Irritability is one of the characteristics of living things, which involves the ability to respond to certain changes that can alter the homeostatic nature of a living system. This change is called STIMULUS. Hence, the body of a living things responds to a stimulus in order to maintain a stable internal state i.e. homeostasis.
In this case of Reagan whose heart rate increased (than normal) in order to compensate for his blood loss, the response of heart rate increase was due to the loss of blood. Hence, this is an example of IRRITABILITY as a characteristics.
