Penis is the external male genital organ that serves two purposes: micturition and semen expulsion.
The penis is externally surrounded by skin forming the penile shaft. On its frontside, the shaft has a retractable double-layered fold of skin, the foreskin (prepuce) . When this is closed, there is a pathological condition called phimosis, which is treated with the surgical procedure of circumcision.
Underneath the prepuce (foreskin), there are the 2 corpora cavernosa (cavernous bodies), which look like two cylinders that get filled with blood and cause erection. The corpora split at the point the penis meets the body, forming the crura which are surrounded by the ischiocavernous muscles of the perineum attaching to the pubic symphysis of the pelvis. It is the contraction of these muscles that induces the hard rigid erection that lasts a few seconds.
The two corpora cavernosa are tigthly connected one with the other, forming one single body, while a fibrous septum separates them in right and left. This fibrous septum stops at the peripheral penile segment, allowing the fusion of the two cavernous bodies. Joint together, they form a single cylinder covered with a strong white protective membrane, the tunica albuginea. This plays a highly significant role in erection, for it contributes both to the penile rigidity and shaping.
Incomplete growth of the tunica albuginea or injury during sexual intercourse may result in the known penile curvatures. If the curvature is due to incomplete development during childhood, it is called congenital penile curvature. If it is due to injury, it is called Peyronie's disease. Both cases are treated surgically.
Corpus Spongiosum (Urethrae)
Below the 2 corpora cavernosa, there is a third penile cylindrical chamber, the corpus spongiosum urethrae. This is surrounded on its base by the bulbocavernous muscle. The contraction of this muscle is responsible for the forwarding and expulsion of semen during ejaculation. Corpus spongiosum, located at the top of the penis, shapes the glans around the corpora cavernosa and, in this way, protects them from injury. In fact, the glans plays the role of a 'shock absorber', absorbing pressures to avoid pain during sexual intercourse.
The corpus spongiosum surrounds the urethra, a tube made of muscle fibres ending up to the external orifice and serving both for urination and ejaculation. Urethral injuries may cause urethral stricture (narrowing) and, as a result, difficulty in urinating. Strictures are treated either with urethral dilation (with the use of special instrumentation or catheters) or surgically in the most severe cases (with transurethral or open surgical procedures).
Mechanism of Erection
Corpora cavernosa are sponge-like formations, located within the tunica albuginea. Histologically, they are composed of numerous trabecula and smooth muscle fibres, forming multiple vacuoles communicating one with another, the blood sinuses. Sexual stimuli are transmitted through penile nerves and induce relaxation to the smooth muscle fibres of blood sinuses; as a result, penile perfusion increases and erection is achieved. This blood supply is carried out through the two corpora cavernosa.
In the centre of each one of the two corpora there is a cavernous artery securing the basic blood supply during erection. This artery branches out in many small arteries of spiral shape, the helicine arteries (spiral arteries). Each one of these ends up in a blood sinus, engorging it with blood during erection. Any kind of lesion or obstruction of the arterial network (e.g. diabetes mellitus, hypertension, cardiovascular disease, hypercholisternonemia) causes arterial blood insufficiency and, thus, vascular erectile dysfunction that is manifested as the male's inability to achieve hard and full erection.
Arterial blood supply is not enough on its own for erection. It is also essential that the venous network -starting with small venules from each blood sinus corresponding to arteries- closes down. These venules join together forming larger veins running between the soft erectile tissue and the hard tunica albuginea. The increase of pressure within blood sinuses compresses veins upon the rigid tunica albuginea. Thus, the venous network closes down and, as a result, blood flows into the corpora cavernosa but does not exit. This mechanism of 'trapping' the blood into the penis is called 'the closing mechanism' of the penile venous network. In case this mechanism is impaired due to the same reasons that cause arterial obstruction, then there is venous leakage characterised by inability to maintain erection (erection is achieved but is lost before ejaculation).