The penis is the male sex organ, reaching its full size during puberty. In addition to its sexual function, the penis acts as a conduit for urine to leave the body.
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. Beginning alongside of the bulbourethral glands are a pair of long cylindrical bodies called the corpora cavernosa penis. These continue through the body of the penis, occupying the sides and upper portion directly above the corpus spongiosum; they terminate immediately before the glans penis.
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, because during sexual excitation, their fibrous tissue is expanded by blood that flows into and fills their empty spaces. The blood is temporarily trapped in the penis by the constriction of blood vessels that would normally allow it to flow out. The penis becomes enlarged, hardened, and erect as a result of this increased blood pressure. The corpus spongiosum is also considered erectile tissue. This area, however, does not become as enlarged as the other two during erection, for it contains more fibrous tissue and less space; unlike the corpora cavernosa, the corpus spongiosum has a constant blood flow during erection.
The penis is made of several parts:
- Glans (head) of the penis: In uncircumcised men, the glans is covered with pink, moist tissue called mucosa. Covering the glans is the foreskin (prepuce). In circumcised men, the foreskin is surgically removed and the mucosa on the glans transforms into dry skin.
- Corpus cavernosum: Two columns of tissue running along the sides of the penis. Blood fills this tissue to cause an erection.
- Corpus spongiosum: A column of sponge-like tissue running along the front of the penis and ending at the glans penis; it fills with blood during an erection, keeping the urethra — which runs through it — open.
The urethra runs through the corpus spongiosum, conducting urine out of the body. - An erection results from changes in blood flow in the penis. When a man becomes sexually aroused, nerves cause penis blood vessels to expand. More blood flows in and less flows out of the penis, hardening the tissue in the corpus cavernosum.
