Preservation of life is perhaps the most crucial objective of the Shari`ah. If one were to organize the aims of Islam and shari`ah into a pyramid, this would be sitting at the top.
Because of that, We decreed upon the Children of Israel that whoever kills a soul unless for a soul or for corruption [done] in the land - it is as if he had slain mankind entirely. And whoever saves one - it is as if he had saved mankind entirely.
(On a side note, it is extremely ironic that today so many people think that shari`ah is all about taking life)
In a situation where life is in danger, one is granted concessions on a number of laws that would be absolute and binding otherwise (the scholars have said that eating pork is OK if that's the only thing remaining between you and death). Of course each situation is different and exactly what laws are relaxed depends from situation to situation. However, rules concerning other people's lives are not relaxes - so one is for example not permitted to take someone else's life in order to save one's own.