Complex sentence: The peritoneal or abdominal cavity is located anterior to the duodenum, that is why if the ulcer grows deep enough, it will perforate; whereas if a posterior ulcer grows deep enough, it will perforate the gastroduodenal artery and bleed.
Split into simpler sentences: The peritoneal or abdominal cavity is located anterior to the duodenum. Therefore, if the ulcer grows deep enough, it will perforate; whereas if a posterior ulcer grows deep enough, it will perforate the gastroduodenal artery and bleed.