Behind the erection or non-erection of the penis, a lot of processes are executed.
Penis erection is the flow of blood into the penis during arousal.
If the penis receives a good supply of blood the penis will become rock-like hard and erect or else no matter how much you are excited the penis remains loose. This situation is called Erectile Dysfunction.
When Black Generic Viagra or more accurately Sildenafil Citrate dissolves in the blood it eases the tissues of the penile region and walls of the blood vessels of the penis.
Overall a situation is produced where the blood faces no obstruction in reaching the penis.
Along with all this PDE5 (phosphodiesterase type 5 is made non-functional. Usually, PDE5 slows the erection by letting the blood exit from the penis, this is a normal process after ejaculation.
Now when the PDE5 levels are lower than in normal situations. This time the penis becomes not only erect but also remains erect for 3 to 4 hours.