I believe it is K or rather potassium
When a client comes into the emergency room (er) after hitting his head while playing any sport, if he is alert and oriented. immobilize the client´s head and neck would be a priority nursing intervention, because a head injury is treated as if a cervical spine injury is present until x-rays confirm their absence, then call for an immediate head computed tomography to look for any spine or head injury.
Organisms on earth live in the Biosphere
Used by people is the answer. :-)