Brain thermodynamics

Thermodynamics is the branch of physics concerned with heat, temperature and energy. There are three major thermodynamic systems: open, closed, and isolated systems. Our brain is an open system which follows four laws of thermodynamics: a) zeroth law defines thermal equilibrium; b) first law is about energy quantity; c) second law is about energy quality; and d) third law is related to thermal fluctuations. Since thermodynamics is dealing with thermal energy in equilibrium and in transit, heat production and dissipation via heat transfer processes are also important to comprehend. Pertaining to the brain, heat production is directly linked to brain metabolism and information processing, while heat dissipation occurs through conduction involving blood vessels, through CSF and air convection, and through radiation which is largely prevented by the presence of glistening inner dura and layers covering the brain. Therefore, the following features may modulate brain temperature: a) brain blood supply; b) presence of venous sinuses and plexuses surround or near main arteries; c) glymphatic system; d) air and air sinuses at skull base; and e) the black box-like cranium. The insights gained from this review are a) decompressive craniectomy could also work by augmenting heat dissipation through radiation; b) early cranioplasty to recreate optimal brain thermodynamic system; c) direct brain cooling for the injured brain; d) basal cisternostomy could also reduce brain heat via convection, radiation and conduction; e) innovation related to brain thermodynamics such as focal cooling and light therapy; and f) provide basic knowledge related to MR brain-thermometry.