Getty/Jason Hosking. A niche is a term that is used in ecological biology to define an organism's role in an ecosystem. Not only does a niche include the environment a given organism lives in, it also includes the organism's job in that environment.
Perilymph (also known as Cotunnius' liquid, and liquor cotunnii) is an extracellular fluid located within the cochlea (part of the inner ear) in two of its three compartments: the scala tympani and scala vestibuli. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major cation (positively charged ion) in perilymph is sodium: The values of sodium and potassium concentration in the perilymph are 138 mM and 6.9 mM, respectively.
Here are the four valves in order of circulation: TRICUSPID VALVE. Closes off the upper right chamber (or atrium) that holds blood coming in from the body. Opens to allow blood to flow from the top right chamber to the lower right chamber (or from right atrium to right ventricle). Prevents the back flow of blood from the ventricle to the atrium when blood is pumped out of the ventricle.