1, blood reflux resistance becomes large The human anus is located between the perineum and the tailbone in the midline of the buttocks, usually contracted into a front and back longitudinal fissure, expanding into a circle when defecating. Because of muscle contraction to form a lot of radial folds. The upper part of the anus is connected to the rectum. When the human body is upright, the heart is higher than the anorectum, and the blood flow back to the anorectum is easily stagnated due to resistance and the influence of geocentric attraction. In contrast, the anorectal area of reptiles is higher than the heart, and blood is returned from the top to the bottom, so it is not easy to produce depressed blood in the anus due to obstruction of the return flow and the influence of gravity. So animals do not have hemorrhoid disease. 2, the rectum became curved rectum the name from the animal, because the rectum of the animal is not curved, is a straight tube-like channel, so named. After the human body is upright, the rectum-anal part is bent due to the downward pressure of the abdominal organs. In the sagittal plane, the rectum descends along the front of the sacrococcyx, forming a rectosacral curve. Further the rectum wraps around the tip of the coccyx and turns posteriorly, forming another recto-perineal curve. These curves have a physiological role in the control of stool in humans, but they also bring anatomical potential for the formation of hemorrhoids. 3, the anorectal part of the vein structure is special Most of the human veins have venous valves within them, which can help venous blood return to the heart. However, the superior rectal vein and its branches do not have a venous valve, so the blood returning to the heart from the bottom up through the rectal muscle layer can easily stagnate here. This is why resting flat on your back during a hemorrhoid attack is beneficial to the recovery of hemorrhoids.