The penis is involved in two important functions, namely sexual activity and urination, but it is also a symbol of masculinity and fertility. At the same time it is a complex organ, composed of various structures that need to work in synch to perform optimally.
The structures mainly responsible for an erection are chambers called the corpus cavernosa, which run from the head of the penis into the body and along bony structures called ischiopubic rami.
The corpus cavernosa are composed of the tunica albuginea (tunica for short), which is where the erectile tissue resides. The erectile tissue has lots of spaces, which are small when the penis is flaccid, but which fill with blood when the penis becomes erect. The spaces are lined by endothelium, which controls blood flow into the penis, most importantly by producing nitric oxide. Nitric oxide is the substance that allows smooth muscle to relax, which in turn lets the spaces expand and fill with blood.
Outside the endothelium is muscle which, when stimulated by sexual arousal, relaxes and allows the spaces to open up and facilitate an erection. The penis also consists of another chamber called the corpus spongiosum. This chamber houses the urethra, which is the tube through which urine leaves the body. More on how erections work
On the outer surface of the penis, there are several blood vessels–a dorsal vein and a right and a left artery (dorsal artery). The dorsal vein is the main transportation pathway for blood leaving the penis, while the dorsal arteries provide the blood to the penile skin, shaft, head, and subcutaneous tissue. The dorsal arteries also have several branches that reach into the various chambers.
For an erection to occur, one major requirement is an adequate blood supply, which is provided by the cavernosal arteries. The cavernosal artery originates in the pelvis, winds its way under the ischiopubic ramus, and then joins up with the dorsal nerves in a special canal. The cavernosal arteries run up the right and left side of the penis and branch off to provide blood to the spaces in the corpus cavernosa.
The penis also has a rich supply of nerves. The dorsal nerves, also referred to as sensory nerves, are dedicated to supplying sensation to the penis. They run alongside the erection artery on the left and right sides of the penis. The erection nerves travel next to the prostate and are the nerves that surgeons attempt to spare (hence the term “nerve-sparing prostatectomy”) to help avoid erectile dysfunction. These erection nerves, which are also referred to as the cavernosal nerves, originate in the spinal cord and make their way toward and alongside the prostate and into the penis to supply the erection tissue.