Cerebrospinal fluid circulates around the brain in the __ space.
A Epidural
B Subarachnoid
C Subdural
D Endoneurial
E Central Canal
The correct answer and explanation is:
Correct Answer: B. Subarachnoid
Explanation (300 words):
Cerebrospinal fluid (CSF) circulates around the brain and spinal cord within the subarachnoid space. This space lies between two layers of the meninges: the arachnoid mater (middle layer) and the pia mater (innermost layer). The subarachnoid space not only contains cerebrospinal fluid but also blood vessels that supply the brain and spinal cord.
CSF is a clear, colorless fluid that serves multiple critical functions:
- Protection: It cushions the brain and spinal cord from trauma by absorbing shocks.
- Buoyancy: It reduces the effective weight of the brain, preventing it from compressing nervous tissue.
- Homeostasis: It helps maintain chemical stability by removing waste products from the brain and distributing nutrients.
- Immune function: It acts as a barrier against toxins and pathogens.
The circulation of CSF starts in the choroid plexuses of the brain’s ventricles, flows through the ventricles, and enters the subarachnoid space via the foramina in the fourth ventricle. It then circulates around the brain and spinal cord and is eventually absorbed into the bloodstream through the arachnoid villi into the superior sagittal sinus, a major venous structure in the brain.
Why the Other Options Are Incorrect:
- A. Epidural: This is the space between the skull or vertebral bones and the dura mater; it does not contain CSF.
- C. Subdural: Lies between the dura mater and arachnoid mater, but normally contains only a small amount of fluid and no circulating CSF.
- D. Endoneurial: Found within peripheral nerves, surrounding individual nerve fibers—not part of CSF circulation.
- E. Central Canal: This narrow canal in the spinal cord contains CSF but is not the main area of circulation around the brain.
Conclusion:
The subarachnoid space is the correct answer because it is the primary area where CSF circulates around the brain and spinal cord.