What effect does glucagon have on blood glucose levels?

Glucagon is a hormone produced by the alpha cells of the pancreas that plays a crucial role in regulating blood glucose levels. When blood sugar levels are low, glucagon acts to increase these levels by prompting the liver to convert stored glycogen into glucose and release it into the bloodstream. This process is known as glycogenolysis. Additionally, glucagon stimulates gluconeogenesis, the production of glucose from non-carbohydrate sources, further contributing to an increase in blood glucose levels.

This mechanism is especially vital during fasting or between meals when glucose levels can drop, ensuring that the body has a continuous supply of energy. Therefore, the primary effect of glucagon is to raise blood glucose levels, making this option the correct answer.