Formation of hemroids
If the hemroids originates at the top (rectal side) of the Excretory Orifice canal, it is referred to as an internal hemroids. If it originates at the lower end of the Excretory Orifice canal near the Excretory Orifice, it is referred to as an external hemroids. Technically, the differentiation between internal and external hemroids is made on the basis of whether the hemroids originates above or below the dentate line (internal and external, respectively).
As discussed previously, hemroidsal cushions in the upper Excretory Orifice canal are made up of blood vessels and their supporting tissues. There usually are three major hemroidsal cushions oriented right posterior, right anterior, and left lateral. During the formation of enlarged internal hemroids, the vessels of the Excretory Orifice cushions swell and the supporting tissues increase in size. The bulging mass of tissue and blood vessels protrudes into the Excretory Orifice canal where it can cause problems. Unlike with internal hemroids, it is not clear how external hemroids form.
Anatomy of hemroids
The arteries supplying blood to the Excretory Orifice canal descend into the canal from the rectum above and form a rich network of arteries that communicate with each other around the Excretory Orifice canal. Because of this rich network of arteries, hemroidsal blood vessels have a ready supply of arterial blood. This explains why bleeding from hemroids is bright red (arterial blood) rather than dark red (venous blood), and why bleeding from hemroids occasionally can be severe. The blood vessels that supply the hemroidsal vessels pass through the supporting tissue of the hemroidsal cushions.
The Excretory Orifice veins drain blood away from the Excretory Orifice canal and the hemroids. These veins drain in two directions. The first direction is upwards into the rectum, and the second is downwards beneath the skin surrounding the Excretory Orifice. The dentate line is a line within the Excretory Orifice canal that denotes the transition from Excretory Orifice skin (anoderm) to the lining of the rectum.