Anatomy of an erection

Erection

Erection, also called Penile Erection, enlargement, hardening, and elevation of the male reproductive organ, the penis. Internally, the penis has three long masses of cylindrical tissue, known as erectile tissue, that are bound together by fibrous tissue. The two identical areas running along the sides of the penis are termed corpora cavernosa; the third mass, known as the corpus spongiosum, lies below the corpora cavernosa, surrounds the urethra—(a tube that transports either urine or semen),—and extends forward to form the tip (or glans) of the penis. All three masses are spongelike; they contain large spaces between loose networks of tissue. When the penis is in a flaccid, or resting, state, the spaces are collapsed and the tissue is condensed. During erection, blood flows into the spaces, causing distention and elevation of the penis. The amount of blood entering the penis can be increased by physical or psychological stimulation. As blood enters, there is a temporary reduction in the rate and volume of blood leaving the penis. The arteries carrying blood to the penis dilate; this, in turn, causes tissue expansion. The veins leading from the penis have funnel-shaped valves that reduce the outflow of blood. As the erectile tissue begins to enlarge, the additional pressure causes the veins to be squeezed against the surrounding fibrous tissue, and this further diminishes the outflow of blood. Essentially, blood becomes temporarily trapped in the organ.

male reproductive organsStructures involved in the production and transport of semen.Encyclopædia Britannica, Inc.

The corpus spongiosum does not become as erect as the corpora cavernosa. The veins are more peripherally located, so that there is a continual outflow of blood in this region. This constant circulation prevents the urethra from being collapsed by the adjacent tissue, which would prevent release of the semen.

The penis returns to its flaccid state when the arteries relax and begin to contract. Blood flow is once again reduced to its usual rate and volume. As blood drains from the erectile tissue spaces, pressure is reduced on the veins, and flow continues at its normal pace. See also ejaculation.

Ejaculation

Ejaculation, the release of sperm cells and seminal plasma from the male reproductive system. Ejaculation takes place in two phases: in the first, or emission, stage, sperm are moved from the testes and the epididymis (where the sperm are stored) to the beginning of the urethra, a hollow tube running through the penis that transports either sperm or urine; in the second stage, ejaculation proper, the semen is moved through the urethra and expelled from the body.

male reproductive organsStructures involved in the production and transport of semen.Encyclopædia Britannica, Inc.

Sperm cells that are stored in the male body are not capable of self-movement because of the acidity of the accompanying fluids. When the sperm receive fluids, called seminal plasma, from the various internal accessory organs (prostate gland, ejaculatory ducts, seminal vesicles, and bulbourethral glands), the acidity decreases. As they leave the body, the sperm receive oxygen, which is vital to motility. Unable to leave the male body by their own motivation, the sperm cells are transported by muscular contractions. During the emission phase, the muscles around the epididymis and ductus deferens (the tube extending from the epididymis) contract to push the sperm into the prostate and urethra. During ejaculation, the semen is expelled by strong spasmodic contractions of the bulbocavernosus muscle, which encircles the corpus spongiosum (the structure in the penis that encloses the urethra). The whole process of ejaculation is accomplished by nerve impulses received from the penis; once ejaculation is started it becomes a reflex reaction that cannot be voluntarily interrupted.

The seminal fluid is not passed from the various accessory glands simultaneously. A small amount of mucuslike secretion is first passed from the bulbourethral and urethral glands to flush out the urethra and prepare it for the sperm. Next follows the fluid from the prostate gland, and then that from the seminal vesicles. Finally, the fluid actually containing the sperm is ejaculated. After the bulk of the sperm cells have passed, more fluids follow and again flush out the urethra. The total volume of the ejaculate averages between 2 and 5 millilitres (0.12 to 0.31 cubic inch) in the human; of this, only about 1 to 5 percent are actually sperm cells. The other constituents of semen include nutrients, water, salts, waste products of metabolism, and cellular debris. The secretions of the testes and accessory glands are produced under the influence of the male hormone testosterone; without sufficient testosterone the glands degenerate and cannot secrete fluids. See also erection.

The Anatomy of an Erection

Most of the time, and for most men, erections come naturally and with little or no effort. But a number of physical and emotional factors are involved in the process of getting and maintaining an erect penis long enough to achieve satisfaction for you and your partner during sexual intercourse. When one or more of these processes aren’t working, the result can be erectile dysfunction (ED).

Anatomy of the Erect Penis

The penis has three main structures: the corpora cavernosa, the corpus spongiosum, and the tunica albuginea. The corpora cavernosa, which is filled with sponge-like tissue, extends the entire length of the penis. The tunica albuginea is the membrane that covers and protects the corpora cavernosa. The corpus spongiosum, in the middle of the penis, houses the urethra, the opening at the end of the penis through which a man urinates and ejaculates. There is also a network of tissues, muscles, arteries, and veins running through the penis, which allows it to fill with blood and stiffen for an erection.

A man achieves an erect penis from stimulation, such as touching or rubbing the penis, or seeing or thinking about something sexually stimulating and exciting. When stimulation occurs, the brain takes over and works with the nerve endings in the penis. The brain and nerves send messages to the penis to relax, resulting in increased blood flow to the veins and arteries in the penis.

The blood fills the penis, leading to pressure that is contained and supported by the tunica albuginea. Blood entering the penis is essentially locked in by the supporting tissues and membranes, which allows the pressure to be maintained, creating and holding an erection. The testicles also release the hormone testosterone, which play a role in determining when an erection happens and how firm it is.

An erection goes away when the penile muscles tighten up and stop blood from flowing into the penis. This usually happens after orgasm or once the stimulation is no longer present.

Why Erectile Dysfunction Happens

Erectile dysfunction can occur for many different reasons, including nerve sensitivity, poor circulation and other problems with blood flow, a slowed heart rate, and low testosterone production.

If you have a neurological problem that affects the messages between the brain and the nerves of the penis, erectile dysfunction may occur. If you have high blood pressure or are taking medications to treat high blood pressure, blood flow and circulation can be affected, making it difficult to achieve an erection because there’s not enough blood flowing in to fill the penis. Diabetes can cause circulatory problems that inhibit blood flow and damage nerves that receive messages from the brain signaling the penis to relax and fill with blood.

Psychological issues like depression, fear, and anxiety about sex and sexual performance can also affect the process of sexual stimulation and the messages sent to the brain.

In order to achieve an erection, your mind and body have to work together. When a breakdown in communication occurs — whether physical or emotional — working with your doctor to find exactly where the problem occurs is key to treating erectile dysfunction and, just as importantly, its underlying cause.

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Erectile Dysfunction (ED) Anatomy & Physiology

Anatomy of the Penis

The internal structure of the penis consists of two cylinder-shaped vascular tissue bodies (corpora cavernosa) that run throughout the penis; the urethra (tube for expelling urine and ejaculate); erectile tissue surrounding the urethra; two main arteries; and several veins and nerves. The longest part of the penis is the shaft, at the end of which is the head, or glans penis. The opening at the tip of the glans, which allows for urination and ejaculation, is the meatus.

Physiology of Erection

The physiological process of erection begins in the brain and involves the nervous and vascular systems. Neurotransmitters in the brain (e.g., epinephrine, acetylcholine, nitric oxide) are some of the chemicals that initiate it. Physical or psychological stimulation (arousal) causes nerves to send messages to the vascular system, which results in significant blood flow to the penis. Two arteries in the penis supply blood to erectile tissue and the corpora cavernosa, which become engorged and expand as a result of increased blood flow and pressure.

Because blood must stay in the penis to maintain rigidity, erectile tissue is enclosed by fibrous elastic sheathes (tunicae) that cinch to prevent blood from leaving the penis during erection. When stimulation ends, or following ejaculation, pressure in the penis decreases, blood is released, and the penis resumes its normal shape.

Exercise & Sexual Function in Men

In March 2015, results of a study published in the Journal of Sexual Medicine indicated that men of all races who exercise more have improved erectile and sexual function. According to the study, men who reported more frequent exercise had higher sexual function scores and those who reported less exercise experienced lower sexual function. Higher rates of exercise involved a total of 18 metabolic equivalents (METS) per week, which involved 2 hours of strenuous exercise, 3.5 hours of moderate exercise, or 6 hours of light exercise.

Other factors that contribute to lower levels of sexual function include

  • Age
  • Cardiovascular disease
  • Diabetes
  • Past or current smoking

Updated by Remedy Health Media

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 09 Jun 1998

Last Modified: 14 Sep 2015

Erection Ejaculation: How It Occurs

What is the anatomy of the penis?

The penis is made up of:

  • Two chambers called the corpora cavernosa, which run the length of the organ and contain a maze of blood vessels shaped like cavernous spaces (like a sponge)
  • The urethra, or channel for urine and sperm, which runs along the underside of the corpora cavernosa
  • Erectile tissue, which surrounds the urethra, two main arteries and several veins and nerves
  • The shaft, the longest part of the penis
  • The head (glans), which is at the end of the shaft
  • The meatus, or opening at the tip of the head where urine and semen are discharged

How does an erection occur?

When the blood vessels of the corpora cavernosa relax and open up, blood rushes in through the cavernosus arteries to fill them. The blood then gets trapped under high pressure, creating an erection.

  • An erection begins with sensory and mental stimulation. During sexual arousal, nerve messages begin to stimulate the penis. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the open spaces. The blood creates pressure in the corpora cavernosa, making the penis expand and creating an erection.
  • The tunica albuginea (the membrane surrounding the corpora cavernosa), helps to trap the blood in the corpora cavernosa, sustaining the erection. Erection is reversed when muscles in the penis contract, stopping the inflow of blood and opening outflow channels.

How does ejaculation occur?

Sexual stimulation and friction provide the impulses that are delivered to the spinal cord and into the brain. Ejaculation is a reflex action controlled by the central nervous system. It is triggered when the sexual act reaches a critical level of excitement. It has two phases.

  • In the first phase, the vas deferens (the tubes that store and transport sperm from the testes) contract to squeeze the sperm toward the base of the penis and the prostate gland and seminal vesicles release secretions to make semen. At this stage, the ejaculation is unstoppable.
  • In the second phase, muscles at the base of penis contract every 0.8 seconds and force the semen out of the penis in up to 5 spurts.

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