Body fluids surrounding all cells in the body are the magic that makes the systems and organs work. This river of life carries in it gasses (oxygen, nitrogen), fuels (glucose, ketone bodies), messengers (hormones, pre-hormones), workers (enzymes, building and repair materials), scouts (proteins looking for damage of foreign materials) and defenders (immune system parts).
When two or more cells joined to make a living organism billions of years ago, they developed a system covering all survival systems, including a way to communicate. This communication system evolved into the fluid system we now see bathing every cell within the body. One of the subsystems they use within this fluid is called hormones. Together, hormones create the interactions between and within cells called metabolism. So another way to think of metabolism is to think of it as life, and lack of metabolism causes death.
A quick example of a hormone and its relationship to metabolism is the hormone called insulin and how it regulates glucose disposal in the body.
These topics cover how metabolism works and what happens when they don’t work. Specifically, messages that regulate body functions report misinformation if the hormones work incorrectly. The most common problems with metabolism are not errors but adaptations to the environment; they adapt to changing living conditions. When the setting changes (an ice age or the overabundance of food), the body adapts to this to help the horse survive. This adaptation to the changes is good when it occurs over thousands of years, but it isn’t good when it happens within a lifetime.