Volume homeostasis

The Monroe-Kelley doctrine states that the volume of the craniospinal compartment which comprises blood, cerebrospinal fluid, and brain parenchyma remains constant. Increased intracranial volume will initially lead to displacement of these three components and reduced production of cerebrospinal fluid (volume compensation). Initially, intracranial pressure (ICP) will rise slowly, but progressive exhaustion of the compensatory mechanisms causes an exponential rise in ICP. Normal recumbent ICP is between zero and 13 mmHg. A sustained ICP in excess of 20 mmHg is pathological. Elevation of ICP reduces the cerebral perfusion pressure, which is defined as the difference between mean arterial pressure and ICP. At cerebral perfusion pressures below 50 mmHg, autoregulation of cerebral blood flow is compromised. If cerebral blood flow falls below 18 ml/100 g/min, the metabolic requirements of brain cells are not met, the cells become ischemic, and energy-dependent membrane pumps fail (Fernandez^ A vicious cycle develops, with intracellular migration of water (hydrostatic edema) causing a further rise in ICP. Falling cerebral perfusion pressure may invoke a compensatory rise in mean arterial pressure in an attempt to maintain it (Cushing response).

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