The<unk>  of the penis, clitoris or a nipple is its enlarged and firm state. It depends on a complex interaction of psychological, neural, vascular and endocrine factors.
The ability to maintain the erectile state is key to the reproductive system and many forms of life could not reproduce in a natural way without this ability.
Erection
Excretion
Swelling 
Extraction 
Secretion
Blocking 
Quenching 
Emotion