The Monro-Kellie doctrine posits that the cranial cavity is a rigid structure with a fixed volume, containing brain tissue, cerebrospinal fluid (CSF), and blood, and that any increase in the volume of one component must be compensated by a decrease in one or more of the others to maintain intracranial pressure (ICP) equilibrium. This principle is crucial in understanding and managing conditions that affect intracranial dynamics, such as traumatic brain injury and hydrocephalus.