Haemorrhoids are swelling that can occur in the anus. There is a network of small veins inside lining of the anus and lower rectum. These veins sometimes become wider and engorge with more blood than usual. These engorge veins and the overlying tissue may then form into one or more small swelling called haemorrhoids.
The exact reason why these changes occur and lead to haemorrhoids forming is not clear. Some haemorrhoids 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. It is thought that an increase in pressure, in and around the anus may lead to the development of haemorrhoids.