17.26: Cerebrospinal Fluid
Cerebrospinal fluid (CSF) is a colorless liquid that flows around the brain and the spinal cord, playing a vital role in the protection, support, and overall function of the central nervous system (CNS). CSF production, circulation, and absorption are tightly regulated processes essential for the brain and spinal cord to function properly.
CSF Production
CSF is produced mainly in the choroid plexus, a network of capillaries and ependymal cells located within the ventricular system of the brain. The total volume of CSF in an adult human is approximately 125-150 mL. About 500 mL of CSF is produced daily, meaning the entire CSF volume is replaced roughly every 5 to 6 hours. The composition of CSF is primarily water (about 99%), with the remaining 1% consisting of proteins, glucose, electrolytes, and other substances. The composition of CSF differs from that of blood plasma, as it has fewer proteins, a lower concentration of glucose, and a higher concentration of chloride ions.
The production of CSF by the choroid plexus involves active secretion, filtration, and selective diffusion. Ependymal cells transport ions, glucose, and other substances from the blood into the ventricular system. They also actively secrete specific molecules and maintain the unique composition of the fluid. The blood-brain barrier (BBB), formed by the tight junctions between endothelial cells of the capillaries and ependymal cells, prevents the passage of many large molecules, pathogens, and toxins from the bloodstream into the CSF.
CSF Circulation
CSF circulates through a network of four interconnected cavities in the cerebrum - two lateral ventricles, the third ventricle, and the fourth ventricle. It flows from the lateral ventricles into the third ventricle through the interventricular foramina. From there, it flows through the cerebral aqueduct into the fourth ventricle. Finally, CSF exits the ventricular system through the median aperture and the two lateral apertures, entering the subarachnoid space of the brain and spinal meninges.
CSF is ultimately absorbed back into the bloodstream via arachnoid granulations, specialized structures that protrude into the venous sinuses. Several factors, including arterial pulsations, respiratory pacing, postural changes, and intracranial pressure, influence the flow of CSF.
Functions of CSF
CSF serves several essential functions in the human body. It acts as a cushion, absorbing mechanical shocks and distributing pressure evenly around the brain and spinal cord. This property helps protect these delicate structures from injury. The relatively heavy brain is suspended in CSF, reducing its effective weight and preventing it from compressing the base of the skull or damaging the underlying neural tissues. Additionally, CSF maintains the optimal chemical environment for neuronal function by regulating the concentrations of ions, nutrients, and waste products. It also helps maintain a stable pH and temperature within the CNS. Finally, CSF serves as a medium for distributing immune cells and antibodies, providing a first line of defense against potential infections and other threats to the CNS.
