← Anatomy & Physiology

The Monro–Kellie doctrine

The adult skull is a closed box of fixed volume. Inside it sit three things: brain tissue (roughly 80% of the volume), blood (about 10%) and cerebrospinal fluid (about 10%). Because the container cannot expand, the sum of those three volumes is constant.

The consequence is the whole of neurocritical care in one sentence: anything that adds volume must be matched by something else giving way, or the pressure rises. A growing haematoma is accommodated first by squeezing CSF out into the spinal subarachnoid space, then by reducing cerebral venous blood volume. Both buffers are small, and once they are spent the pressure climbs steeply.