"Hemorrhoids are swellings that can occur in the anus and lower rectum (back passage).
There is a network of small veins (blood vessels) within the inside lining of the anus and lower rectum. These veins sometimes become wider and engorged with more blood than usual. These engorged veins and the overlying tissue may then form into one or more small swellings called Hemorrhoids.
The exact reason why these changes occur and lead to Hemorrhoids forming is not clear. Some hemorrhoids seem to develop for no apparent reason. However, it is thought that the pressure in and around the anus can be a major factor in many cases. If the pressure in and around the anus is increased, then it is thought that this can lead to Hemorrhoids developing.