The normal blood vessels get traumatized by the forceful passage of hard stools. This causes ulceration which leads to bleeding.
Later the bulging blood vessels cause obstruction to the passage of stools through the anal opening. This causes the patient to strain or apply force to evacuate. This force leads to breaking of the supporting tissue of the blood vessels & they start sagging and sliding down(Prolapse). With each motions the sliding progresses.
OR
The tissues supporting the blood vessels stretch while straining to pass stools especially hard stool.
As a result, the blood vessels dilate(swell); their walls become thin and bleed.
If the stretching and pressure continue, the weakened vessels start sagging and sliding and protrude out of the anal opening which progresses with every motion.
Thus there are different stages (stage-I to stage-IV) depending on which the patient’s treatment needs to be tailored
