Or why vampires don’t drink from engorged penises.
As we know, blood vampires need to take in blood to stay healthy. And some need more than others. So of course, most would initially think that taking from highly vascular areas would be ideal. And there are those who would try to have their dick sucked off of in an attempt to fulfill some movie fantasy, as the penis is one of those that is has a LOT of blood supply. But here’s why that’s not a good idea.
Let’s tackle the anatomy first.
The body of the penis is composed of three cylindrical masses of tissue, each surrounded by fibrous tissue called the tunica albuginea. The two dorsolateral masses are called the corpora cavernosa penis (corpora = main bodies; cavernosa = hollow). The smaller midventral mass, the corpus spongiosum penis, contains the spongy urethra and functions in keeping the spongy urethra open during ejaculation. Fascia and skin enclose all three masses which consist of erectile tissue permeated by blood sinuses(1).
Upon sexual stimulation, which may be visual, tactile, auditory, olfactory, or imagined, the arteries supplying the penis dilate, and large quantities of blood enter the blood sinuses. Expansion of these spaces compresses the veins draining the penis so blood outflow is slowed. These vascular changes, due to local release nitric oxide and a parasympathetic reflex result in an erection, the enlargement and stiffening of the penis. The penis returns to its flaccid state when the arteries constrict and pressure on the veins is relieved(2).
Simply put, your dick acts like a highly absorbent sponge with a high volume capacity, and if engorged, supplied with blood like it’s been pumped by an open faucet on full blast. ON FULL BLAST.
As mentioned above, erection happens when arteries in the penis DILATE. That means increased blood flow.
And not just any blood flow. You see, the pressure in arteries are higher than in veins as illustrated, like so:
Arteries carry the blood that supplies oxygenated blood around the body and veins carry the unoxygenated blood.
The highest blood flow pressure is in the aorta which directly branches from the heart and the biggest artery in the body, and lessens as it passes to the arteries, arterioles, capillaries, venules, veins until the vena cava and back to the heart. Simply put, blood flowing from the arteries would be gushing and faster than that of blood from the veins.
Vampires usually take from the veins or surface capillaries where the blood vessels are closer to the surface and the blood pressure significantly less than the arteries. The veins is also where most of the blood volume is also located. Hence, safer generally.
If you open an artery, you would be losing more blood and at a faster rate than when you open a vein. If you cut more than one and if you cut deep, then you’re in trouble.
As if that’s not enough, the arteries in an engorged penis would actually be dilated as opposed to a normal artery. Could you say blood fountain?
(1) Principles of Anatomy and Physiology 10th Edition by Tortora and Grabrowski