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An erection is the result of increased blood flow to the penis. But you may be wondering exactly how that works, and whether there’s a specific amount of blood that your penis needs to achieve an erection.

In some cases, reduced blood flow to the penis can cause noticeable changes in the penis. But many other parts of your body, like your nervous system and hormones, also impact how and when your penis gets hard.

Read on to learn more about what blood has to do with erections. We’ll also cover what you can do if you feel unsatisfied by your erection when you masturbate or have sex.

The actual amount of blood needed to get hard varies among people. On average, it’s thought to be about 130 milliliters (mL), or 4.4 ounces. It’s a small fraction of the 1.2 to 1.5 gallons (4,500 to 5,600 mL) of blood circulating throughout the adult human body at any given time.

Because an erection needs a relatively small amount of blood, there’s no increase in blood production in the body. But blood is redirected to supply tissue in the penis, which means that a little less blood can be directed elsewhere in the body.

Here’s exactly what happens to the penis physiologically during an erection and how blood is involved in this process:

Inside the shaft of the penis are two columns of spongy tissue called corpora cavernosa. This tissue contains blood vessels. When your penis is flaccid, arteries are constricted, supplying just enough blood flow to keep the tissue in the corpora cavernosa healthy.

When you become aroused, the smooth muscles of the arteries in the penis relax, allowing the blood vessels to expand and fill with more blood. This expands corpora cavernosa tissue too, making your penis larger and firmer.

To make an erection happen, the brain, nervous system, blood vessels and certain hormones are recruited. Here’s how this part works:

  • Nerve signals from the brain based on arousing stimuli, like visual imagery or erotic thoughts, can cause the muscles in the corpora cavernosa to relax.
  • Sensory stimulation of the penis or surrounding area can trigger a similar response, with nerve impulses signaling to the tissue in the corpora cavernosa to prepare for sexual intercourse.
  • During sexual stimulation, the body releases a chemical called nitric oxide (NO). This helps dilate the blood vessels and activate an enzyme called guanylate cyclase to trigger the release of cyclic guanosine monophosphate (cGMP). This chemical relaxes the spongy tissue and allows it to become engorged as arteries expand with greater blood flow.
  • After an orgasm, the additional blood that engorges the corpora cavernosa will start to flow out through veins in the penis. The same amount that entered at the start of the process will also exit.

What if blood doesn’t flow back out?

Blood that doesn’t properly flow back out of the penis can result in a condition called priapism. Blood pooling in the penis this way can damage tissue in the corpora cavernosa.

Priapism is more common in people with blood disorders, like sickle cell anemia, but can also be brought on by medications or other factors, like the use of cocaine or conditions like leukemia.

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In addition to blood, the hormones testosterone and oxytocin may both play a role in getting and maintaining an erection.

A 2016 review in the The Journal of Sexual Medicine notes that testosterone may play a role in the timing of an erection by helping to relax the penile arteries so they can fill with blood.

Some individuals with ED and low levels of testosterone may benefit from testosterone therapy, but levels below the normal range are still enough to achieve a healthy erection. Testosterone also drives sexual desire, and low levels may cause a drop in libido.

Oxytocin has also been identified as an important component in arousal. But researchers in the 2016 review noted that the use of oxytocin to create sexual arousal needs to be studied more.

Several factors can affect blood flow to the penis or the ability of the penis to become erect, like:

  • Circulation problems. Cardiovascular disease, high blood pressure, diabetes, and high cholesterol can reduce blood flow to the penis and other parts of the body.
  • Nervous system dysfunction. Neurological disorders like multiple sclerosis, Alzheimer’s disease, and Parkinson’s disease can interfere with proper signaling from the brain to initiate the sexual arousal process.
  • Tissue damage. Radiation treatment to the bladder or prostate can sometimes harm the nerves and blood vessels that bring nerve and chemical signals to the area for arousal and blood vessel dilation. This can make it difficult or impossible for the penis to engorge itself with blood.

A lifestyle that focuses on good physical, mental, and emotional health promotes good circulation. This is one way to help increase the likelihood of erectile function.

Try these tips to support healthy erections and overall well-being:

  • Consider quitting or cutting back on smoking. The chemicals in cigarette smoke can injure your blood vessels.
  • Get regular aerobic exercise. Exercising throughout the week helps improve circulation, energy, and overall fitness and self-confidence.
  • Eat a balanced diet. Focus on vegetables, fruits, whole grains, and lean protein sources.
  • Address mental health issues like depression and anxiety. These can affect not just your sexual health, but your overall health.

A 2018 study found that following a Mediterranean diet was associated with a reduction in ED symptoms, compared with a typical Western diet high in fat and processed sugars.

Another 2018 study also found that an exercise regimen of 40 minutes done four times a week decreased ED within 6 months, especially for individuals experiencing ED caused by cardiovascular disease, obesity, or a sedentary lifestyle.

An occasional episode of ED or erectile dissatisfaction, an erection that isn’t firm enough for satisfactory intercourse, is normal. This can occur when you are:

  • tired
  • distracted
  • stressed
  • under the influence of alcohol

If you notice frequent ED or dissatisfaction even with lifestyle changes, especially if there’s no obvious trigger, talk with a primary care professional or a urologist.

Other signs that you should see a doctor include:

  • Noticeable changes in your sex drive. These could be triggered by hormonal changes or factors like stress, depression, poor sleep, or relationship troubles.
  • Premature ejaculation. This is especially the case if you’re ejaculating much earlier than you typically expect.
  • Painful erections. These can result from tissue damage or infection.
  • Pain when urinating. This might be a sign of an infection or other conditions that can affect your urinary tract.

The most common ED treatments are medications like PDE5 inhibitors. These include tadalafil (Cialis) and sildenafil (Viagra). These drugs work by protecting cGMP, which encourages blood flow to the penis and greater blood retention in the corpora cavernosa during sexual activity.

Another possible treatment is a vacuum erection device (or penis pump), a tube that you place over your penis.

A handheld pump pulls air out of the tube, creating a vacuum that triggers blood flow to the penis. A ring is then slipped around the end of your penis when you remove the pump to help maintain the erection during sex.

Penile injections or penile implant surgery can also help treat severe cases of ED or those that are caused by another condition like diabetes (known as refractory cases).

Healthy blood flow to the tissue within the penis helps produce an erection, and it only takes about 130 mL to get you hard.

But creating the right environment for proper blood flow involves the brain and nervous system, plus certain hormones and chemicals. Many factors go into healthy sexual function, and many issues can interfere with it, too.

If you notice changes in your erectile function, see a doctor. It’s a common concern and often one with a variety of effective treatments.