Perilymph is an extracellular fluid located within the cochlea

Cochlea

The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....

 (part of the ear

Ear

The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

) in two of its three compartments: the scala tympani

Scala tympani

Scala tympani is one of the perilymph-filled cavities in the cochlear labyrinth of the human ear. It is separated from the scala media by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as scala vestibuli....

 and scala vestibuli

Scala vestibuli

Scala vestibuli is a perilymph-filled cavity inside the cochlea of the inner ear that conducts sound vibrations to the scala media.It is separated from the scala media by Reissner's membrane and extends from the vestibule of the ear to the helicotrema where it joins scala tympani.-External links:* ...

. The ionic composition of perilymph is comparable to that of plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

 and cerebrospinal fluid

Cerebrospinal fluid

Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

. The major cation in perilymph is sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

: The values of sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 and potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 concentration in the perilymph are 138 mM and 6.9 mM, respectively.

Perilymph versus Endolymph

The inner ear

Inner ear

The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...

 has two parts: the bony labyrinth

Bony labyrinth

The Bony labyrinth or osseous labyrinth consists of three parts: the vestibule, semicircular canals, and cochlea. These are cavities hollowed out of the substance of the bone, and lined by periosteum; they contain a clear fluid, the perilymph, in which the membranous labyrinth is situated....

 and the membranous labyrinth

Membranous labyrinth

The receptors for the senses of equilibrium and hearing are housed within a collection of fluid filled tubes and chambers known as the membranous labyrinth...

. The membranous labyrinth is contained within the bony labyrinth, and within the membranous labyrinth is a fluid called endolymph. Between the outer wall of the membranous labyrinth and the wall of the bony labyrinth is the location of perilymph.

Perilymph and endolymph

Endolymph

Endolymph is the fluid contained in the membranous labyrinth of the inner ear. It is also called Scarpa's fluid, after Antonio Scarpa.-Composition:...

 have unique ionic compositions suited to their functions in regulating electrochemical impulses of hair cells. The electric potential of endolymph is ~80-90 mV more positive than perilymph due to a higher concentration of K compared to Na.

Perilymph is the fluid contained within the osseus labyrinth, surrounding and protecting the membranous labyrinth; perilymph resembles extracellular fluid in composition (sodium salts are the predominate positive electrolyte) and, via the perilymphatic duct, is in continuity with cerebrospinal fluid.

Syn: Perilympha [NA], Cotunnius' liquid, and liquor cotunnii.

Endolymph is the fluid contained within the membranous labyrinth of the inner ear; endolymph resembles intracellular fluid in composition (potassium is the main positively-charged ion).

Syn: Endolympha [NA], Scarpa's fluid, and Scarpa's liquor.

Clinical significance

It has also been suggested that perilymph and endolymph participate in a unidirectional flow that is interrupted in Ménière's disease

Ménière's disease

Ménière's disease is a disorder of the inner ear that can affect hearing and balance to a varying degree. It is characterized by episodes of vertigo and tinnitus and progressive hearing loss, usually in one ear. It is named after the French physician Prosper Ménière, who, in an article published...

.