When a man is sexually stimulated by sight, thought, or touch, the brain sends signals that relax the smooth muscles around the arteries that supply blood to the spongy and cavernous bodies. The veins draining the bodies can't keep up, resulting in swelling.  As the swelling reaches the limit of the penile skin, the penis becomes firm.  The pressure of the spongy and cavernous bodies against the skin partially closes the veins, helping to maintain the erection.  Erection continues until the signals from the brain stop, but erections are not consistent; waking and waning are normal, even during intercourse.   Erection  can occur throughout life, happening before birth and into the 90's in healthy men. Nocturnal erections occur during all male dreams (regardless of what the dream is about), unless the man has physical problems (this is the easiest way to determine if impotence is physical or emotional in nature).  The so called "morning erection" is the result of being wakened during, or just after a dream; and it can be a very persistent erection.  While a morning erection is not a sign of arousal, it's presents and the pleasurable sensations it can create may result in arousal.   Men have only very limited control over their erections.  During puberty the young man is often embarrassed by erection in public settings, but he gradually becomes able to suppress erections when the stimulation is mild.  Likewise, it is impossible to "will" an erection, although sexual thoughts can cause erection.  During prolonged foreplay a man's erection may go away; this is normal, and is not a sign of lessening interest. General health and physical exhaustion can affect erection; when very tired, a man may be able to have only a partial erection, but still be able to climax. Erection is lost in two stages; the initial stage is very quick, but usually leaves the penis firm enough to continue intercourse.  The second stage is somewhat slower and is effected by a variety of things including age (which tends to speed it), and arousal level before climax, with higher (or longer) pre-climax arousal generally resulting in slower lose of erection Although the head (or glands) of the penis is very sensitive to touch, touch alone does not bring about an erection. The epicenter responsible for such essential arousal is actually within the brain. Only after the brain receives visual, audio or mentally stimulating input will it transmit (via the central nervous system) instructions to the smooth muscles along the penis to relax. Specifically the release of nitric oxide in the corpora cavernosa relaxes the smooth muscles. At the same time, the artery to the penis widen to twice its diameter, increasing the blood flow sixteen-fold, and the veins which carry blood away from the penis are blocked. As a result, the two spongy-tissue chambers in the shaft of the penis fill with blood and the penis becomes firm. The corpora cavernosa, acting like a sponge, fill with blood. In fact, the corpora absorb up to eight times more blood than when the penis is flaccid. As your penis swells and lengthens, the filled corpora cavernosa press against the veins. 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. This condition normally keeps this erection firm enough for intercourse. At this most basic level this function is considered to be normal if a man is able to maintain his erection sufficiently long to engage in satisfying intercourse and the subsequent ejaculation. The length of time a man might stay continuously erect can be on the average be about 30 - 45 minutes. Of course the duration of his erection vary greatly, in which case it may be a shorter or a great deal longer.  After ejaculation or cessation of further stimulation to the penis itself or to the brain excess blood will be allowed to drain away, while the level of blood flow into the penis returns to normal. The penis once again becomes flaccid as it loses the built up pressure. Erectile quality or intensity may depend on the nature in which the brain is stimulated. Acts as simple as kissing or "petting" are at times sufficiently arousing to bring about an erection. Viewing a sexually titillating movie or photographs will arouse most men to erection. Beyond this the particular nature of erotic images will have varying effects, as men tend to have a broad range of sexual tastes and desires. While some men may become highly aroused by mundane stimulation, others thrive on fetishes. The male libido is often receptive to a great variety of stimulation. To consider a man sexually dysfunctional solely by measuring his arousal during intercourse, with a long term partner, is simply too limited as well as illogical. Certainly there are men who are not only fulfilled, but thrive in life-long monogamous relationships. This confirms the notion that there is great variation between the needs and sensibilities among men. It would be a mistake however to insist that all men can achieve the same purpose, blissfully maintaining a long-term relationship, which is both sexually and emotionally satisfying. What if things aren't quite working that way. There are a number of conditions which may diminish or otherwise influence this process, these are known and considered under one very general catch all term:  Erectile Dysfunction, which is technically defined as "the inability to achieve or maintain an erection sufficient for sexual intercourse". This is one of the most common sexual ailments in men. Although erectile dysfunction can be primarily psychological in origin, for most men it's more likely a physical  disorder, often with some psychological overlay. While some men assume that erectile failure is a natural part of the aging process and tolerate it;  others find it devastating. Withdrawal from sexual intimacy because of fear of failure can damage relationships and have a profound effect on overall well being for the couple. The Massachusetts Male Aging Study measured several health related variables in 1290 men aged 40 to 70 years. Erectile dysfunction was very common. Fifty two per cent of the men reported some degree of impotence-mild in 17.1%, moderate in 25.2%, and complete in 9.6%. Complete impotence was reported by 5% of men at 40 years of age and 15% at 70 years of age. Over the past decades, the medical perspective on the causes of impotence has shifted. Common wisdom used to attribute almost all cases of impotence to psychological factors. Now investigators estimate that between 70% and 80% of impotence cases are caused by medical problems. It is often difficult to determine if the cause of erectile dysfunction is physical or psychological, or even some combination. The following may be helpful in understanding the difference. Psychological impotence tends to be abrupt and related to a recent situation. The invidual may be able to have an erection in some circumstances but not in others. The inability to experience or maintain an erection upon waking up in the morning suggests that the problem is physical rather than psychological. Physical impotence occurs gradually but continuously over a period of time. If impotence persists over a three-month period and is not due to a stressful event, drug use, alcohol, or medical conditions, the individual may needs to seek medical attention from a urologist specializing in impotence. In virtually every case of impotence, there are emotional issues that can seriously affect the man's self-esteem and relationships, and may even cause or perpetuate erectile dysfunction. Many men tend to fault themselves for their impotence even if it is clearly caused by physical problems over which they have little control. Anxiety has both emotional and physical consequences that can affect erectile function. It is among the most frequently cited contributors to psychological impotence. Anxiety over sexual performance is often referred to as performance anxiety and may provoke an intense fear of failure and self-doubt. It can sometimes set off a cycle of chronic impotence. In response to anxiety, the brain releases chemicals known as neurotransmitters that constrict the smooth muscles of the penis and its arteries. This constriction reduces the blood flow into and increases the blood flow out of the penis. Simple stress may even promote the release of brain chemicals that negatively affect potency in a similar way. Depression is strongly associated with erectile dysfunction. In one study, 82% of men who reported moderate to severe erectile dysfunction also had symptoms of depression. Depression can certainly reduce sexual desire, but it is often not clear which condition came first. Problems in Relationships often have a direct impact on sexual functioning. Partners of men with erectile dysfunction may feel rejected and resentful, particularly if the affected man does not confide his own anxieties or depression. Both partners commonly experience guilt for what they each perceive as a personal failure. Tension and anger frequently arise between people who are unable to discuss sexual or emotional issues with each other. It can be very difficult for the man to perform sexually when both partners harbor negative feelings. Socioeconomic Issues like losing a job or having lower income or education increases the risk for impotence. Smoking (particularly heavy) is frequently cited as a contributory factor in the development of impotence. Alcohol has also been implicated in causing impotence. In small doses, alcohol releases inhibitions, but in doses larger than one drink, it can depress the central nervous system and impair sexual function. Lack of Frequent Erections deprive the penis of oxygen-rich blood. Without daily erections, collagen production increases and eventually may form a tough tissue that interferes with blood flow. The spontaneous erection men experience while sleeping or awake may be a natural protection against this process. The Physical causes of Erectile Dysfunction Blockage of Blood Vessels  The primary cause of oxygen deprivation is ischemia, the blockage of blood vessels. The same conditions that cause blockage in the blood vessels leading 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 slowly constrict, reducing blood flow. This process, known as arteriosclerosis, is the major contributor to the development of coronary heart disease. Diabetes may contribute to as many as 40% of impotence cases . Between one third and one half of all diabetic men report some form of sexual difficulty. High Blood Pressure  Erectile dysfunction is more common and more severe in men with hypertension than it is in the general population. Many of the drugs used to treat hypertension are thought to cause impotence as a side effect; in these cases, it is reversible when the drugs are stopped. Parkinson's Disease As a risk factor for impotence, Parkinson's disease (PD) is an under-appreciated problem. It is estimated that about one-third of men with PD experience impotence. Multiple Sclerosis affects the central nervous system, also precipitates sexual dysfunction in as many as 78% of males suffering for MS. Prostate Cancer and its Treatments can damage nerves needed for erectile function. Radiation the side effects of radiation therapy include most of those of surgery, but the risks for impotence and incontinence are considerably lower. Medications about a quarter of all cases of impotence can be attributed to medications. Many drugs pose a risk for erectile dysfunction. Among the drugs that are common causes of impotence are the following: Drugs used in chemotherapy. Many drugs taken for high blood pressure, particularly diuretics and beta blockers. Most drugs used for psychological disorders, including anti-anxiety drugs, anti-psychotic drugs, and antidepressants. Physical Trauma, Stress or Injury Injury : spinal cord injury and pelvic trauma, such as a pelvic fracture, can cause nerve damage that results in impotence. Bicycling : studies have indicated that regular bicycling may pose a risk for erectile dysfunction by reducing blood flow to the penis. Vasectomy : does not cause erectile dysfunction. When impotence occurs after this procedure, it is often in men whose female partners were unable to accept the operation. Hormonal Abnormalities : Hypogonadism in men is a deficiency in male hormones, usually due to an abnormality in the testicles, which secrete these hormones. It affects 4 to 5 million men in the United States. Low Testosterone Levels : only about 5% of men who see a physician about erectile dysfunction have low levels of testosterone, the primary male hormone. In general, lower testosterone levels appear to reduce sexual interest, not cause impotence. A 1999 study, however, suggests that testosterone levels are not an accurate reflection of sexual drive. Lifestyle changes to help prevent Erectile Dysfunction Maintain General Health : Because many cases of impotence are due to reduced blood flow from blocked arteries, it is important to maintain the same lifestyle habits as those who face an increased risk for heart disease. Diet : Everyone should eat a diet rich in fresh fruits and vegetables, whole grains, and fiber and low in saturated fats and sodium. Because erectile dysfunction is often related to circulation problems, diets that benefit the heart are especially important. Exercise : A regular exercise program is extremely important. One study reported that older men who ran 40 miles a week boosted their testosterone levels by 25% compared to their inactive peers. Another study found that men who burned 200 calories or more a day in physical activity (which can be achieved by two miles of brisk walking) cut their risk of erectile dysfunction by half compared to men who did not exercise. Limit Alcohol and Quitting Smoking : Men who drink alcohol should do so in moderation. Quitting smoking is essential. Stay Sexually Active : Staying sexually active can help prevent impotence. Frequent erections stimulate blood flow to the penis. It may be helpful to note that erections are firmest during deep sleep right before waking up. Autumn is the time of the year when male hormone levels are highest and sexual activity is most frequent. Kegel Exercises :  The Kegel exercise is a simple exercise commonly used by people who have urinary incontinence and by pregnant women. It may also be helpful for men whose erectile dysfunction is caused by impaired blood circulation. The exercises consist of tightening and releasing the pelvic muscle that controls urination Changing or Reducing Medications : If medications are causing impotence, the patient and physician should discuss alternatives or reduced dosages. Psychotherapy and Behavioral Therapy Interpersonal, supportive, or behavioral therapy can be of help to a patient during all phases of the decision-making process regarding possible methods of treatment. Therapy may also ease the adjustment period after the initiation or completion of treatment. It is beneficial to have the partner involved in this process. The value of sex therapy is questionable. In one study, 12 out of 20 men whose dysfunction had a psychological basis and who were advised to enter a sex clinic resisted sex therapy out of embarrassment or because they felt it wouldn't help. Of the eight who entered therapy, only one actually achieved satisfactory sex. Treatment with medication Viagra : is now prescribed in over 90% of erectile dysfunction cases. Studies indicate that it helps 70% of patients achieve sexual function. In one 1999 study, overall male satisfaction was 65%. Not surprisingly, the best results occurred in men who had the fewest sexual problems before treatment, but even men with severe erectile dysfunction had a 41% satisfaction rate. A 2000 study of men who had responded well initially to sildenafil found that 96% of them were satisfied with the treatment after two to three years. Administration and Effect  Sildenafil is effective within 20 to 40 minutes. The drug works only when the man experiences some sexual arousal. Mechanism of Actions  Sildenafil was originally developed for heart disease, but was found to have a unique mechanism of action that targeted only factors in the penis. The drug blocks the enzyme phosphodiesterase. This action maintains persistent levels of cyclic GMP, a chemical that is produced in the penis during sexual arousal and which is the primary chemical that relaxes smooth muscles and increases blood flow. Common Side Effects  Common side effects include the following: Flushing. Muscle aches. Gastrointestinal distress. Headache. Nasal congestion. Cialis :  is a potent and highly-selective PDE5 inhibitor and may not affect other parts of the body, including the brain, heart, kidney and eyes. Clinical trials are reporting significant success rates in up to 88% of patients. It appears to take effect in 15 minutes and the effects last up to 24 hours. Improved results were reported in men suffering from erectile dysfunction of varying severity and causes. Common side effects include headache, muscle pain, stomach upset following meals, and back pain. Vardenafil : is another PDE5 inhibitor currently being investigated. A small study concluded that it increased penile rigidity and tumescence. Further evaluation is warranted. Testosterone Replacement Therapy Testosterone replacement therapy may be effective in inducing puberty in adolescent boys with hypogonadism and may also be helpful for some adult patients with the condition. Some experts believe testosterone replacement therapy also may be helpful for older men whose testosterone levels are deficient. Other procedures and devices Vacuum Devices : Vacuum devices, or external management systems, are effective, safe, and simple to use for all forms of impotence except when severe scarring has occurred from Peyronie's disease. Using the Device The man places the penis inside a plastic cylinder. A vacuum is created, which causes blood to flow into the penis, thereby creating an erection. A band is tightly secured around the base of the penis, which retains the erection, and the cylinder is removed. It takes about three to five minutes to produce an erection. Drawback: Lack of spontaneity is this method's only major drawback. The erection involves only part of the penis shaft, and the process will certainly seem peculiar in the beginning. When these psychological obstacles are overcome, many couples find the result highly satisfactory. Success rate: Studies have found that success with the vacuum device is equal to other methods. Between 56% and 67% of men using it reported the device to be effective. In one study of men who had used the vacuum device for many years, almost 79% reported improvement in their relationships with their sexual partners, and 83.5% said they had intercourse whenever they chose. Nevertheless, drop out rates are high. In one 1999 study, for example, the overall drop out rate was 65%. Side effects: include blocked ejaculation and some discomfort during pumping and from use of the band. Minor bruising may occur, although infrequently. It is very important to use a medically approved pump. Venous Flow Controllers (Cock rings) : Vacuumless devices that trap blood within the penis are also available. They are called venous flow controllers or simple constricting devices. These devices are typically rubber or silicone rings or tubes that are placed at the base of the erect penis to trap the erection. They can be used by men who can achieve erections but lose them easily. These devices should not be used for longer than 30 minutes or lack of oxygen can damage the penis. Penile Implants Three types of surgical implants are currently being used for the treatment of erectile dysfunction: A hydraulic implant consists of two cylinders placed within the erection chambers of the penis and a pump. The pump releases a saline solution into the chambers to cause an erection, and removes the solution to deflate the erection. A penile prosthesis is composed of two semi-rigid but bendable rods that are placed inside the erection chambers of the penis. The penis can then be manipulated to an erect or non-erect position. A third implant uses interlocking soft plastic blocks that can be inflated or deflated using a cable that passes through them. Other alternative treatments  Many alternative agents are marketed for impotence. Very few have been studied and some can be harmful. Yohimbine (Yocon, Yohimex) has been used as folk medicine for years. It appears to improve blood flow. Studies have been inconclusive about its benefits. Aphrodisiacs Aphrodisiacs are substances that are supposed to increase sexual drive, performance, or desire. Some examples include the following: Viramax is a well-marketed product that contains Yohimbine and three herbal aphrodisiacs: Catuaba, Muira puama, and Maca. It has not been proven to be either effective or safe and interactions with medications are unknown. Foods that some people claim have aphrodisiacal qualities include chilies, chocolate, licorice, lard, scallops, oysters, olives, and anchovies. No evidence exists for these claims and certainly no one would ever advocate eating large amounts of such foods, which in cases such as licorice and lard, can be dangerous. Spanish fly, or cantharides, which is made from dried beetles, is the most widely-touted aphrodisiac and is particularly harmful. It irritates the urinary and genital tract and can cause infection, scarring, and burning of the mouth and throat. In some cases, it can be life threatening. No one should try any aphrodisiac without consulting a physician. Still other alternative remedies In one small study, 78% of men who had impotence caused by impaired blood flow regained erections after taking ginkgo. More research is needed. Ginseng root is a traditional Asian remedy for stimulating sexual function, although no studies have been conducted on its effectiveness.