NO is produced by the enzyme NO synthase (NOS).  NOS plays many roles, ranging from homeostasis to immune system regulation. To date, 3 subtypes have been identified: nNOS, iNOS, and eNOS, which are produced by the genes NOS1, NOS2, and NOS3, respectively. This nomenclature is derived from the sources of the original isolates: neuronal tissue (nNOS), immunoactivated macrophage cell lines (iNOS), and vascular endothelium (eNOS). The subtypes are not, however, limited to the tissues from which they were first isolated.
Vascular damage may result from radiation therapy to the pelvis and prostate in the treatment of prostate cancer.  Both the blood vessels and the nerves to the penis may be affected. Radiation damage to the crura of the penis, which are highly susceptible to radiation damage, can induce ED. Data indicate that 50% of men undergoing radiation therapy lose erectile function within 5 years after completing therapy; fortunately, some respond to one of the PDE5 inhibitors.
The recommended starting dose of vardenafil is 10 mg taken orally approximately one hour before sexual activity. A doctor may adjust the dose higher or lower depending on efficacy and side effects. The maximum recommended dose is 20 mg, and the maximum recommended dosing frequency is no more than once per day. Patients can take vardenafil with or without food. As with sildenafil, for vardenafil to be effective, sexual stimulation must occur.
Erectile dysfunction is a surprisingly common experience. It has been estimated that at least 1 in 10 men is affected to some extent yet, because of the embarrassment and even shame which has been attached to this condition, many men do not seek treatment. Growing older is a factor, with the number of those experiencing erectile dysfunction increasing with age, but it affects the entire range from the youngest to the oldest.
Long-term predictions based on an aging population and an increase in risk factors (eg, hypertension, diabetes, vascular disease, pelvic and prostate surgery, benign prostatic hyperplasia, and lower urinary tract symptoms) suggest a large increase in the number of men with ED. In addition, the prevalence of ED is underestimated because physicians frequently do not question their patients about this disorder.
Taking one of these tablets will not automatically produce an erection. Sexual stimulation is needed first to cause the release of nitric oxide from your penile nerves. These medications amplify that signal, allowing some men to function normally. Oral erectile dysfunction medications are not aphrodisiacs, will not cause excitement and are not needed in men who get normal erections.
Cavernosography measurement of the vascular pressure in the corpus cavernosum. Saline is infused under pressure into the corpus cavernosum with a butterfly needle, and the flow rate needed to maintain an erection indicates the degree of venous leakage. The leaking veins responsible may be visualized by infusing a mixture of saline and x-ray contrast medium and performing a cavernosogram. In Digital Subtraction Angiography (DSA), the images are acquired digitally.
Besides treating the underlying causes such as potassium deficiency or arsenic contamination of drinking water, the first line treatment of erectile dysfunction consists of a trial of PDE5 inhibitor (such as sildenafil). In some cases, treatment can involve prostaglandin tablets in the urethra, injections into the penis, a penile prosthesis, a penis pump or vascular reconstructive surgery.
Since you’re the talker, this is an argument that you’re going to have to win. Really let him know that you feel insecure and unloved when he doesn’t say “I love you.” Tell him it makes you worry about how he really feels when he doesn’t say anything. Tell him that it hurts you that he won’t step the slightest bit out of his comfort zone to say three words that would make you feel so much better. Let him know this doesn’t mean he has to suddenly get all lovey-dovey and give you a cheesy nickname and lay on the sugar so sweet your teeth rot, you adorable little honeybee — because then you might both puke. (I just threw up a little in my mouth myself while typing that.) But that’s not what you’re asking. Let him know you just want an “I love you” now and then. That’s not unreasonable. He doesn’t have to go overboard and you may not get the constant affirmation you prefer — but you can both compromise.
#6 It’s performance anxiety. Many men suffer from performance anxiety, and that’s another reason you can’t get hard. Simply put, you’re too nervous to get your dick up. And that’s okay, it happens! This is likely to happen if you haven’t had sex in a while or if you’re starting up with a new partner. Sex is supposed to be fun, but worrying over your prowess between the sheets can make sex the exact opposite of what’s it’s supposed to be. [Read: 13 ways to overcome sexual anxiety and perform]
While we may think the penis has a mind of it’s own, it’s actually heavily dependent to the brain, and too much stress can interfere with a man’s ability to obtain and hold an erection. According to Healthline, stress can interrupt how your brain sends messages to the penis to allow extra blood flow. Even though a man may want to have sex, too much stress can make this impossible.
Remember what I said before about how it's not you? Okay, sometimes it is you. But it's not that you're not sexy — it's that for men, as well as women, relationship problems (like fighting all the time, or having clashing expectations about where things are going) can severely mess up your sex drive and ability to become aroused. Which makes sense — if you're spending 90 percent of your time together fighting about whether you're going to move in, switching gears to make 10 percent of your time together into a sexy sex party is pretty damned difficult.
Erectile dysfunction (ED) is commonly called impotence. It’s a condition in which a man can’t achieve or maintain an erection during sexual performance. Symptoms may also include reduced sexual desire or libido. Your doctor is likely to diagnose you with ED if the condition lasts for more than a few weeks or months. ED affects as many as 30 million men in the United States.
Organic Impotence. Diabetes mellitus, thyroid disease, and dysfunction of the pituitary gland or testes can cause impotence, as can certain medications. Other organic causes include arterial ischemia associated with atherosclerosis of the aorta and common iliac arteries, extensive pelvic surgery such as radical prostatectomy, spinal cord injury and other neurologic disorders, and a history of cigarette smoking. Because certain medications can cause impotence, it is recommended that in cases of recent impotence it be determined whether the patient has started on a new drug. The most common offenders are diuretics, antihypertensives, and vasodilators. Alcohol, which sometimes is ignored as a drug, is often a contributor to the problem of impotence.
In some cases, ED can be a warning sign of more serious disease. One study suggests ED is a strong predictor of heart attack, stroke, and death from cardiovascular disease. The researchers say all men diagnosed with ED should be evaluated for cardiovascular disease. This does not mean every man with ED will develop heart disease, or that every man with heart disease has ED, but patients should be aware of the link.