The corpora cavernosa consist of empty spaces divided by partitions of tissue. The tissue consists of muscle, collagen (a fibrous protein), and elastic fibre. The corpora cavernosa are termed erectile tissue (see erection), because during sexual excitation, their fibrous tissue is expanded by blood that flows into and fills their empty spaces
The human penis is anatomically divided into two continuous areas—the body, or external portion, and the root. The root of the penis begins directly below the bulbourethral glands with a long cylindrical body of tissue known as the corpus spongiosum (or corpus cavernosum urethrae). This tissue extends through the body of the penis to the tip, where it expands into a mushroom-shaped structure called the glans penis. Running through the centre of the corpus spongiosum is the urethra, a common passage for semen and urine; the urethra ends in a slitlike opening at the tip of the glans penis.