Secretion refers to cellular release of substances (ions and small and large molecules) to the external environment of the cell. Secretion may be accomplished by exocytosis (fusion of transport vesicles with the plasma membrane and release of vesicle contents to the external environment), by transport of molecules across the plasma membrane (via the activity of transport proteins such as pumps, transporters, and channels), or by simple diffusion of fat-soluble molecules through the plasma membrane out of the cell.
For example, endocrine cells secrete hormone molecules that then enter the bloodstream. Neurons release (i.e., secrete) neurotransmitter molecules into the synaptic cleft. Some neurons secrete neurohormones; which similar to hormones, travel in the bloodstream to reach distant target cells. Epithelial cells secrete molecules in luminal spaces, such as digestive enzymes secreted into the digestive tract by various cell types.