tropic

trooppinen

kääntöpiiri Either of the two parallels of latitude 23°27′north and south of the equator; the farthest points at which the sun can be directly overhead; the boundaries of the torrid zone or tropics.
Of, or relating to the tropics; tropical.
(qualifier) hot and humid.
puhekieltä (noncomparative) Having the quality of indirectly inducing a biological or chemical change in a system or substrate.

The binding of oxygen to hemoglobin is allosterically regulated by various tropic factors, such as BPG and acidity.