What is the primary effect of cortisol when released during stress?

The primary effect of cortisol when released during stress is to increase blood glucose levels. Cortisol is a hormone produced by the adrenal glands in response to stress, part of the body's fight-or-flight response. When stress is detected, cortisol mobilizes energy reserves by promoting gluconeogenesis, which is the synthesis of glucose from non-carbohydrate sources, to ensure that there is enough energy available for vital functions. This increase in blood sugar levels provides immediate energy to muscles and other tissues, preparing the body to respond to the stressor effectively.

Additionally, cortisol plays a significant role in several metabolic processes, including promoting the breakdown of fats and proteins, although its primary action under stress is the elevation of glucose levels. Enhanced blood glucose is critical for maintaining energy during stressful situations, making this the logical correct answer.