For example:

• If you eat a big meal, your body needs to send more blood to the stomach and intestines to help with digestion. 
• If you are running in a marathon, your body needs to send more blood to your arm and leg muscles, and it may want to cut most of the blood flowing to the stomach (and other nonessential organs) in order to save oxygen for the legs.

What your body needs, in other words, is a set of valves that it can use to increase and decrease blood flow to certain parts of the body. And your brain needs a way to control those valves so it can turn them on and off when necessary.

The penis is one of the places in the body where the brain needs to be able to turn the blood flow on and off with a valve. To understand how the brain controls this particular valve, let’s start with the basic concept at work: How does the brain control blood flow to different parts of the body?

Turning Valves On and Off
In the human body, the “valves” open and close using muscles in the walls of arteries. When these muscles relax, the arteries open up and blood flow increases. The valves respond to chemical messages that the brain can control.

The mechanism that the body uses to “open a valve” in any part of the body involves four steps:

1. The brain sends a signal down a particular nerve fiber. This nerve fiber ends in an NANC nerve cell in an artery, somewhere near the point where blood flow needs to change. NANC stands for nonadrenergic-noncholinergic, and what it means is that the NANC nerve cell is able to create nitric oxide. 
2. The NANC nerve endings inject nitric oxide into the blood and surrounding cells. 
3. The nitric oxide stimulates an enzyme called guanylate cyclase in nearby cells, and this enzyme starts producing a chemical called cyclic guanosine monophosphate (cGMP). 
4. cGMP tells smooth muscles that line an artery to relax. When they relax, blood flow increases.

This mechanism is a simple little chemical machine, and the brain uses it to increase blood flow in several different parts of the body. But there is one final part to this chemical machine: Another enzyme called phosphodiesterase (PDE) deactivates the cGMP.

The brain sends signals to NANC cells in the artery. The NANC cells release nitric oxide (NO). Nitric oxide acts as a signaling molecule and stimulates an enzyme called guanylate cyclase in nearby cells. The guanylate cyclase converts a chemical called GTP into another chemical called cGMP. cGMP causes muscles in the walls of the arteries to relax. This relaxation increases blood flow. Meanwhile, PDE is decomposing the cGMP and turning it back into GTP. There is a cycle — guanylate cyclase turns GTP into cGMP, and PDE turns cGMP into GTP. Nitric oxide turns the cycle on.

­cGMP is produced as long as the brain is sending messages down the nerve fibers in the artery, which generate nitric oxide and keep the cycle going. When the brain stops sending the signal, all of the cGMP goes away because PDE is deactivating it. This way, the brain can turn valves on and off whenever it wants to.

So how does this relate to an erection?

When the brain gets aroused, it sends a signal to the penis. Nerve cells in the penis’ corpora cavernosa start producing nitric oxide, which leads to the creation of cGMP. The cGMP causes arteries in the corpora cavernosa to dilate, causing lots of blood to flow into the penis. The extra blood flowing in causes the penis to inflate like a balloon. An erection occurs.

When a man suffers from erectile dysfunction, there can be many reasons for the problem. But one of the most common reasons, especially in older men, is that the arteries in the penis aren’t dilating enough when the brain sends the signal. The man is aroused, and the nerves in the penis are producing NO; but the amount of cGMP produced is not enough to maintain an erection.

The way that Viagra goes about solving this problem is quite ingenious, and involves the following question: How can you create a drug that affects only the penile valve?

If you want to create a drug that increases blood flow to the penis, there are at least three ways to do it:

1. Increase the amount of nitric oxide produced in the penis
2. Increase the amount of cGMP produced in the penis in response to the nitric oxide
3. Eliminate the PDE in the penis so that the cGMP builds up instead of getting decomposed by the PDE

 ­Viagra uses method No. 3 — it eliminates the PDE that is decomposing the cGMP, so cGMP builds up in the penis and has a larger effect on the artery walls. The greater the amount of cGMP the greater the blood flow, and the greater the blood flow the greater the degree of the erection.

The reason that Viagra uses this technique is because of an interesting quirk of PDE.

It turns out that the human body has at least 11 different kinds of PDE that it produces. Only one of those kinds of PDE — PDE5 — is found primarily in the penis. Once scientists discovered this fact, the creation of Viagra®was relatively simple. All that Pfizer needed to find was a chemical that would selectively block PDE5 and nothing else. With the PDE5 blocked, cGMP could build up in the penis and increase the blood flow there without affecting other parts of the body.

Oral medications
Oral medications available to treat ED include:

• Sildenafil (Viagra)
• Tadalafil (Cialis)
• Vardenafil (Levitra)

All three medications work in much the same way. Chemically known as phosphodiesterase inhibitors, these drugs enhance the effects of nitric oxide, a chemical that relaxes muscles in the penis. This increases the amount of blood flow and allows a natural sequence to occur — an erection in response to sexual stimulation.

These medications don’t automatically produce an erection. Instead they allow an erection to occur after physical and psychological stimulation. Many men experience improvement in erectile function after taking these medications regardless of the cause of their impotence.

These medications share many similarities, but they have differences as well. They vary in dosage, duration of effectiveness and possible side effects. Other distinctions — for example, which drug is best for certain types of men — aren’t yet known. No study has directly compared these three medications.

Not all men benefit
Although these medications can help many people, not all men can or should take them to treat erectile dysfunction. You should not take these medications if:

• You take nitrate drugs for angina, such as nitroglycerin (Nitro-Bid, others), isosorbide mononitrate (Imdur) and isosorbide dinitrate (Isordil)
• You take a blood-thinning (anticoagulant) medication
• You take certain types of alpha blockers for enlarged prostate (benign prostatic hyperplasia) or high blood pressure

Viagra, Levitra or Cialis may not be a good choice for you if:

• You have severe heart disease or heart failure
• You’ve had a stroke
• You have very low blood pressure (hypotension)
• You have uncontrolled high blood pressure (hypertension)
• You have uncontrolled diabetes

Don’t expect these medications to fix your erectile dysfunction immediately.

Work with your doctor to find the right treatment and dose for you. Dosages may need adjusting. Or you may need to alter when you take the medication.

Before taking any medication — including Viagra, Levitra or Cialis — make sure to discuss with your doctor:

• Potential benefits and side effects of the medication you are considering
• Any illnesses or serious health problems you have now or have had in the past
• Any prescription or over-the-counter medications you take (including herbal remedies)

Prostaglandin E (alprostadil)
Two treatments involve using a drug called alprostadil. Alprostadil is a synthetic version of the hormone prostaglandin E. The hormone helps relax muscle tissue in the penis, which enhances the blood flow needed for an erection. There are two ways to use alprostadil:

• Needle-injection therapy. With this method, you use a fine needle to inject alprostadil (Caverject, Edex) into the base or side of your penis. This generally produces an erection in five to 20 minutes that lasts about an hour. Because the injection goes directly into the spongy cylinders that fill with blood, alprostadil is an effective treatment for many men. And because the needle used is so fine, pain from the injection site is usually minor. Other side effects may include bleeding from the injection, prolonged erection and formation of fibrous tissue at the injection site. The cost per injection can be expensive. Injecting a mixture of alprostadil and other prescribed drugs may be a less expensive and more effective option. These other drugs may include papaverine and phentolamine.
• Self-administered intraurethral therapy (Muse). This treatment involves using a disposable applicator to insert a tiny alprostadil suppository, about half the size of a grain of rice, into the tip of your penis. The suppository, placed about two inches into your urethra, is absorbed by erectile tissue in your penis, increasing the blood flow that causes an erection. Although needles aren’t involved, you may still find this method painful or uncomfortable. Side effects may include pain, minor bleeding in the urethra, dizziness and formation of fibrous tissue.

Hormone replacement therapy
For the small number of men who have testosterone deficiency, testosterone replacement therapy may be an option.

Penis pumps
This treatment involves the use of a hollow tube with a hand-powered or battery-powered pump. The tube is placed over the penis, and then the pump is used to suck out the air. This creates a vacuum that pulls blood into the penis. Once you achieve an adequate erection, you slip a tension ring around the base of your penis to maintain the erection. You then remove the vacuum device. The erection typically lasts long enough for a couple to have sex. You remove the tension ring after intercourse.

Vascular surgery
This treatment is usually reserved for men whose blood flow has been blocked by an injury to the penis or pelvic area. Surgery may also be used to correct erectile dysfunction caused by vascular blockages. The goal of this treatment is to correct a blockage of blood flow to the penis so that erections can occur naturally. But the long-term success of this surgery is unclear.

Penile implants
This treatment involves surgically placing a device into the two sides of the penis, allowing erection to occur as often and for as long as desired. The inflatable device allows you to control when and how long you have an erection, the semirigid rods keep the penis in a rigid state all the time. These implants consist of either an inflatable device or semirigid rods made from silicone or polyurethane. This treatment is often expensive and is usually not recommended until other methods have been considered or tried first. As with any surgery, there is a small risk of complications such as infection.

Psychological counseling and sex therapy
If stress, anxiety or depression is the cause of your erectile dysfunction, your doctor may suggest that you, or you and your partner, visit a psychologist or counselor with experience in treating sexual problems (sex therapist). Even if it is caused by something physical, erectile dysfunction can create stress and relationship tension. Counseling can help, especially when your partner participates.