Erectile dysfunction (ED) is the inability of a man to achieve or maintain an erection sufficient for his sexual needs or the needs of his partner. Most men experience this at some point in their lives, usually by age 40, and are not psychologically affected by it.

Some men, however, experience chronic, complete erectile dysfunction (impotence), and others, partial or brief erections. Frequent erectile dysfunction can cause emotional and relationship problems, and often leads to diminished self-esteem. Erectile dysfunction has many causes, most of which are treatable, and is not an inevitable consequence of aging.

Incidence and Prevalence
The term “erectile dysfunction” can mean the inability to achieve erection, an inconsistent ability to do so, or the ability to achieve only brief erections. These various definitions make estimating the incidence of erectile dysfunction difficult. According to the National Institutes of Health in 2002, an estimated 15 million to 30 million men in the United States experience chronic erectile dysfunction.

According to the National Ambulatory Medical Care Survey (NAMCS), approximately 22 out of every 1000 men in the United States sought medical attention for ED in 1999.

Incidence of the disorder increases with age. Chronic ED affects about 5% of men in their 40s and 15�25% of men by the age of 65. Transient ED and inadequate erection affect as many as 50% of men between the ages of 40 and 70.

Diseases (e.g., diabetes, kidney disease, alcoholism, atherosclerosis) account for as many as 70% of chronic ED cases and psychological factors (e.g., stress, anxiety, depression) may account for 10�20% of cases. Between 35 and 50% of men with diabetes experience ED.

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.

Artery -  A blood vessel that carries blood away from the heart
Atherosclerosis - Deposition of lipids and plaque formation in arteries, leading to hardening and decrease in size of arterial lumen
Autonomic nerve - A nerve with involuntary neural function, such as vasodilation

Body of the penis - The main segment of the penis. It contains the erectile tissues that fill up with blood during sexual excitement
Blue vision - Distortion of color perception that results in increased blue color

cGMP - Abbreviation for cyclic guanosine monophosphate. A compound that promotes vascular smooth muscle relaxation
Corpus cavernosum (Corpus caverNOsum) - Spongy erectile tissue of the penis
Corpus spongiosum  (Corpus sponjiOsum)- Midventral mass of erectile tissue that encloses the penile portion of the urethra
Creatinine (creAtineen) - A compound that is present in the blood, urine, and skeletal muscle

Diabetes mellitus - Disease caused by insulin deficiency or diminished tissue response to insulin

Erection - A penile condition characterized by enlargement and filling of erectile tissue with blood
Erectile bodies - Erectile tissue of the penis to become erect by filling with blood
Erectile dysfunction - The inability to attain or maintain an erection satisfactory to complete intercourse

Flaccid (FLAsid)- Referring to the non-erect, flabby state of the penis when a man is not sexually aroused.

Glans penis - The distal end of the penis where the corpus spongiosum enlarges to form an acorn-shaped region

Hyperlipidemia (hyperlipiDEEMia)- Abnormally high levels of lipids in the blood
Hypertension - High blood pressure

Libido - Sexual drive

Multiple sclerosis - A debilitating disease affecting nerve cells

Neurogenic - Originating in the nervous system
Neurotransmitter - A chemical messenger used by nerve cells to transmit nerve impulses from one nerve cell to another
Nitric Oxide (Nitrik oxide) - A substance that mediates vascular smooth muscle relaxation

Phosphodiesterase - 5 (PDE5)(fahsfodaiESterase) Enzyme that breaks down cGMP during an erection, ultimately returning the penis to a flaccid state
Prostate gland - An accessory reproductive gland, located under the urinary bladder and responsible for the secretion of a milky fluid that is important for the production and viability of sperm
Priapism  (PRYapism) - Persistent abnormal and painful erection of the penis
Peyronie�s disease -(pay raNEEZ disease) A condition of unknown etiology in which there is a structural deformity of the erectile tissue of the penis
Psychogenic - Psychological

Sensory - Relating to afferent nerves that transmit signals to the central nervous system so that an organism has perception of self and the environment
Smooth muscle - A type of involuntary muscle

Testis - The male sexual organ that is responsible for the production of reproductive cells (sperm) and testosterone, the principal male sex hormone. (Plural = testes)
Testosterone - The primary male sex hormone. It is produced by the testes, and its main function is mediating the development, growth, and maturation of the male sex organs

Vasodilator (Va-zoDYlater) - An agent that causes dilation of blood vessels
Vein - A blood vessel that carries blood toward the heart
Venous plexus - A vascular network formed by numerous connections between veins

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SOURCE: The Journal of Urology