A normal male erection can occur when physical or psychological stimulation triggers impulses from the brain to travel down to the nerves of the penis. These signals cause the muscles within the arteries to relax, making the arteries bigger, and resulting in more bloodflow through the penis. This is called an erection.
Sometimes physical causes such as disease or injury can prevent the signals from reaching the arteries. This condition is known as erectile dysfunction. Erectile dysfunction can be treated with a medication called sildenafil citrate. This medication blocks the effects of an enzyme, causing the smooth muscle in the walls of the arteries to relax. The arteries grow in size to allow more blood to flow through the penis. At the same time, the enlarged arteries put pressure on nearby veins, which prevents blood from exiting and maintains an erection.
The effects of sildenafil citrate are only temporary, and can not be considered a permanent cure for erectile dysfunction.