The corpora cavernosum are a pair of specialized, sponge-like structures found within the shaft of the penis, serving as the primary mechanism for achieving and maintaining an erection. These two cylindrical columns of tissue are made up of millions of tiny, interconnected spaces that can rapidly fill with blood, causing the penis to stiffen and expand. Their function is dependent on a complex neurovascular process that controls blood flow into and out of these chambers.
The Physical Structure
The penis contains three columns of erectile tissue, with the two corpora cavernosa positioned side-by-side along the top, or dorsal, aspect of the shaft. Each corpus cavernosum is filled with a honeycomb-like network of spaces known as sinusoids or lacunar spaces. These spaces are lined with endothelial cells and separated by walls of connective tissue and smooth muscle fibers, collectively called trabeculae. The amount of smooth muscle within the trabeculae dictates the state of the penis, whether relaxed or erect.
A dense, non-elastic sheath of fibrous tissue, called the tunica albuginea, tightly encases each of the two corpora cavernosa. This sheath is composed primarily of collagen and acts as a rigid container, which is necessary for creating the pressure required for rigidity during an erection.
The third column, the corpus spongiosum, runs along the underside of the penis and contains the urethra. Its tunica albuginea is much thinner. This difference means the corpus spongiosum remains more flexible during an erection, which prevents the urethra from being compressed and allows for the passage of semen.
How Erection Happens
The process of an erection is initiated by sexual stimulation, which triggers the release of chemical signals from nerves and blood vessels in the penile tissue. The most important signaling molecule released is nitric oxide (NO), a potent messenger that acts on the smooth muscle within the trabeculae and the walls of the penile arteries. Nitric oxide causes these smooth muscle cells to relax, which is the first step in the transformation to an erect state.
The relaxation of the smooth muscle allows the helicine arteries, which feed the corpora cavernosum, to widen dramatically, leading to a massive increase in arterial blood flow into the sinusoidal spaces. The blood rushes into and fills these expanding lacunar spaces, causing the corpora cavernosum structure to swell and lengthen. This rapid engorgement can increase blood flow by up to 40 times the resting rate.
As the sinusoidal spaces fill and expand, they press outward against the dense, unyielding tunica albuginea. This outward pressure compresses the small veins, known as emissary veins, that normally drain blood out of the corpora cavernosum. The compression effectively traps the blood inside the chambers, a mechanism called veno-occlusion, which prevents the blood from draining away. This sustained trapping of blood under high pressure within the rigid tunica albuginea provides the necessary stiffness and rigidity of a full erection.
Health Issues Related to the Corpora Cavernosum
Damage to the physical structure of the corpora cavernosum can impair erectile function. A penile fracture is a mechanical injury that occurs when the erect penis is subjected to blunt trauma or forceful bending. This trauma causes a tear or rupture in the tunica albuginea, the strong outer sheath. When the tunica ruptures, blood leaks out of the erectile chambers, resulting in immediate loss of rigidity, pain, and bruising.
Another structural condition is Peyronie’s disease, which involves the formation of a fibrous scar, or plaque, within the tunica albuginea. The cause is theorized to be micro-trauma during sex, leading to an abnormal wound-healing response that forms an inelastic scar. When the corpora cavernosum fills with blood, the section of the tunica containing the scar tissue cannot stretch like the healthy tissue.
This lack of elasticity pulls the shaft toward the plaque during an erection, causing a characteristic curvature, indentation, or deformity. The disease can lead to erectile dysfunction if the plaque prevents the full expansion of the cavernosum or causes a failure of the veno-occlusion mechanism. Conditions that compromise the health of the smooth muscle within the trabeculae, such as diabetes or aging, can also prevent the necessary relaxation required for full blood inflow, which is a major contributor to erectile dysfunction.