The penis is an external male organ involved in urinary and reproductive functions. Its capabilities result from a specialized structure composed of several types of tissue working in concert. This anatomy allows for changes in size and rigidity, which are fundamental to its roles. The organ’s design blends soft, expandable tissues with strong, supportive layers.
The Erectile Bodies
The penis contains three cylindrical columns of spongy tissue. The two largest chambers are the corpora cavernosa, which run parallel along the top of the shaft. These bodies are made of a dense network of interconnected vascular spaces known as sinusoids. The sinusoids are separated by walls of smooth muscle and connective tissue called trabeculae, giving the tissue its sponge-like quality.
A third erectile body, the corpus spongiosum, is a single chamber below the two corpora cavernosa. This tissue surrounds the urethra, the tube that carries urine and semen out of the body. While it also contains expandable tissue, its structure is less dense than the corpora cavernosa. This ensures the urethra remains open during an erection for the passage of ejaculate.
The Mechanism of Erection
An erection begins with sexual stimulation, which can be mental or physical. This stimulation prompts the brain to send nerve signals to the arteries in the penis. These signals cause the release of chemical messengers, primarily nitric oxide, which instructs the smooth muscles of the trabeculae within the erectile bodies to relax.
This muscular relaxation allows the penile arteries to widen, increasing blood flow into the penis. Blood fills the sinusoids within the corpora cavernosa, causing them to expand. As these chambers fill, they press against the veins that normally drain blood away from the penis. This compression traps the blood, leading to a rise in internal pressure that produces a rigid erection.
Protective and Structural Layers
Several layers of tissue surround the erectile bodies to provide structure and protection. Encasing the two corpora cavernosa is a dense sheath of fibrous tissue called the tunica albuginea. This strong covering is composed of collagen and elastic fibers, allowing it to be flexible when flaccid but providing strength when stretched during an erection to maintain rigidity.
External to the tunica albuginea is a layer of deep fascia known as Buck’s fascia. This layer bundles all three erectile bodies together into a single unit. It supports the internal structures and contains the deep dorsal vein, a main pathway for blood drainage. The outermost layer is the skin, which is loose to accommodate changes in penile size.
Conditions Affecting Penile Tissue
Certain medical conditions directly involve the tissues responsible for penile function. Peyronie’s disease is characterized by the development of fibrous scar tissue, or plaques, within the tunica albuginea. These non-elastic plaques can cause the penis to bend during an erection, sometimes leading to pain or difficulty with intercourse. The scarring prevents one side of the organ from expanding evenly.
Another condition is corporal fibrosis, which involves the replacement of healthy smooth muscle tissue inside the corpora cavernosa with scar tissue. This scarring reduces the ability of the sinusoids to fill with blood, impeding the erectile process and often leading to erectile dysfunction. This condition can be associated with aging, diabetes, or complications from surgical procedures.