陰莖動脈裏的血液能迅速地充滿3條海綿體，因而促成海綿體變硬。結果呢，陰莖便勃起了。反過來，當血液離開海綿體，勃起便消失。陰莖的勃起通常由性刺激所激發，但是也能因為別的原因所引起。在勃起期間，陰莖增大尺寸。因為陰莖的皮膚鬆弛，尺寸的增大是可能的。

Erection
The penis arteries can quickly fill the three spongy bodies with blood, thus causing them to stiffen. As a result, the penis becomes erect. Conversely, when the blood leaves the spongy tissue the erection subsides. Erections of the penis are usually triggered by sexual excitement, but they can also occur for other reasons. During an erection the penis increases in size. This is possible because the covering skin is very loose.