My "laymans" understanding of what happens is this.
1. Apnea causes O2 levels to fall drastically.
2. The body responds to protect the brain and vital organs by pumping more blood in an attempt to meet O2 needs
3. As the heart pumps harder, the blood pressure elevates, reaching dangerous levels
4. The body reacts to the elevated pressure by releasing diuretic hormones (ANP) to reduce blood volumne in an attempt to lower blood pressure.
5. We wake up to urinate.
Well, obviously, I am not a medical professional, but hope this illustrates the importance of using cpap regularly to prevent stroke and heart attacks during sleep.
http://www.sleepapnea.org/resources/pubs/noct.html
A google search for "apnea+ANP" lists pages of articles