The Basics of Erectile Function

David Knowles, M.D. Health Pro
  • This week, I would like to discuss the basic principles of normal erectile function. By understanding the basic mechanisms of achieving an erection, we will be able to better understand the different things that can go wrong and cause erectile dysfunction. We will discuss the problems that occur with this system in later blogs.

    Erections start in the brain. When a man is sexually excited, a message is sent from the front portion of the brain down the spinal cord toward the penis. From the spinal cord, the message is sent through nerves that run behind the bowels and down the pelvis. They run behind the bladder, next to the prostate and travel along and into the erectile tissue in the penis. The penis has three different and separate nerve systems. Each of these nerve systems involves different nerves from the spinal cord. These nerves travel different paths through the pelvis, but they all end up terminating near or in the penis. One nerve system causes an erection, one involves feeling or sensation of the penis, and the third involves ejaculation. Damage may be done to each system individually or sometimes to all three. This is why you may have normal sensation but problems with erection. You may have problems with sensation but normal erections. And you may have normal erections and sensation but you can not ejaculate.
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    The nerves involved in erection stimulate the blood vessels that enter and exit the penis. When the nerves responsible for erection fire, they cause the blood vessels that enter the penis and arteries to dilate or become larger and increase the blood flow into the penis. The outgoing vessels and veins contract or become smaller and the outflow of blood from the penis decreases. This causes an increase in pressure in the penis. When the pressure reaches a critical point, the walls of the erectile bodies, corpora cavernosa, press together and close most of the veins that exit the penis. This decreases the amount of blood exiting the penis even more. With a high inflow of blood and little outflow, the penis begins to thicken, become longer, and stiffer. An erection is born. The stretching, elastic nature of the tissue of the penis is mandatory to allow the walls to press together and close the blood vessels that exit the penis.

    Erection can be best compared to the filling of a sink. The arteries are the faucet and the veins are the drain. In order to get an erection, you have to fill the sink. Dilation of the arteries is equal to increasing the water flow from the faucet. Dilation of the veins is equal to partially closing a flap valve in the drain. Once the sink fills with a small amount of water, this water forces the flap valve completely closed, and the sink begins to fill at a very fast rate. A full sink is equal to a strong, rigid erection.

    Erectile function is a complex series of events which all have to occur in order to start, achieve and maintain an adequate erection. Any problem with this process can cause erectile dysfunction. This is part of the reason why erectile dysfunction is so common.

  • After ejaculation, the blood vessels that enter the penis and arteries are contracted by the same nerves that trigger ejaculation. By decreasing the inflow of blood, the pressure is decreased. When the pressure drops below a critical point, the walls of the erectile bodies, corpora cavernosa, separate. This separation opens up the exiting vessels and veins and allows blood to exit the penis rapidly, and the penis becomes soft again.
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    When using our sink analogy, ejaculation turns off the faucet and the water level begins to drop. Once the sink is only partially full, the drain snaps open, and the water drains out of the sink at a rapid rate.

    I hope this gives you a little better understanding of the complex nature of erectile function. By having better knowledge of normal erectile function we will be able to better understand the causes of erectile dysfunction.
Published On: June 28, 2006