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.
Pericardial Effusion. Pericardial effusion is abnormal fluid build up in the membrane surrounding the heart called the pericardial sac. The clinical signs owners report depends on the amount of fluid accumulation as well as the rate of development of the fluid. Slowly accumulating fluid allows time for the pericardial sac to stretch and expand. Since the heart has time to adjust to the new fluid accumulation the intracardiac pressure does not increase to affect the filling ability of the heart until a critical point is reached.