The corpus cavernosum are placed along the length of the penis. They are found along the penis shaft, from the pubic bones to the head of the penis. The spongy tissue fills with blood. The blood comes from arteries down the length of the penis. The blood fills the corpora cavernosa. This causes it to swell. When swollen, it holds 90% of the blood involved in an erection. This increases both the length and circumference of the penis.