Introduction:

Erectile dysfunction (impotence) is the inability to achieve or maintain an erection sufficiently rigid for sexual intercourse, ejaculation, or both. Sexual drive and the ability to have an orgasm are not necessarily affected. Because all men have erection problems from time to time, doctors consider impotence to be present if attempts at intercourse fail at least 25% of the time.

Erectile dysfunction is not new in either medicine or human experience, but it is not easily or openly discussed. Cultural expectations of male sexuality inhibit many men from seeking help for a disorder that can usually benefit from medical treatment.

The Penis and Erectile Function

The Structure of the Penis. The penis is composed of the following structures:

• Two parallel columns of spongy tissue called the corpus cavernosa, or erectile bodies.
• A central spongy chamber called the corpus spongiosum, which contains the urethra, the tube that carries urine from the bladder through the penis.

These structures are made up of erectile tissue. Erectile tissue is rich in tiny pools of blood vessels called cavernous sinuses. Each of these vessels are surrounded by smooth muscles and supported by elastic fibrous tissue composed of a protein called collagen.

Erectile Function and Nitric Oxide. The penis is either flaccid or erect depending on the state of arousal. In the flaccid, or unerect, penis, the following normally occurs:

• Small arteries leading to the cavernous sinuses contract, reducing the inflow of blood.
• The smooth muscles regulating the many tiny blood vessels also stay contracted, limiting the amount of blood that can collect in the penis.

During arousal the following occurs:

• The man's central nervous system stimulates the release of a number of chemicals, including nitric oxide, which is now considered the main contributor for eliciting and maintaining erection.
• Nitric oxide stimulates production of cyclic GMP, a chemical that relaxes the smooth muscles in the penis. This allows blood to flow into the tiny pool-like cavernous sinuses, flooding the penis.
• This increased blood flow nearly doubles the diameter of the spongy chambers.
• The veins surrounding the chambers are squeezed almost completely shut by this pressure.
• The veins are unable to drain blood out of the penis and so the penis becomes rigid and erect.
• After ejaculation or arousal, cyclic GMP is broken down by an enzyme called phosphodiesterase-5 (PDE5), and other compounds are released that cause the penis to become flaccid (unerect) again.

Important Substances for Erectile Health

A proper balance of certain chemicals, gases, and other substances is critical for erectile health.

Collagen. The protein collagen is the major component in structural tissue in the body, including in the penis. Excessive amounts, however, form scar tissue, which can impair erectile function.

Oxygen. Oxygen-rich blood is one of the most important components for erectile health. Oxygen affects two substances that are important in achieving erection:

• Oxygen suppresses transforming growth factor beta 1 (TGF-B1). TGF-B1 is a component of the immune system called a cytokine and is produced by smooth muscle cells. It appears to stimulate collagen production in the corpus cavernosum, which can lead to erectile dysfunction.
• Oxygen enhances the activity of prostaglandin E1. Prostaglandin E1 is produced during erection by the muscle cells in the penis. It activates an enzyme that initiates calcium release by the smooth muscle cells, which relaxes them and allows blood flow. Prostaglandin E1 also suppresses production of collagen.

Oxygen levels vary widely from reduced levels in the flaccid state to very high in the erect state. During sleep, oxygen levels are high and a man can normally have three to five erections per night, each one lasting 20 - 40 minutes.

Testosterone and Other Hormones. Normal levels of hormones, especially testosterone, are essential for erectile function, though their exact role is not clear.

Erectile Dysfunction and Oxygen Deprivation

Erectile dysfunction most commonly occurs when the penis is deprived of oxygen-rich blood. When oxygen levels to the penis are low, an imbalance occurs in two important substances, TGF-B1 and prostaglandin E1:

• TGF-B1 levels increase, which trigger production of collagen, a tough protein that forms all types of connective tissue, including scar tissue.
• In addition, there is a reduction in prostaglandin E1, a chemical that suppresses collagen production and relaxes the smooth muscles to allow blood flow, resulting in an erection.

When TGF-B1 levels increase and prostaglandin E1 levels decrease, smooth muscles waste away and collagen is overproduced, causing scarring, loss of elasticity, and reduced blood flow to the penis. A number of conditions can deprive the penis of oxygen-rich blood.

Blockage of Blood Vessels (Ischemia). The primary cause of oxygen deprivation is ischemia -- the blockage of blood vessels. The same conditions that lead to heart problems may also contribute to erectile dysfunction. For example, when cholesterol and other factors are imbalanced, a fatty substance called plaque forms on artery walls. As the plaque builds up, the arterial walls gradually narrow, reducing blood flow. This process, known as atherosclerosis, is the major contributor to the development of coronary heart disease. It may also play a role in the development of erectile dysfunction.