In the average human, the total body water is about 60% of the body weight . The brain is highly sensitive to the amount and distribution of water in the intracranial space. The intracranial space enclosing the brain has a capacity of about 1600–1700 ml. About 100–150 ml of this capacity is occupied by cerebrospinal fluid (CSF) . Other components in the intracranial cavity include blood (100–150 ml). The remaining volume is comprised of brain parenchyma and intracellular and interstitial spaces. The brain parenchyma, CSF, and blood all interact with one another in a pressure–volume relationship known as the Monro–Kellie doctrine. According to the Monro–Kellie doctrine, the volume inside the cranium is a fixed volume and the intracranial components are in equilibrium such that an increase in one component (i.e. brain parenchyma) must lead to a compensatory decrease in another component. Since the brain parenchyma is minimally compressible, the primary buffers for increased intracranial volumes are CSF and to a lesser extent blood. The total intracranial and extracranial volume of CSF is divided between the ventricular system (35 ml, 25%), the spinal canal (30–70 ml, 20–50%), and the cranial subarachnoid space (35–75 ml, 25–55%) [2,3]. Cerebrospinal fluid is formed at a rate of approximately 500 ml each day. This volume is three to four times as much as the total volume in the entire CSF system and reflects that CSF is constantly being replaced . CSF function CSF provides physical protection for the brain and spinal cord through buoyancy and thus by reducing the active weight of the nervous structures. Immersed in CSF, the active weight of the brain is reduced from 1500 g to as few as 50 g thus reducing tension on the nerve roots and vessels and the risk of injury in case of trauma . More importantly, CSF functions as a reservoir to get rid of metabolites and toxins from the brain. It also plays a role in homeostatic hormonal signaling, chemical buffering, circulations of nutrients, and neurodevelopment [4–7].
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