Where is CSF secreted?

• A. In the subarachnoid space
• B. In the ventricles of the brain
• C. In the brain parenchyma
• D. In the spinal canal

Answer: (B)

CSF is produced mainly by the choroid plexus within the brain’s ventricular system. The choroid plexus, located in the lateral, third, and fourth ventricles, is a specialized network of ependymal cells paired with a rich capillary bed. These cells actively transport ions, and water follows, creating a clear CSF filtrate that fills the ventricles and then flows through the ventricular system into the subarachnoid space. From there, CSF circulates around the brain and spinal cord and is ultimately absorbed into the venous system via arachnoid granulations.

The subarachnoid space is where CSF resides after production, not where it is secreted. The brain parenchyma and the spinal canal are not primary sites of CSF secretion. Production mainly in the ventricles explains why CSF originates there and then circulates outward.