What do high levels of CO2 in water cause

It reacts with water to produce carbonic acid.  

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As it combines with water, it forms carbonic acid, making the blood acidic. So CO2 in the bloodstream lowers the blood pH. When CO2 levels become excessive, a condition known as acidosis occurs. This is defined as the pH of the blood becoming less than 7.35.

Explanation:

answer; the tunica intima also called tunica interna, consists of elastic fibers,connective tissue and smooth muscle cells;