Erectile Function and Blood Flow

The Back Story on Viagra

Over the past 20 years, more than one hundred million men have taken medications like Viagra and Cialis (termed PDE-5 inhibitors) to improve their sexual performance. Ironically, the researchers at Pfizer, who identified the erectile benefits of Viagra (generic form: Sildenafil), were actually developing the medication to treat heart problems. By blocking the PDE-5 enzyme, they thought they would be able to open up the heart’s blood vessels.

Pfizer was disappointed by the results of the early clinical trials of Viagra, and by 1993, the pharmaceutical giant was ready to abandon the project. However, an observant nurse noticed that many of the men were lying on their stomachs after receiving experimental Sildenafil. It turns out that the men were embarrassed because they were getting erections. At that moment, Pfizer knew they were on to something HUGE!

Also, in 1993 the NIH recognized that impotence was a public health problem and suggested that erectile dysfunction replace the term impotence. Later that decade, three doctors received the Nobel Prize in Medicine for discovering the role of nitric oxide in opening up blood vessels throughout the body, including the arteries that support penile erections.

In 1998, the FDA approved Viagra for the treatment of men with ED in the United States. Following the release of Viagra, the FDA approved similar medications like Cialis and Levitra, all variations in a class of drugs described as PDE-5 inhibitors. Now that you know the Viagra story, let’s examine how the penis works.

Blood Flow Is the Key

When you get an erection, your penis swells like a balloon—but it swells with blood rather than air. Understanding how this happens will help you take your overall health and sexual fitness to the next level.

Your body continually transfers blood around from organ to organ. For example, when you wake up in the morning and exercise, your blood moves to the muscles of your arms and legs. When you eat breakfast, blood flows to your intestines. After breakfast, you go to work, and blood shoots up to your brain. So how does your body shift blood from organ to organ?

The On Switch: Your nerves release chemicals—neurotransmitters—that make things happen and cause reactions. In many systems, there are specific chemicals that act like an Off or On Switch. In the case of the neurotransmitter Nitric Oxide (NO), that release increases another chemical called cGMP that functions like an On Switch, causing blood vessels to open, improving circulation.

The Off Switch: However, you don’t want these blood vessels to remain open forever. (Imagine life with an erection that won’t go away—that could be very inconvenient at times.) A series of enzymes called the PDE system inactivate cGMP, functioning like an Off Switch. As a result, blood vessels becoming narrower, decreasing blood flow.

Penile Anatomy Pt. II

Nerves and Penile Blood Flow: [or…The Role of the Relaxation Response]

The autonomic nervous system is a control system that functions automatically, usually beyond our control. This system regulates essential body functions like your heart rate, digestion, the rhythm of your breathing (respiratory rate), urination, and sexual arousal—all without your conscious awareness. The system has two modes:

  • The Stress Response—fight or flight—(also referred to as the sympathetic nervous system)
  • The Relaxation Response (the parasympathetic nervous system).

The story I use to explain the effects of the stress response and the relaxation response goes like this:

Say you are a caveman, and you are hanging out in your cave with your cavewoman. The only problem is that there is an 800 pound, hungry saber-tooth tiger pacing outside your cave. You are in a full-blown stress response (maximum sympathetic nervous system outflow) because this nasty tiger wants to eat you and your cavewoman. Now is not a good time to get an erection! An erect penis gives the tiger something easy to bite. Seriously, while you are in stress response, you are not going to get an erection—your body needs maximum blood flow to your legs if you need to run, and to your arms if you need to attack the tiger.

Three days go by, and you and your cavewoman start to get hungry. She starts comparing you to her old cave-boyfriend. Of course, he would not be afraid of a sabretooth tiger. He would have gone out there and taken care of business. Two more days go by, and your cavewoman is getting really hungry, and she is giving you all sorts of grief. At this point, you would prefer to take your chances with a vicious sabretooth tiger than spend one more minute in the cave with your cavewoman.

Penile Anatomy Pt. II

In a flash, you pick up a sharpened spear, pissed off at the world, and charge out of the cave. With a massive rush of adrenaline, you hurl the spear right into the eye of the tiger. You emit a triumphant roar as the giant tiger falls to the ground! Exhausted, with the task completed, your adrenaline plummets (lowering sympathetic nervous outflow), flipping you out of the Stress Response.

Your body moves into the Relaxation Response (parasympathetic mode), giving you a chance to catch your breath, to rest and digest, and to repair wounds and bruises you sustained in the battle. Blood flow is draining out of your large muscles and traveling to other organs.

You skin the tiger and roll out a velvety tiger skin rug. You then light a fire and put the tiger filets on a skewer over the fire as you and your cavewoman enjoy fresh tiger steaks. Blood flow is moving to the intestines to digest food and make waste, and to the kidneys to produce urine to clean your bloodstream of toxins and balance the minerals in your blood (electrolytes).

Fresh from your victory, you invite your cavewoman down to your tiger skin rug to make a baby. You are relaxed (in full parasympathetic mode), so blood flows smoothly to your penis. That is until you reach climax. Orgasm releases adrenaline (sympathetic mode), which immediately reduces blood flow to your penis.

Nature only invented one stress hormone system. Regardless of whether you are running away from a tiger, under a deadline at work, or anxious about whether you will keep an erection, your body produces adrenaline as its stress hormone.  Adrenaline prevents an erection by reducing the amount of blood flowing into the penis. That is why when you are nervous or stressed, your erections go away. To optimize your erections, put your cell phone away, turn off the news, set aside your cares temporarily, and focus on being in the moment. It really will improve your erections and your enjoyment of physical intimacy.

Penile Anatomy Pt. II

Putting It All Together:


When we are in relaxation mode (parasympathetic), our five senses work together to stimulate the release of feel-good neurotransmitters like dopamine and oxytocin. They trigger other brain centers to send signals down your spinal cord.

·      The tips of those nerves release Nitric Oxide into blood vessels. That flips the On Switch, starting the process of an erection.

·      Nitric Oxide release leads to the production of cGMP in the endothelial cells that line the arteries in the penis.

·      The presence of cGMP expands the arteries in the penis, which results in increased blood pressure in the penis.

·      As the blood pressure rises in the penis, the outflow of blood to the body thru veins is blocked, as tiny veins are squeezed between the layers of tissue that line the penis.

This venous obstruction leads to a rigid erection of the penis, necessary for penetration.

The end of the erection:

·      At the same time that this is happening, PDE-5 is turning cGMP into an inactive form so that the blood vessels lose their stimulation to remain open. (After all, at some point, you will probably want to go to sleep, and in the morning, you’ll want to head off to work or school without the erection.)

·      An erection requires continuous stimulation, either visual or tactile, to sustain the flow of Nitric Oxide to keep the blood vessels open to maintain the erection.

We can help our blood vessels keep the blood flowing by doing two things:

  1. Boost Nitric Oxide with a Citrulline-based Nitric Oxide booster like AFFIRM by AFFIRM Science. Nitric Oxide declines with age, and we need a lot of Nitric Oxide to maintain our whole body circulation and our erections. Taking a Citrulline-based Nitric Oxide booster like AFFIRM improves erections, athletic performance, and mental sharpness.
  2. Use a PDE-5 inhibitor like Viagra or Cialis to block the PDE-5 enzyme. As opposed to Nitric Oxide, PDE-5 is specific to the penis. Other PDE enzymes in the body are specific to different locations. In the case of a PDE-5 inhibitor like Viagra, these medications only affect the circulation to the penis.

PDE-5 Inhibitors:

All PDE-5 inhibitors work in the same way, but there are slight differences between them. All medications have two names, their trade name and their generic name, and here I will use both interchangeably.

  1. Viagra (generic: Sildenafil) has an onset of action of 30 minutes after the initial dose and a maximum duration of action of 4 to 6 hours. Visual disturbances like a bluish haze have been reported most commonly with Sildenafil over other PDE 5 inhibitors. The maximum dose of Viagra is 100 mg. Taking more than 100 mg of Viagra (the maximal dose) does not have much of an additive effect, but that amount increases side effects, including headaches, facial flushing, visual changes, and reflux. The generic Sildenafil is typically sold as 20 mg tablets allowing the patient to escalate the dose as needed. Viagra should be taken on an empty stomach, or at least avoid high-fat meals. Grapefruit juice can delay the onset of effect but can also increase blood levels.
  2. Cialis (generic: tadalafil) has an onset of action of 20 minutes and a duration of action of 24 to 36 hours. The maximum dose is 20 mg. Because of its long half-life, tadalafil 5 mg is sold as a daily dose. Cialis should ideally be taken on an empty stomach or at least avoid high-fat meals. It is FDA approved for improving urinary flow as well as erections. One unusual side effect of Cialis is muscle aches.
  3. Levitra (generic: vardenafil) has the quickest onset of action at 10 minutes and lasts around 6 hours. The maximum dose is 20 mg. It has a side effect profile similar to Viagra and Cialis. There is a sublingual form of Levitra (taken under the tongue) called Staxyn, which supposedly has a more rapid onset of action.
  4. Stendra (generic: avanafil) was approved in 2012, has a quick onset of action, and its effects last about 8 hours. It has a lower incidence of side effects. The dose options are 100 and 200 mg.

These medications do not always work on the first occasion that you use them. If your first attempt was not successful, try a few more times before calling it a failure.

PDE-5 inhibitors do not cause heart disease. However, a concerning side effect of PDE-5 inhibitors is that they cannot be combined with nitroglycerine. In terms of recreational drug use, this means that taking nitroglycerine (“poppers”) with a PDE-5 inhibitor can be very dangerous.

Heaven forbid, in the event of a heart attack while using a PDE-5 inhibitor, you must not take sublingual nitroglycerine because that can lower your blood pressure to dangerous levels.

Bringing It Home

Knowledge is POWER. This is especially true when it involves knowledge about your body, how it works, and the medications that you put into it. This knowledge will hopefully help you maximize the enjoyment of your intimate life!

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