What is a gland?

A gland is defined as a cell or group of cells that secretes substances for use in the body. This can include hormones, enzymes, mucus, and other important substances that play various roles in bodily functions. Glands can be categorized into two main types: endocrine glands, which release their products directly into the bloodstream (such as hormones), and exocrine glands, which release substances through ducts to the surface of organs or skin (such as sweat or saliva). This ability to produce and release specific substances is crucial for maintaining homeostasis and regulating various physiological processes.

In contrast, the other options refer to different biological structures or systems that do not fit the definition of a gland. For instance, neurotransmitters are chemical messengers used in the nervous system, while muscle tissue refers to the types of tissues responsible for movement. Additionally, the nervous system itself is comprised of neurons and glial cells, which are distinct from glands and their secretory functions.