Obstruction of the lumen of the appendix is the main cause of acute appendicitis. Faecolith (a hard mass of faecal matter), normal stool, or lymphoid hyperplasia are the main causes for obstruction. Faecolith alone causes simple appendicitis in 40%, gangrenous non-perforated appendicitis in 65%, and perforated appendicitis in 90% of cases. 
There is evidence suggesting a neuroimmune aetiology in some cases, but this is still being investigated. 
The lumen distal to the obstruction starts to fill with mucous and acts as a closed-loop obstruction. This leads to distension and an increase in intraluminal and intramural pressure. As the condition progresses, the resident bacteria in the appendix rapidly multiply. The most common bacteria in the appendix are Bacteroides fragilis and Escherichia coli . 
Distension of the lumen of the appendix causes reflex anorexia, nausea and vomiting, and visceral pain.
As the pressure of the lumen exceeds the venous pressure, the small venules and capillaries become thrombosed but arterioles remain open, which leads to engorgement and congestion of the appendix. The inflammatory process soon involves the serosa of the appendix, hence the parietal peritoneum in the region, which causes classical right lower quadrant pain.
Once the small arterioles are thrombosed, the area at the anti-mesenteric border becomes ischaemic, and infarction and perforation ensue. Bacteria leak out through the dying walls and pus forms (suppuration) within and around the appendix. Perforations are usually seen just beyond the obstruction rather than at the tip of the appendix. 
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