream. This process is essential for maintaining adequate glucose levels, especially during fasting or prolonged periods without food intake.

In contrast, the liver storing excess glucose, the pancreas secreting insulin, and the kidneys filtering out glucose would all be inappropriate responses to falling blood glucose levels. Insulin, specifically, lowers blood glucose by promoting the uptake of glucose into cells and encouraging the storage of glucose as glycogen in the liver and muscle. Therefore, glucagon's role in increasing blood glucose makes it the correct choice in this context.