Humans are hardwired for two basic functions: survival and reproduction. Nature’s forces have made the reproductive process a pleasurable one, and by so doing have ensured the greatest likelihood of reproduction being successful. What a clever bait and switch scheme! In the seeming pursuit of a feel-good activity—determined by this evolutionary sleight of hand—we have been hoodwinked into reproducing!
The goal of reproduction is fusion of DNA from two individuals to perpetuate the species. The penis functions as a “pistol” to penetrate the female’s reproductive tract and inject DNA. A flaccid penis is unable to complete this task.
Many mammals—including the gorilla and chimpanzee—have a bone in the penis (the baculum), which functions to keep the penis hard enough for vaginal penetration and injection of the DNA. (There is also a bone in the clitoris called the os clitoridis.) However, the human penis is boneless (as is the human clitoris). While we can debate whether or not this is a good thing, it certainly helps to keep the penis concealed!
Creating a “Bone” Where One Doesn’t Exist
How did nature overcome this challenging design problem: creating bone-like rigidity in a boneless organ?
The answer is hydraulics—using blood as a hydraulic mechanism—not the typical use of blood, which is for the transportation of oxygen, carbon dioxide, hormones, nutrients, and waste products to and from our organs. This use of blood as a hydraulic mechanism for erections—both penile erections in men and clitoral erections in women—is nothing short of brilliant…our bodies having evolved to use blood the way a tire uses air, to inflate deflated organs to allow them to function!
An example of an animal that uses hydraulics is the jumping spider, which uses blood forced into the legs to straighten them out to facilitate powerful jumps, avoiding the need for muscular legs that are bulky and clearly not spider-like.
Erection hydraulics requires a special means of regulating flow. To do so, the inflow needs to turn on like a gushing faucet and the outflow needs to shut off like a plugged drain in a sink. This is not the usual state of affairs for blood flow to an organ, which typically requires a relatively small amount of inflow to meet basic metabolic needs and an equal amount of outflow, creating a dynamic state of equilibrium. An erection demands that the arteries of the penis function as high-pressure faucets (inflow increasing many times over baseline) and the penile veins to close off completely.
So how has our body evolved this capacity?
The penis is a marvel of design and engineering, capable of increasing its blood flow by a factor of 40-50 times over baseline! This surge happens within seconds and is accomplished by relaxation of the smooth muscle within the arteries supplying the erection chambers and within the sinuses of the erectile chambers. This is not the case of non-genital organs, in which blood flow can be increased upon demand (for example, to our muscles when exercising), but not anywhere to this extent.
Now for a little deviation off course for some interesting trivia:
- The spongy sinus tissue in the erectile chambers is virtually identical to the spongy sinus tissue in our facial sinuses. (My pathologist friend claims that he can’t tell the difference under a microscope.)
- When this spongy tissue in the penis or clitoris becomes congested with blood, an erection occurs; when it happens in ours facial sinuses it is known as sinus congestion or a stuffed nose.
- The spongy tissue in the erectile chambers is surrounded by connective tissue known as the tunica albuginea, the second toughest connective tissue in our bodies, the toughest being the dura mater that surrounds our brains and spinal cords.
- A side effect of the ED meds like Viagra is nasal congestion…now you understand why.
- Prolonged erections (priapism) are often treated with the same medications used to treat a stuffed nose, e.g., phenylephrine.
The Important Role of the Pelvic Muscles
So, under the right circumstances the penis becomes swollen (tumescent) with blood. How has our body evolved the capacity to trap the blood so it does not return to the circulation? How does the penis go from swollen to rock-hard rigid?
First, as the sinuses within the erectile chambers fill with blood, they pinch off the veins, which traps blood in the penis. Second, nature—in its typical brilliant way—has designed a means of increasing the blood pressure in the erectile chambers to sky-high levels by means of a “muscular tourniquet” that not only chokes off the exit of blood, but with each squeeze causes a surge of blood with increased filling of the erectile chambers, the end result being bone-like rigidity.
What are the names of these specialized muscles and what muscle group are they part of?
These are the ischiocavernosus and bulbocavernosus muscles (man’s best friends) that are part of the group of muscles known as the pelvic floor muscles, which form the floor of the important group of muscles known as the “core” muscles. When a man has a rigid erection and he voluntarily contracts these muscles, the erection will lift up and point majestically towards the heavens, thank you pelvic floor muscles. It is no wonder that in the classic Gray’s Anatomy textbook in 1909 referred to the ischiocavernosus muscle as the “erector penis muscle!”