Anatomically, the superior rectal vein in the human body is a blood vessel with a long stroke, and it lacks a venous valve that can prevent venous blood reflux. Therefore, the blood in the blood vessel can generate downward hydrostatic pressure. In addition, the pressure in the abdomen increases during defecation, and the distal end of the rectal vein can expand to form hemorrhoids.
There are 3 thickened areas on the surface of the lower rectum and upper anal canal, which are called "anal vascular liners", which are composed of blood vessels, smooth muscles, elastic fibers and structural tissues. Normally, the vascular liner is only loosely attached to the muscle ring below it. When the muscle ring is relaxed during defecation, the vascular liner will turn outwards towards the anal orifice and cannot be completely reset. Hemorrhoids will be formed over time. Some people have abnormal contraction of the muscles of the anal orifice, which makes the anal orifice narrow and increases the pressure. It is necessary to increase the force of defecation to pass stool. This affects the normal bowel function and process, increases intra-abdominal pressure, venous congestion, and forms hemorrhoids.