two-thirds of the penis is external (that is, outside of the body) and one-third is internal (Figure 1). The erection chambers are paired cigar-shaped structures that travel from the head of the penis into the body and run along the bony structures known as the ischiopubic rami. These bony structures are what we sit on when we sit on a bicycle seat, for example. The paired erection chambers are technically known as the corpora cavernosa (singular: corpus cavernosum). They are composed of two major types of tissue. The outer lining, which is approximately 2mm in thickness, is known as the tunica albuginea, more commonly called, simply, the tunica. Housed within the tunica is the erectile tissue, which itself is composed of numerous types of tissues (Figure 2). This tissue includes muscle, blood vessels, nerves, elastic tissue, collagen, and a tissue very important to erectile function known as the endothelium. I will talk more about this tissue toward the end of this chapter and you will see it pop up again and again in this book.
The erectile tissue is fashioned into a lattice-work of spaces. The simplest way to think of it is that erectile tissue is like Swiss cheese; you have solid tissue interspersed with spaces. In the flaccid (soft) state, these spaces are tiny. However, when men are aroused, these spaces expand, become large, and actually fill with blood to allow an erection to occur (Figure 3). Each of these spaces (known as sinusoids or lacunar spaces) is lined on its inside by endothelium. The endothelium is a major regulator of blood flow into the penis. It produces numerous chemicals, the most important one of which is nitric oxide, which I will discuss in greater detail shortly. Outside of the endothelium is the muscle. This muscle is similar in nature to the bladder and bowel muscle in that it is not under your direct control. In the flaccid state, it is kept contracted to make sure the spaces are tiny, but under arousal, it relaxes greatly and allows these spaces to expand significantly. Coursing outside of the smooth muscle between the spaces are blood vessels and nerves, which, of course, are essential to erection.
The third chamber inside the penis is known as the corpus spongiosum. This sits beneath the paired erectile bodies and houses the urethra (the urine channel). It travels from the opening of the bladder (bladder neck) as the urethra passes through the prostate, all the way out to the head of the penis. In fact, the head of the penis (known as the glans) is in direct continuity with the corpus spongiosum and has no direct connection with the erectile bodies. Strictly speaking, the corpus spongiosum is not a true erectile body as it has very little spongy tissue in contrast to the corpora cavernosa. The corpora cavernosa are tightly joined along their external two-thirds and share an intervening wall known as the septum. This septum has numerous channels passing from right to left side so that there is equalization of blood flow and pressure between the two erectile bodies. For example, with a penile injection (where a patient injects his penis to get an erection; see Chapter 11), an injection on the left side of the penis causes an erection in both right and left erectile bodies.
Figure 1 • Side view of the male pelvis illustrating relationship between the bladder, prostate, and penis (corpora cavernosa).
Outside the erectile bodies, there are numerous layers of tissue, but most importantly, on the top surface of the penis (that is, the surface of the penis that a man looks down on), there is a vein (dorsal vein), a right and a left artery (dorsal artery), and a right- and left-sided set of nerves (dorsal nerves). The deep dorsal vein is a major route of blood flow leaving the penis. The dorsal arteries supply the penile shaft, skin, and subcutaneous tissue, as well as the head of the penis,with fresh blood.They also have some branches that travel into the erectile chambers as the dorsal artery passes from the base to the head of the penis. In some men, these arteries (known as perforators, as they perforate the tunica) are significant contributors to erectile function. The nerves running on the surface of the penis beneath the skin, known as the dorsal nerves, are sensory in nature. That is, they supply sensation and only sensation to the penis. Thus, they are not intrinsically involved in erectile function.
Figure 2 • Cross section of the penis demonstrating the corpora cavernosa and corpus spongiosum. The outer lining of the tunica albuginea is a multi-layered structure. The erectile tissue has a honey-comb appearance.
Blood Supply
The major source of blood flow for erection is the deep or cavernosal artery. Given how erections work (to be discussed later), increased blood flow during erection is critical to gaining maximum rigidity and maximum sustaining capability. The cavernosal artery starts its journey in the pelvis, where it is known as the internal pudendal artery. This artery takes a circuitous course and travels underneath the ischiopubic ramus (remember, that bony structure that we sit on while on a bicycle seat). It travels in a special canal along with the dorsal (sensory) nerves. The right artery travels on the right side, the left artery travels on the left, and they pass about one-third of the way into the erection chamber and give off numerous branches to supply blood to the lacunar spaces inside the erection tissue. This artery at rest is approximately 0.5 mm in diameter and during erection dilates (expands) to approximately 1 to 1.2 mm in diameter. When you appreciate that the coronary arteries are 1.5 to 3 mm in diameter, you can appreciate why we now believe that there is a link between penile blood flow problems and hidden or future coronary artery disease. There is accumulating evidence to suggest that men who have erectile problems are more likely currently or in the future to develop blockage of their heart arteries, which, of course, is a risk factor for heart attack.
Figure 3 • High power view of erectile tissue. The lacunar spaces are lined by endothelium beneath which is smooth muscle. In the space between the lacunar spaces run the nerves.
To make matters more complicated, there are a series of arteries that travel very closely to the prostate, known as accessory pudendal arteries. It is important to understand that the prostate sits on a layer of muscle known as the urogenital diaphragm known officially as the levator ani muscles but more commonly referred to as the pelvic floor. The two erection arteries, the cavernosal arteries, sit beneath this and cannot be injured at the time of prostatectomy. However, with radiation, these arteries fall into the field of radiation exposure. The accessory pudendal arteries, on the other hand, sit above the pelvic floor muscles and travel very close to the prostate. They are also potentially threatened and injured at the time of prostatectomy and during radiation therapy. How common these arteries are is variable. It is generally believed that somewhere in the range of one in four men have an accessory pudendal artery, and in a majority of these men, these arteries are contributors to erection. Indeed, in some, these blood vessels are the major source of arterial blood flowing into the penis for the purpose of erection. Thus, you can see that, if they are injured at the time of radical prostatectomy or exposed to radiation, this may, in fact, impair erection function recovery after both of these treatments.
The anatomy of the venous drainage from the penis is complicated and highly variable. There are numerous veins that leave the penis traveling on the top and bottom surfaces. These veins carry blood from exits the lacunar (erection) spaces through the emissary veins. These veins are tiny veins that travel from the erection tissue through the tunica albuginea into the subcutaneous veins that drain blood back into the general circulation.
Nerve Supply
I previously mentioned the dorsal nerves of the penis, which are purely sensory. However, erection nerves are entirely different. While the sensory nerves travel with the erection artery underneath the ischiopubic ramus, the erection nerves travel very much like the accessory pudendal arteries I mentioned earlier alongside the prostate. Many men equate penis sensation or even orgasm with erection nerve function. In fact, the erection nerves can be completely damaged and sensation will be unaffected.
