Human beings are inherently wired for two fundamental purposes: survival and procreation. Nature, in its wisdom, has made the process of reproduction a delightful one, ensuring its success through pleasure. In a cunning twist, the pursuit of pleasure, facilitated by this evolutionary sleight of hand, leads to the act of reproduction itself.
The primary goal of reproduction is to merge DNA from two individuals, perpetuating the species. The penis serves as a biological "tool" to penetrate the female's reproductive tract and deposit DNA. A flaccid penis lacks the capability to fulfill this crucial task.
In contrast to many mammals, such as gorillas and chimpanzees, which possess a penis bone (baculum) to maintain rigidity during vaginal penetration and DNA injection, the human penis, like the human clitoris, is boneless. The absence of a bone aids in concealing this essential organ.
Creating a "Bony" Rigidity in a Boneless Organ
How did nature tackle the complex challenge of creating a bone-like rigidity in an organ devoid of bones?
The solution lies in hydraulics—utilizing blood as a hydraulic mechanism, deviating from its typical role of transporting oxygen, carbon dioxide, hormones, nutrients, and waste products to and from our organs. Blood, in this context, functions as a hydraulic mechanism for achieving erections, both penile erections in men and clitoral erections in women. It's a brilliant adaptation, allowing our bodies to use blood in a manner analogous to how a tire employs air to inflate deflated organs, enabling them to function optimally.
An interesting example of hydraulic use in nature is observed in jumping spiders, which employ blood pressure to straighten their legs, facilitating powerful jumps without the need for bulky, unspider-like muscles.
Erection hydraulics necessitate specialized flow regulation. In this case, blood inflow must increase rapidly, like a gushing faucet, while outflow must be restricted, akin to plugging a sink's drain. This differs from the typical regulation of blood flow to organs, where equilibrium is maintained with a relatively steady inflow and outflow to meet basic metabolic needs. Erections require arteries in the penis to act as high-pressure faucets, significantly increasing inflow, while penile veins must constrict fully to prevent outflow.
So, how has our body evolved to achieve this capability?
The penis is a marvel of design and engineering, capable of boosting its blood flow by a staggering factor of 40 to 50 times above baseline. This transformation occurs within seconds, achieved through the relaxation of smooth muscles in the arteries supplying the erectile chambers and within the sinusoidal spaces of these chambers. This level of blood flow augmentation is unique to genital organs and far exceeds the capabilities of non-genital organs.
Here's an interesting tidbit to diverge slightly from the topic:
The spongy sinus tissue in the erectile chambers is nearly indistinguishable from the spongy sinus tissue in our facial sinuses when viewed under a microscope (as vouched for by a pathologist friend).
Congestion of this spongy tissue in the penis or clitoris leads to an erection, while the same occurrence in our facial sinuses results in nasal congestion or a stuffy nose.
The spongy tissue in the erectile chambers is encased in connective tissue known as the tunica albuginea, the second toughest type of connective tissue in our bodies, with the toughest being the dura mater that surrounds our brains and spinal cords.
It's worth noting that one side effect of ED medications like Viagra is nasal congestion—a connection that now makes perfect sense.
In cases of prolonged erections (priapism), the same medications used to alleviate nasal congestion, such as phenylephrine, are often employed.
The Crucial Role of Pelvic Muscles
Under the right conditions, the penis becomes engorged with blood, transitioning from a flaccid to a rock-hard, rigid state. How has nature equipped our bodies to retain this trapped blood, preventing it from returning to the circulation? What processes convert a swollen penis into a robustly rigid one?
First, as blood fills the sinuses within the erectile chambers, it compresses the veins, effectively trapping blood within the penis. Second, nature has ingeniously devised a mechanism to elevate blood pressure within the erectile chambers significantly. This "muscular tourniquet" not only restricts blood outflow but, with each contraction, propels more blood into the erectile chambers, resulting in bone-like rigidity.
What are these specialized muscles called, and which muscle group do they belong to?
Meet the ischiocavernosus and bulbocavernosus muscles—your body's allies in this endeavor. These muscles are part of the pelvic floor muscle group, forming the foundation of the "core" muscles. When a man voluntarily contracts these muscles during a rigid erection, the penis rises majestically—a testament to the remarkable pelvic floor muscles. It's no wonder that the 1909 edition of Gray's Anatomy referred to the ischiocavernosus muscle as the "erector penis muscle!"