When these veins become swollen, they form a hemorrhoid. Because the veins are thin and rupture easily, they can bleed during a bowel movement.
Hemorrhoids can develop internally or externally.
Internal hemorrhoids occur underneath the mucous membrane lining the rectum. External hemorrhoids form outside the canal in the anal lining, and are more easily visible to the eye. Both of these types may bleed, and it's not unusual for a blood clot to form in the enlarged vein, particularly with external hemorrhoids. When this happens, the hemorrhoid is referred to as being thrombosed, and can cause a painful lump at the end of the anal opening. Causes Increased pressure on the rectal veins causes them to enlarge.