Rectal Plexus 

HEMORRHOIDS are really varicose veins on the inside. The opening of the bottom is called the anus. Just above here, in three spots are gathered veins which carry blood back from this region to the heart to be recycled. The cause of hemroids is this process getting sluggish and blood that should be on its way back up pools instead at the three areas we just mentioned. This extra blood blows out these veins like balloons which then hurt. 

The reason this all happens in the first place is the same as in varicose veins - blood flowing the wrong way. Blood courses down from the heart (1) to supply the anus tissues with nutrition. This blood is then supposed to return to the heart along veins (2). The veins have valves in them to stop blood on its way up to the heart from rushing back down. But in hemroids the valves stop working properly and the blood falls back down and collects at the lowest point - the veins of the anus (3). This causes them to engorge and expand creating a swelling of the area (3) called a hemroids.