An endocrine gland is generally considered a closed system in the sense that it does not have ducts that open to the outside of the body or to other open cavities. Instead, endocrine glands secrete hormones directly into the bloodstream or the surrounding interstitial fluid, which then transports the hormones to target cells throughout the body. This is in contrast to exocrine glands, which use ducts to release their secretions (like sweat or digestive enzymes) onto body surfaces or into body cavities.