Which system is primarily responsible for regulating metabolism and stress responses?

The endocrine system is primarily responsible for regulating metabolism and stress responses because it utilizes hormones to communicate and control various bodily functions. Hormones are chemical messengers that are secreted into the bloodstream and travel to target organs and tissues, where they exert their effects.

For metabolism regulation, hormones such as insulin, glucagon, thyroid hormones, and cortisol play critical roles. For instance, insulin regulates glucose levels in the blood, while thyroid hormones control the rate of metabolism across various tissues. The stress response, often referred to as the "fight or flight" response, involves the release of hormones like adrenaline and cortisol from the adrenal glands. These hormones prepare the body to respond to stressors by increasing heart rate, enhancing energy availability, and modulating various physiological functions.

In contrast, while the circulatory system is vital for transporting hormones and nutrients, it does not directly regulate metabolism or stress responses. The nervous system is important for immediate responses and coordination but operates through electrical impulses rather than hormonal changes. The digestive system is primarily responsible for nutrient absorption and processing rather than regulation of metabolism and stress responses, although it does interact with the endocrine system in terms of hormonal signals related to hunger and satiety.