Cochlear Fluids Composition

Perilymph is a typical extracellular fluid, with ionic composition comparable to plasma or cerebrospinal fluid (CSF), as shown above. The main cation is sodium. The compositions of ST perilymph and SV perilymph are not the same. SV perilymph has a significantly higher potassium level, and slightly lower sodium.

Endolymph is a totally unique extracellular fluid, with an ion composition unlike that that found anywhere else in the body. The major cation in endolymph is potassium, and there is virtually no sodium. Because endolymph is so unlike other body fluids, the processes by which it is maintained and regulated cannot be extrapolated from other fluid systems. As shown in the chart above, the total ion content of endolymph is significantly higher than that of perilymph.

Cochlear endolymph is also unique in other respects, as shown in the table below.
It has an extremely low calcium content (approximately 20 microMolar).
It is held at a positive voltage with respect to perilymph, of approximately 85 mV. This is called the endocochlear potential
Both the low Ca level and endocochlear potential are extremely important for the cochlea to function normally. Even small changes from the normal state result in a decrease of hearing sensitivity.

Chemical Composition of the Cochlear fluids

                       ST           SV
                   Perilymph    Perilymph   Endolymph    CSF
 Sodium (mM)          148          141          1.3     149
 Potassium (mM)         4.2          6.0      157         3.1
 Chloride (mM)        119          121        132       129
 Bicarbonate (mM)      21           18         31        19
 Calcium (mM)           1.3          0.6        0.023     -
 Protein (mg/dl)      178          242         38        24
 pH                     7.3          7.3        7.4       7.3
 Potential (mV)         0           <3         85         0 
ST = scala tympani, SV = scala vestibuli, CSF = cerebrospinal fluid

Data summary from Wangemann & Schacht, 1996

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Page generated by: Alec N. Salt, Ph.D.,
Cochlear Fluids Research Laboratory,
Washington University, St. Louis