Physical Causes of Erectile Dysfunction
The process of achieving an erection is complex and problems may occur for a variety of reasons. These problems can be psychological, physical, or a combination of the two.
Physical causes of ED are related to a breakdown or damage to the sequence of events that lead to an erection. This sequence involves nerve impulses in the brain, spine, and penis as well as the subsequent response in the muscles, fibrous tissues, veins and arteries in and near the corpora cavernosa.
Often times the breakdown or damage in the sequence affects the arteries, muscles, and surrounding tissues of the penis, and this breakdown is most commonly the result of a disease. Diseases that commonly cause ED include:
Diabetes: Diabetes can cause nerve and artery damage that can make achieving an erection difficult. According the National Institutes of Health, between 20% and 75% of men with diabetes experience ED. Having diabetes more than doubles the risk of having erectile dysfunction.
Kidney disease: Kidney disease can cause chemical changes to occur in your body that affect hormones, circulation, nerve function, and energy level. Often times these changes will lower a person's libido (sex drive) or sexual ability. Drugs used to treat kidney disease may also cause ED. Some estimates report that over 50% of men with kidney failure suffer from erectile dysfunction, decreased sex drive, and a significantly decreased sex life.
Neurological (nerve and brain) diseases: The nervous system (the body's system of nerves) plays a vital part in achieving and maintaining an erection and it is common for men with diseases such as stroke, multiple sclerosis (MS), Alzheimer's disease, Parkinson's disease and spinal cord injuries to experience ED. This is due to an interruption in the transmission of nerve impulses between the brain and the penis.
Vascular disease: Vascular diseases are those that affect the blood vessels. These diseases include atherosclerosis (hardening of the arteries), hypertension, and high cholesterol. These diseases, which account for 70% of physically-related causes of ED, all restrict blood flow to the heart, the brain and, in the case of ED, the penis.
Prostate cancer: Prostate cancer doesn't cause ED on its own, but treatment (radiation, hormonal manipulation, or surgery to remove the cancer) can lead to erectile problems.
The physical causes of ED are not only disease-related. There are many other potential causes, including:
Surgery: Surgery performed to treat diseases such as prostate cancer and bladder cancer often require the removal of nerves and tissues around the affected area which can lead to ED. Some of these surgeries result in only temporary problems (lasting 6-18 months) while others result in permanent damage to the nerves and tissue around the penis and require treatment in order for an erection to be achieved.
Injury: Injuries to the pelvis, bladder, spinal cord, and penis that require surgery also commonly cause ED.
Hormonal imbalances: Imbalances of hormones, such as thyroid hormones, prolactin, and testosterone, can affect a man's response to sexual stimulation. These imbalances can be the result of a tumor of the pituitary gland, kidney disease, liver disease, or hormonal treatment of prostate cancer.
Venous leak: If the veins in the penis cannot prevent blood from leaving the penis during an erection, an erection cannot be maintained. This is known as a venous leak, and can be a result of injury or disease.
Tobacco, alcohol or drug use: All three of these substances can damage a person's blood vessels and/or restrict blood flow to the penis, causing ED. Smoking in particular plays a large role in causing ED in people with arteriosclerosis.
Prescription drugs: There are more than 200 types of prescription drugs that may cause ED.
Prostate enlargement: Prostate enlargement has recently been implicated in being associated with varying degrees of ED.
Psychological causes of erectile dysfunction
The brain plays a key role in triggering the series of physical events that cause an erection, starting with feelings of sexual excitement. A number of things can interfere with sexual feelings and cause or worsen erectile dysfunction. These include:
Depression, anxiety or other mental health conditions
Relationship problems due to stress, poor communication or other concerns
Not defending pr0n but something is missing from the above links. I actually had not known all the physical aspects of what would cause it and had wondered myself as to why someone couldn't get it up.