The human brain is exposed to large variations in gravitational forces both on the arterial and the venous side of its circulation when changing from supine to upright position and vice versa.

As the venous pressure outside the dura is clearly lower than the 10 mmHg inside the dura, this pressure fall will create a passive venous collapse just inside the dura. The collapse will increase upon an increase in intracranial pressure and decrease in venous pressure. A venous pressure fall just outside the dura following a change from supine to upright position, therefore, will not be transferred to the brain due to the compensatory passive increase in the venous collapse. A similar collapse of the veins outside the dura will have similar protecting effect (1). The normal brain is protected from arterial pressure variations via an active autoregulatory mechanism, which limits blood flow and hydrostatic pressure variations upon variation in position. The autoregulatory mechanism can be abolished following head injury. The venous protecting system presupposes a closed spinal canal. If the distal spinal canal is opened, as sometimes occurs in clinical practice after spinal lumbar puncture, the gravitational force on the venous side will be compensated for by a similar large force of the spinal canal, and head elevation may dilate the venous vessels (no venous collapse). This will increase the blood volume close to the pain sensitive meningeal structures. This scenario may result in a violent headache appearing after changing from supine to upright position and disappearing when returning to the supine position (2). 1) Kongstad L, Grände PO. Acta Anaesth Scand 1999; 43:501–8 2) Grände PO. Acta Anaesth Scand. 2005; 49:619–26