What major function does a gland in the endocrine system perform?

The primary function of a gland in the endocrine system is to secrete hormones. Hormones are chemical messengers that are released directly into the bloodstream and travel to various organs and tissues throughout the body to regulate physiological processes. This includes critical functions such as metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood. The specialized nature of endocrine glands, such as the pituitary, thyroid, adrenal glands, and others, is specifically designed for the production and release of these vital hormones, which are essential for maintaining homeostasis in the body.

In contrast, other functions mentioned in the choices do not align with the primary role of endocrine glands. For instance, providing structural support relates to the function of connective tissues, facilitating cellular respiration is a metabolic process primarily involving the mitochondria in cells, and protecting against pathogens is a function typically associated with the immune system. Each of these processes is important for overall health, but they are not the primary role of the endocrine glands, which distinctly focus on hormonal secretion.