The Sex Organs - The Male Sex Organs

The External Sex Organs: The Penis

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.