Lungs

Pair of lungs chilling in space

Lungs are the organs interfacing between the atmospherics and circulatory systems. They intake gases, including oxygen, and transfer a part of it into the circulatory system.

Lungs have a specific rate at which they do this intake. If the equilibrium between amount of oxygen needed by body parts and amount of oxygen available gets negative (e.g when too much blood is lost or when oxygen in air is low), then the intake rate may increase.

Lungs are working independently from each other, meaning a human can survive with only one lung left, even though it will be able to intake only half the amount of oxygen it needs, and so will have to breath two times faster.