Hemorrhoids are swollen veins in the anal canal. This common problem can be painful, but it’s usually not serious. Too much pressure on the veins in the pelvic rectal area causes hemorrhoids. Normally, tissue inside the anus fills with blood to help control bowel movements. If you strain or sit on the toilet a long time to move stool, the increased pressure causes the veins in this tissue to swell and stretch. This can cause hemorrhoids.