What distinguishes an exocrine gland from an endocrine gland?

An exocrine gland is characterized by its method of secretion, specifically that it releases its secretions through ducts to external surfaces, such as onto the skin or into the lumen of an organ. This feature distinguishes exocrine glands from endocrine glands, which release hormones directly into the bloodstream without the use of ducts.

Exocrine glands serve important functions in processes such as digestion, thermoregulation, and lubrication, as they can release substances like enzymes, sweat, and saliva directly to where they are needed. This direct pathway allows for localized effects of the secretions on external surfaces, rather than the systemic effects typically associated with hormones released by endocrine glands.

Considering the other options, the release of hormones into the bloodstream is a hallmark of endocrine glands, while regulating metabolic functions and nerve impulse transmission are not specific to exocrine glands. Metabolic regulation is primarily the role of certain hormones produced by endocrine glands, and nerve impulses are transmitted using different mechanisms entirely. Therefore, understanding that exocrine glands function through ductal secretion to external surfaces is crucial in distinguishing them from endocrine glands.