A man gets an erection when the penis enlarges and stiffens. It is a complex process that happens as a result of changes within the muscles, nerves and blood vessels of the penis. The regulation of blood flow into and out of the penis is what makes an erection possible. On average, it takes two tablespoons of blood to get a man's penis erect.
During an erection pressure in the penis is at least twice the pressure of blood in the main circulation.
This is possible because the muscles of the pelvic floor contract around the base of the corpora cavernosa.