What Is CSF? - Definition, Function, Pressure & Analysis

Instructor: Aileen Staller
Cerebrospinal fluid (CSF) is a clear, colorless fluid that bathes, nourishes, and protects the brain and spinal cord. Imagine the brain as an object in a snow globe; floating freely in the clear fluid (minus the snowflakes). In this lesson we will discuss how CSF is produced, its function, the normal contents and pressure, and how it is analyzed to provide clues to disease.


Cerebrospinal fluid (CSF) is clear, colorless, and filtered from the blood by cells in the ventricles (fluid spaces). The body produces 500 ml/day, but reabsorption is slower, so only 100-150 ml is found circulating around the brain and spine. CSF flows from the ventricles to the subarachnoid space (a space between the covering membranes) and then around the brain and spine where it is reabsorbed by openings in the membrane (arachnoid granulations).


Cerebrospinal fluid provides buoyancy, allowing flotation of the brain without pressure. In CSF, a 1400 gram brain responds as though it weighs only 25 grams. This protects the brain from external forces, allowing your brain to bounce away from the skull when you are hit in the head. CSF is also believed to cleanse the brain of toxins and metabolic by-products, but the exact mechanism is unknown.

CSF Contents and Pressure

CSF contains more Na+ and Cl- than blood but less potassium, (K+), calcium (Ca++), proteins and glucose. Normal CSF glucose is usually 2/3 of blood glucose. Normal CSF protein is 15-60 mg/100 ml and contains no red blood cells (RBC) or clotting factors and only 0-3-white blood cells (WBC).

CSF is in a closed system, so pressure is an important component of analysis. When lying down, the pressure is 10-18 cm H2O (8-15 mm Hg); in the sitting position it is 20-30 cm H2O (16-24 mm Hg).

Clinical Analysis

CSF pressure is measured by lumbar puncture (LP) or spinal tap. The skin is anesthetized and a thin needle is introduced, below the end of the spinal cord, allowing removal of CSF without damaging the spinal cord. Elevated pressure usually indicates swelling, infection, or mass (tumor or clot).

Elevated CSF protein may indicate meningitis, other infections, tumors, or neurologic diseases. Analysis of protein types is needed for more definitive diagnosis. In infection and inflammation, there are increased WBC's as the body tries to fight the disease. In bacterial meningitis the CSF glucose is decreased, as the bacteria use up the glucose for nutrition during growth.

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