These can be factors that contribute to development of hemorrhoids. Sometimes the veins just inside the anus become swollen if there is pressure on the vein itself to prevent blood flow from returning to the heart. The veins may become constricted by a large amount of stool that stays in the rectum just above the anus in someone who has constipation. A job in which a person sits all of the time can produce hemorrhoids just as a person who is on their feet may get varicose veins of the leg. Straining and sitting a long time on the toilet trying to have a bowel movement is also thought to produce hemorrhoids.
Chronic diarrhea causes hemorrhoids by the same mechanism.
Heavy lifting or a chronic cough from asthma, smoking or any chronic lung disease causes hemorrhoids because of increased intraabdominal pressure. This increased pressure is just like straining at stool in that the pressure obstructs the flow of blood through the veins causing them to swell. Obesity can also cause an increased intraabdominal pressure just by the weight of the abdominal wall. It is associated also with hemorrhoids. Pregnancy produces swollen veins and hemorrhoids due to a moderately increased blood volume, and also because the pregnancy itself may slightly block some of the return venous blood flow from the lower pelvis and rectum.