With psychological or sexual stimulation, neurotransmitters cause penile smooth muscles to relax, increasing blood flow to the body of the penis.
Maximum rigidity is attained. The body and the tip of the penis enlarge until the penile veins are forcefully compressed. This increases engorgement and maintains maximum penile rigidity, which will later be followed by ejaculation.
Muscle contractions result in increased blood outflow from the penis, decreasing its length and girth and then returning to its flaccid state.