10.3 Cerebrospinal fluid

• Cerebrospinal fluid (CSF) is a clear, colorless liquid.
• Protects the brain and spinal cord from chemical and physical injuries.
• Contains oxygen, glucose, from the blood to neurons and neuroglia (also contains proteins, lactic acid, urea, cations (Na+, K+, Ca²+, Mg²+), and anions (Cl^– and HCO^3-).
• CSF continuously circulates through cavities in the brain and spinal cord and around the brain and spinal cord in the subarachnoid space (the space between the arachnoid mater and pia mater).
• The total volume of CSF is 80 to 150 mL in an adult.
• It also contains some white blood cells.

10.3.1 Functions of CSF

The CSF has three basic functions:

1. Mechanical protection – CSF serves as a shock-absorbing medium that protects the delicate tissues of the brain and spinal cord.
2. Homeostatic function – The pH of the CSF affects pulmonary ventilation and cerebral blood flow, which is important in maintaining homeostatic controls for brain tissue.
3. Circulation – CSF is a medium for minor exchange of nutrients and waste products between the blood and adjacent nervous tissue.

10.3.2 Formation of CSF in the Ventricles

• The majority of CSF production is from the choroid plexuses (Choroid Plexus networks of blood capillaries in the walls of the ventricles).
• Ependymal cells joined by tight junctions cover the capillaries of the choroid plexuses.
• Blood in the choroid plexus filtered (selected substances mostly water), are secreted by the ependymal cells to produce the cerebrospinal fluid.
• This secretory capacity is bidirectional and accounts for continuous production of CSF and transport of metabolites from the nervous tissue back to the blood.
• Because of the tight junctions between ependymal cells, materials entering CSF from choroid capillaries cannot leak between these cells; instead, they must pass through the ependymal cells.

Figure 15- Choroid Plexus

10.3.3 Circulation of CSF

• The CSF formed in the choroid plexuses of each lateral ventricle flows into the third ventricle through two narrow, oval openings, the interventricular foramina.
• More CSF is added by the choroid plexus in the roof of the third ventricle. The fluid then flows through the aqueduct of the midbrain (cerebral aqueduct), which passes through the midbrain, into the fourth ventricle.
• The choroid plexus of the fourth ventricle contributes more fluid.
• CSF enters the subarachnoid space through three openings in the roof of the fourth ventricle: a single median aperture and paired lateral apertures, one on each side.
• CSF then circulates in the central canal of the spinal cord and in the subarachnoid space around the surface of the brain and spinal cord.
• CSF is gradually reabsorbed into the blood through arachnoid villi.
• Normally, CSF is reabsorbed as rapidly as it is formed by the choroid plexuses, at a rate of about 20 mL/hr (480 mL/day).

(Because the rates of formation and reabsorption are the same, the pressure of CSF normally is constant. For the same reason, the volume of CSF remains constant.)

Figure 16- Summary of the formation, circulation, and absorption of cerebrospinal fluid (CSF)