How can the respiratory system of my genetically engineered people be arranged, having one or two pairs of lungs, one of which uses the tactics of liquid breathing to breathe underwater? (not gills, like ichthyander, but lungs filled with a special fluid that delivers oxygen) Liquid breathing involves filling the lungs with liquid saturated with dissolved oxygen, which enters the bloodstream. The most suitable substances for this purpose are perfluorocarbon compounds that dissolve oxygen and carbon dioxide well, have a low surface tension, are highly inert, and are not metabolized in the body.
So since the creation of the first corresponding fluids, this technology has been improved and the most advanced solution at the moment is called Perflubron or "Liquid" (commercial name). This oil-like transparent liquid with a density twice the density of water has many useful qualities: it can carry twice as much oxygen as ordinary air, has a low boiling point, so after use, its final removal from the lungs is done by evaporation. Alveoli under the influence of this liquid open better, and the substance gains access to their contents, this improves the exchange of gases.
In short, how will a liquid breathing system work?
Note: if someone does not understand, then I want to create a biologically reliable system that synthesizes the above liquid in a natural way, just like our body creates blood or gastric juice. How about creating a special third lung for storing respiratory fluid, where it is drained if it is not needed?