An erection might last indefinitely if it weren’t for chemicals in the body called phosphodiesterases. They work at reversing the process trying to return the penis to being soft (i.e., flaccid). Usually there is a balance between the chemicals that result in an erection lasting for as long as it is needed, which allows the phosphodiesterases to do their job when they should and no sooner. You wouldn’t want an erection forever so it is a good thing that phosphodiesterases exist.
I’m going to share a secret with you. I understand that for men it’s a big (well, perhaps flaccid and at least momentarily small) deal when you can’t get hard. However, in most instances, women don’t give a fuck. We’re understanding creatures who (ahem) know a thing or two about not being able to get off every time. To ease the awkwardness, here are a few go-to phrases to drop when you just can’t get it up.
To a significant degree, the public, particularly older men, is conditioned to accept erectile dysfunction as a condition of progressive aging for which little can be done. In addition, there is considerable inaccurate public information regarding sexual function and dysfunction. Often, this is in the form of advertisements in which enticing promises are made, and patients then become even more demoralized when promised benefits fail to materialize. Accurate information on sexual function and the management of dysfunction must be provided to affected men and their partners. They also must be encouraged to seek professional help, and providers must be aware of the embarrassment and/or discouragement that may often be reasons why men with erectile dysfunction avoid seeking appropriate treatment.
The nerves and endothelium of sinusoids and vessels in the penis produce and release transmitters and modulators that control the contractile state of corporal smooth muscles. Although the membrane receptors play an important role, downstream signaling pathways are also important. The RhoA–Rho kinase pathway is involved in the regulation of cavernosal smooth muscle contraction. 
Impotence: A common problem among men characterized by the consistent inability to sustain an erection sufficient for sexual intercourse or the inability to achieve ejaculation, or both. Impotence can vary. It can involve a total inability to achieve an erection or ejaculation, an inconsistent ability to do so, or a tendency to sustain only very brief erections.
The penis contains three cylinders, the two corpora cavernosa, which are on the top of the penis (see figure 1 below). These two cylinders are involved in erections. The third cylinder contains the urethra, the tube that the urine and ejaculate passes through, runs along the underside of the penis. The corpus spongiosum surrounds the urethra. Spongy tissue that has muscles, fibrous tissues, veins, and arteries within it makes up the corpora cavernosa. The inside of the corpora cavernosa is like a sponge, with potential spaces that can fill with blood and distend (known as sinusoids). A layer of tissue that is like Saran Wrap, called the tunica albuginea, surrounds the corpora. Veins located just under the tunica albuginea drain blood out of the penis.
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The link between chronic disease and ED is most striking for diabetes. Men who have diabetes are two to three times more likely to have erectile dysfunction than men who do not have diabetes. Among men with erectile dysfunction, those with diabetes may experience the problem as much as 10 to 15 years earlier than men without diabetes. Yet evidence shows that good blood sugar control can minimize this risk. Other conditions that may cause ED include cardiovascular disease, atherosclerosis (hardening of the arteries), kidney disease, and multiple sclerosis. These illnesses can impair blood flow or nerve impulses throughout the body.