Anatomy of the Penis, Cross-Section
Normal tunica albuginea is elastic and can stretch. During erection, blood fills the corpora cavernosa, and these structures enlarge and become rigid. In some ways, the penis is similar to a water balloon. Imagine a balloon that is almost empty. The balloon will be very flexible. If the balloon were to be filled with water, as the rubber expands, the elastic balloon will become longer, wider, and more rigid. In the penis, there is elastic tunica albuginea instead of rubber and the penis fills with blood instead of water. There are also many tiny veins within the penis that occlude, or pinch off during an erection, allowing the blood to stay within the penis.