During excitement, the initial stage of the human sexual response that is also called arousal, parasympathetic reflexes of the autonomic nervous system are activated. The parasympathetic impulses increase blood flow to the genitalia and secretion of lubricating fluids. In males, the penis becomes erect. In females, the clitoris and nipples become erect. These responses continue through the plateau stage, when the penis is inserted into the vagina.