penile erection true The hardening, enlarging and rising of the penis which often occurs in the sexually aroused male and enables sexual intercourse. Achieved by increased inflow of blood into the vessels of erectile tissue, and decreased outflow. https://www.ncbi.nlm.nih.gov/books/NBK2605/ GO:0043084 Wikipedia:Erection#Penile_erection biological_process multicellular organismal reproductive process