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How the Penis Works: Erection and Ejaculation
Anatomy of a Penis
The
penis is the male sexual organ.
The shaft is the longest part of it. The head or glans is at the end of the shaft. The opening at the tip of the head, where urine and semen come out, is called the meatus.
Inside, two cylinder-shaped chambers called the corpora cavernosa run the length of the
penis. They have a maze of
blood vessels, tissue, and open pockets.
The urethra, the tube that urine and semen flow through, runs along the underside of them, in the spongy tissue of the corpus spongiosum.
Two main
arteries (one in each of the corpora cavernosa) and several veins move
blood in and out. Nerves relay messages to and from other parts of your body.
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What Is an Erection?
An erection starts in your
brain. Something you saw, felt, smelled, heard, or thought makes your nerves send chemical messages to the blood vessels in your penis. The arteries relax and open up to let more blood to flow in; at the same time, the veins close up. Once blood is in the penis, pressure traps it within the corpora cavernosa. Your penis expands and holds the erection.
When the inflow of blood stops and the veins open, your penis becomes soft.
What Is Ejaculation?
When you're aroused, tubes called the vas deferens squeeze
sperm from the testes toward the back of the urethra. The seminal vesicles also release fluid there.
The urethra senses the sperm and fluid mixture. Then, at the height of sexual excitement, it sends signals to your spinal cord, which in turn sends signals to the muscles at the base of your penis. These contract powerfully and quickly, every 0.8 seconds. This forces the semen out of the penis as you climax.
The Biology Behind an Erection
By
Laura Anne Stuart, MPH
What is the biology behind an erection? Can men have erections without testicles? Without a prostate?
The short answer to your question is that it is physically possible to have an erection without testicles or a prostate, but the removal of either makes it less likely that erections will occur.
Inside the shaft of the penis are three columns of erectile tissue—the two corpora cavernosa, which run parallel to each other along the top of the penis, and the corpus spongiosum, which runs along the bottom of the penis and surrounds the urethra. Despite the fact that erections are often called “boners,” there are no bones within the penis. During an erection, the corpora cavernosa and the corpus spongiosum, which are rich in blood vessels, become engorged with blood and expand, making the penis larger and firmer. The fancy name for this is “vasocongestion.”
Vasocongestion is not caused by the testicles or the prostate; it’s caused by nerve impulses that are sent to and from the penis at two different locations in the spinal cord. The first, called the sacral erection center, is located at the base of the spine, and it responds to direct physical stimulation of the penis. The second, called the thoracolumbar erection center, is located higher in the spine, and it responds to nerve impulses from the brain—sights, sounds and fantasies that are arousing to a particular individual. So, the nerve impulses that signal the body to get an erection can be caused by physical or by psychological stimuli. Reflexive erections that have nothing to do with stimulation can also occur, as anyone who’s ever had a wet dream or spontaneous hard-on can attest.
In addition to producing sperm, the testicles also produce male hormones, including testosterone. Testosterone has a sizable effect on sexual desire, and, in turn, sexual desire is often the first stage in sexual arousal and erection. If the testicles are removed and testosterone production slows or stops, then sexual desire typically decreases, and erections may be fewer or nonexistent.
The prostate is a gland that surrounds the urethra in men and produces about 30% of the fluid that makes up ejaculate or “cum.” It is also a gland that is particularly prone to cancer, especially as men age, which makes it important to get regular prostate examinations if you have a prostate and are 50 years or older. Treatments for prostate cancer, including surgical removal of the prostate, can cause erectile dysfunction (an inability to get an erection). This is not because the prostate is necessary to have an erection, but most likely because of the nerve or blood vessel damage caused by surgery or radiation treatments, or because of the psychological trauma associated with prostate cancer.