Erection and ejaculation should not be confused. Erection is necessary to provide the penis with sufficient rigidity to permit the penetration of the female vagina. Ejaculation, however can be achieved by masturbation in the absence of an erection, although this implies a loss of much of the pleasure associated with sexual relations and presents an obvious, but not insurmountable, problem with respect to having children.

In humans, erection is achieved by a hydraulic mechanism. Hydraulic systems are based on the inherent difficulty of compressing any kind liquid. As a result, when a force is applies to a liquid confined within a limited area, a buildup of pressure occurs, and the liquid will press against its physical boundaries.

The brake system found in modern automobiles are hydraulic in nature. When the driver presses the brake pedal, the considerable pressure buildup activates the braking process. Hydraulic brake systems normally work very well and, for this reason, were adopted many years ago for use in automobiles in preference to simple mechanical braking systems. As every driver knows, hydraulic brakes sometimes fail, particularly when leaks occur in the vicinity of the seals. With the penis, leakage of hydraulic fluid, in this case, blood, is one of the possible causes of loss of an erection.

Incidentally, some types of whales do not employ the hydraulic approach for the formidable task of coitus on the high seas. Instead, such whales are equipped with a penile bone. Many men are now employing something very similar, in the form of a penile implant.

The “corpus cavernosum” (plural, “corpora cavernosa”) tissue, shaped into two long cylinders, parallels the shaft of the penis and the urethral channel over most of their respective lengths. Surrounding, and protecting the corpora cavernosa is a strong fibrous tissue layer known as the tunica albuginea. When conditions call for an erection, the tissues of the corpora cavernosa absorb large quantities of the hydraulic fluid (blood), resulting in the expansion of penis length, diameter, and needed rigidity.

Within the corpus cavernosum are thousands of expandable saclike structures, known as sinuses, each capable of storing very large quantities of blood. The sinuses are surrounded by “smooth” muscle tissue. Smooth muscles are one of the three types of muscles, a familiar example being the bicep muscles that control motion in our forearms, and the cardiac muscles that control motion in our forearms, and the cardiac muscles that make up the tissue of our hearts. In addition to the functioning of the penis, smooth muscles are involved in such automatic processes as the digestion of food, the elimination of body wastes, and the regulation of blood flow. In contrast to the situation that takes place when we consciously flex our biceps, we normally are unaware and have no conscious control over the functioning of our smooth muscles. The smooth muscles, however, play a key role in erection, contributing both to the flow of blood into the sinuses and the retention of blood within the sinuses during the period the erection is sustained.

Exactly how the sinuses fill with blood during an erection is still subject to some uncertainty, although research now underway may soon provide a full explanation. From the standpoint of hydraulics, filling the sinuses with blood logically involves the continuous pumping of upstream or arterial blood into the sinus cavities and some form of valving action to restrict the downstream movement of blood from the cavities into the veins of the venous blood system. Part of the explanation is that as the sinuses fill with blood, the engorged muscular tissue confined within the rigid envelope of the tunica albuginea cannot expand outward and instead presses internally against the veins that normally transport blood away from the penis. This restricts all but a necessary trickle of the downstream flow. This does not account, however, for how the buildup of blood in the sinuses occurs in the first place.

One possible way to increase the flow of blood into the sinuses would be to substantially increase the pumping rate of the heart, The variations in heartbeat that occur during periods of sexual activity are not sufficient in themselves to account for buildup of blood in the sinuses. A better explanation is that the flow of blood into the sinuses is enhanced due to actions that take place in the smooth muscle tissues. An important characteristic of smooth muscle is that they are most of the time under tension and in a contracted state. While thus contracted, the smooth muscles, within the corpus cavernosum, normally constrict the arteries, that is, reduce the inside diameter of the arteries, that carry blood into the penis. When an erection occurs, however, the smooth muscle relax. This, in turn, allows the arteries to expand and greatly increase the flow of blood into sinuses within the penis.

The erection has been defined as multifaceted process initiated by events with the nervous system and then maintained by a complex interplay between the vascular system and the nervous system. The most likely sequence of events is that the erection first gets underway when appropriate messages are sent to the nerve centers in the penis that directly control the smooth muscles of the corpus cavernosum. This, in turn, serves to dilate (increase the inside diameter) of the arteries entering the corpus cavernosum, resulting in increased blood flow. Then, as the erection process gains momentum, the valving action in the exiting veins come into play. It Is also reasonable to conclude that feedback relationships probably exist between the indicated upstream and downstream mechanism. When all goes well, there is a major increase in the internal blood pressure of the corpus carnosum, to well above normal body reading. The rise indicates that both the filling and valving mechanisms are operating properly.

A very important area of current research is the processes involved in the relaxation of the smooth muscles of the penis. For relaxation to take place, chemical substance that make the smooth muscles relax must be made available within the corpus cavernosum. Nitric oxide is receiving attention as the most likely chemical agent, although other chemicals may be involved, or perhaps a combination of chemicals. Nitric oxide was first investigated in connection with treating cardiovascular disease. The substance, which acts as a neurotransmitter, has been found to be blood through the major arteries of the body. When these smooth muscles relax, the arteries dilate, resulting in a lowering of blood pressure. Nitric oxide is now also believed to play an important role in the operation of the immune system, including the deference of the body against cancer.

Nitrix oxide, a very simple chemical, is best known to the public as a major, and very undesirable, constituent of smog and acid rain. The removal of nitric oxide from automobile exhaust emissions is one of the principal functions of the catalytic that this toxic substance may prove to be an invaluable agent in the treatment not only of impotence, but also many other physical disorders.

An important area of current investigation is exactly how nitric oxide is generated within the corpus cavernosum. There is now reason to believe that messages sent from the brain to nerve centers within the smooth muscles of the penis result in the generation and release of nitric oxide. The living cells of the body, including those of the nervous system, are proteins in nature. Proteins, in turn, are formed from simpler substances known as amino acids, chemical whose composition includes nitrogen and oxygen. Interest now centers on the essential amino acid, l-arginine. There are grounds to suspect that the messages sent to the penis result in the breakdown of l-arginine yielding the all-important nitric oxide.

Finally, it is important to consider the processes that take place within the brain and nervous system that result in appropriate messages to the penis. The erection is essentially a reflex action that cannot be consciously willed. Sometimes an erection is triggered by psychological factors when the center in the brain associated with the functions of the somatic nervous system is activated by intimate contact with a sexual partner or by merely viewing or even imagining erotic images. An erection may also be triggered via the parasympathetic nervous system when nerve centers located in the spinal cord are stimulated by the physical stroking of the penis. Typically, both processes take place simultaneously. It is also typical, as a men age, for the erection to become more dependent upon direct physical stimulation of the penis, a factor that should be recognized during sexual relations.

Erection are, of course, expected to take place during periods of sexual activities with a partner or during masturbation. In healthy men, erections also normally occur about three to six times per night, while asleep and during dream periods. Such nocturnal erections may last for up to 30 minutes and are often observed by men when awakening. Studies have shown that nocturnal erections can often occur during dreams that have no erotic content. This seems to indicate that nocturnal erections are largely controlled by the autonomic nervous system.

The reason for nocturnal erections are not fully known, but such erections may represent the way the body checks on whether the erectile mechanism is working properly, in much the same manner that aerospace engineers repeatedly check key mechanical systems prior to launching a complicated space vehicle.