Final answer:
The two large cylinders implicated in the process of erection are called corpora cavernosa, which are engorged with blood during sexual arousal due to the effects of nitric oxide and testosterone.
Step-by-step explanation:
The two large cylinders that cause an erection are called the corpora cavernosa. These cylindrical chambers of erectile tissue are responsible for the rigidity of the pen-is during an erection. During sexual arousal or even REM sleep, vasocongestion occurs, leading to more arterial blood entering the pen-is than is leaving it. This process, aided by the release of nitric oxide, results in the engorgement and swelling of the corpora cavernosa, which with the corpus spongiosum, work in concert to produce an erection. The corpus spongiosum also contains the urethra, enabling ejaculation. Testosterone plays a critical role in maintaining the sex organs and sex drive and is regulated via a negative feedback loop involving the testes and endocrine system.