in general, their ductless nature
their vascularity
and commonly the presence of intracellular vacuoles or granules that store their hormones
In contrast, exocrine glands
such as salivary glands
sweat gland
and glands within the gastrointestinal tract
tend to be much less vascular and have ducts or a hollow lumen.
In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as bone, kidney, liver, heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.