There is three quotations from the encyclopedia entry (written by Elisabeth Hsu) I have united in one paragraph:
“Qixue refers to “breath” (qi) and “blood” (xue), which are viewed as flowing constantly through the body, being mutually transformed into one another. Qi is generally attributed with Yang aspects and xue with Yin aspects, though the terms are sometimes used interchangeably, depending on the context in which they are mentioned. In those cases qi and xue do not designate different entities, but different aspects of the same entity. Jing, qi, xue, and mai are all said to be aspects of qi.” It looks good enough to get all main features of qìxuè 氣血 term instantly.
The brightest example of mutual interchangeability can be found in some medical books of ancient China where the blood is called qi before the “middle burner” (zhōngjiāo 中焦, the stomach system) and there it “transforms into a red liquid.”
Of course, I completely understand that modern science is modern science and biologists have their point of view on breath and blood relationships inside our bodies but the wholeness of knowledge educated people of old times kept in their hearts is still absolutely amazing fact for me.