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🌟 Cerebrospinal fluid

 🎯 What is cerebrospinal fluid (CSF)?

  • a clear, colourless, watery fluid that surrounds and bathes the brain and spinal cord

  • fills the subarachnoid space of the meninges (the space between the arachnoid and pia mater)

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💧 How is CSF produced?

  • CSF is the ultrafiltrate of plasma which is produced within the ventricular system of the brain by the choroid plexus

  • the choroid plexus is a specialised vascular structure composed of modified ependymal cells and fenestrated capillaries (with no tight junctions) that filter blood

    • its function is similar to that of the glomeruli in the kidneys

  • proteins, glucose and cells are too large to pass through

    • glucose enters CSF via GLUT1 transporters at the blood-brain-barrier (the composition of glucose in the CSF is 60% of that of plasma)

    • amino acids enter via specific transporter proteins

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🧐 How much CSF is in the CNS?

  • about 150 ml in an adult, and it can be replenished up to four to five times a day

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⚡️ Functions of CSF

  • mechanical protection to the brain and spinal cord from trauma

  • buoyancy, reducing the weight of the brain (the brain and spinal cord are “floating”) from 1,500 grams to 50 grams (which is 97% less!)

  • removal of waste products of metabolism

  • acts as a transport medium for the exchange of products between the central nervous system and plasma

  • maintenance of homeostasis of the interstitial fluid

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🌊 Circulation of CSF

  • CSF travels through the ventricular system of the brain which are lined by ependymal cells

  • course of CSF:

    • lateral ventricles (there are two) —> third ventricle (via the interventricular foramen of Monro) —> fourth ventricle (via the cerebral aqueduct)

      • CSF can exit the fourth ventricle and enter the subarachnoid space around the brain and spinal cord via three main foramina:

        • lateral foramina of Luschka (there are two)

        • midline foramen of Magendie

      • the rest of the CSF flows down the fourth ventricle into the central canal of the spinal cord

  • absorption of CSF

    • CSF is absorbed back into the blood, specifically into the superior sagittal sinus via specialised structures known as arachnoid granulations

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🛡 Blood-brain-barrier​

  • a protective, highly regulated interface between the bloodstream and central nervous system

  • composition of the blood-brain-barrier:

    • ​astrocytic end-feet

    • endothelial cells

    • tight junctions between endothelial cells

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The blood-brain barrier and systemic infections

  • non-polar (lipophilic) molecules are more likely to pass through the blood-brain barrier than polar (hydrophilic) molecules

  • some antibiotics are hydrophilic, meaning they cannot pass through the barrier

  • although the integrity of the barrier can get disrupted as a result of systemic inflammation, making it easy for hydrophilic antibiotics to passively pass through and penetrate brain tissue

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