With psychological or sexual stimulation, neurotransmitters cause penile smooth muscles to relax, increasing blood flow to the body of the penis.
Penile arteries expand to accommodate the increased blood flow needed to elongate and expand the penis.
The increased volume of blood within the penis is prevented from draining, thus expanding the penis to a full erection.
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.