In order to function as it should, human blood requires an appropriate balance of acidic and alkaline compounds, known as the "acid-base balance." While this is normally a function performed by our kidneys and lungs, sometimes this state of balance can become skewed; an example of this is acidosis, when the blood becomes too acidic, or alkalosis, when the blood becomes too basic.
When we vomit, we are expelling contents of our stomachs—including hydrochloric acid (HCl)—from our body. These acidic contents are lost without an equivalent loss of the alkaline bicarbonates. Thus, the remaining liquid contents of the stomach—which are alkaline rather than neutral—are absorbed by blood vessels and carried off to the bloodstream. While this is not a process that happens with minor bouts of vomiting, it will occur with prolonged vomiting.
The short answer is that repeated vomiting causes the blood to become more alkaline. The acid-base balance of anything is measured on the pH...
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