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.
As the de-saturated blood leaves the right ventricle, it passes through the pulmonary valve, which has been closed as the right ventricle was filling. The pulmonary valve opens, allowing the blood to leave the right ventricle and flow to the lungs via the pulmonary artery. Once the right ventricle has emptied, the pulmonary valve closes, thereby keeping the blood from re-entering the right ventricle. > Next: Valves of the Heart: Circulation of Blood, Part 2.