At the scale of fertilization (–1 mm), sperm encounter eggs while being transported within a laminar (or viscous) shear flow. Varying laminar-shear in a.
Blood flow to the genitals increases, resulting in swelling of the woman's clitoris and labia minora (inner lips), and erection of the man's penis.
J Exp Biol. Oct;(Pt 20) Sex and flow: the consequences of fluid shear for sperm-egg interactions. Riffell JA(1), Zimmer RK. Author information.
Many people have speculated on the evolutionary functions of the human orgasm, but the underlying mechanisms have remained mysterious.