system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does someone freeze to death? What exactly happens inside the body to be fatal?
gpt
Life depends on a number of biochemical reactions that occur in each and every cell in your body.  

All chemical reactions (and this includes those upon which life depends) are affected by temperature.  In general, decreasing temperature tends to slow chemical reactions (particularly exothermic chemical reactions, which describes most of the chemical reactions upon which life depends).  

Dropping the temperature of the body affects virtually every organ system.  It causes the heart to slow (because, in part, the chemical reactions in heart muscles required to cause heart contraction are slowed by lower temperature).  It causes blood to reduce its capacity to carry oxygen.  It causes basic energy-producing chemical reactions in every cell (cellular respiration) to slow, and to produce less energy molecules (i.e. ATP, etc) required for basic cellular function.  It causes nerve conduction to slow and slows down neurotrasmitter release.  

In short, through a number of mechanisms at the cellular and organ level, it causes cellular activity to slow, and, eventually, to be unable to maintain the intracellular environment required for life.  

Cells die, organ systems fail, which causes more cell death, until the body is in a catch-22 of death and becomes irrecoverable.