# Could you theoretically make a Human-like/Human-descended species breathe chlorine instead of oxygen?

What would be the major problems/issues from switching a Human lifeform from O2-based respiration to a Chlorine one?

Would said lifeform require not only adaptations to the respiratory system, but also an recalibration of its circulatory system and installation of extra or "enhanced" filtration systems (ie., more kidneys, bigger kidneys or ultra-efficient ones)? What chemical element would be best bind with Chlorine, within the bloodstream? What kind of adaptations should be expected to the blood vessels and the blood components, themselves?

• Would need to rebuild your biochemistry from the ground up. Lose the ADT-ATP cellular energy source. Likely build your DNA from other molecules. Different cellwall membrane, different... well, different everything – PcMan Feb 16 at 10:05
• A springboard for research might be: Fluorine-Rich Planetary Environments as Possible Habitats for Life if it's halogens you're interested in. – A Rogue Ant. Feb 16 at 16:35

In addition to the problem of maintaining a chlorine-rich atmosphere, any chlorine-breathing animal would need to use different biochemistry from Earthly life. Redesigning all of biochemistry would be a task of extraordinary complexity.

It would be easier to take native chlorine-using life and bioengineer it into sapience. Even if you were starting from the equivalent of bacteria. Modifying an individual human to become chlorine-breathing is a task for magic, or technology so advanced that it might as well be magic.

• Very true, virtually none of the existing metabolic pathways would work as they are sensitive to conditions. – Slarty Feb 16 at 16:43

The main problem you will have is to make and keep the atmosphere chlorine rich.

Chlorine, like oxygen, is highly reactive and will bind chemically to any other atomic species it finds available. The reason we have a lot of oxygen in our atmosphere is that photosynthetic organisms have been dumping it as a waste product for millions of years.

In the photosynthetic process, $$CO_2$$ and $$H_2O$$ are turned into $$C_6H_{12}O_6$$ and $$O_2$$.

$$C_6H_{12}O_6$$ has a ring structure with one oxygen atom in the ring. This happens because oxygen will take two valence electrons to form a chemical bond.

Chlorine, on the other hand, takes only one valence electron when forming a compound: that's highly impractical for making molecules used in life.

Without the equivalent of the photosynthetic organisms, you can't have a chlorine breathing organism of any sort.