**Abstract**

The function of the cerebrospinal fluid (CSF) has long been considered for mechanical protection and recently attributed to the supply of nutrients to the brain. However, we hypothesize that the brain is a water-cooled and water-cleaned system. Recent studies on the glymphatic pathways and the introduction of cisternostomy as a surgical procedure for traumatic brain injury reveal a vast and in-depth functionality of the CSF, which works in synchrony with the cardiopulmonary rhythms to act as a buffer for optimum cerebral function. The nasal sinuses are located around the suprasellar cistern, and the evaporating wet mucosa within them during the breathing contributes to local cooling, whereas the nocturnal activation of AQP4 channels allows CSF-ISF exchange. The resultant "cooling and cleaning" of the brain not only maintains a physiological equilibrium but also opens doors for understanding and treating pathophysiology underlying common degenerative and neuro-inflammatory diseases. This chapter describes the novel theory of brain cooling and cleaning and the clinical and experimental evidence to support this hypothesis.

**Keywords:** glymphatic pathway, cisternostomy, CSF shift, Virchow Robin spaces, subarachnoid cisterns, hydrocephalus, aquaporin-4
