为什么碱中毒会导致低钙血症？

碱中毒时，血浆中的H离子浓度下降，同时游离的Ca2+离子的浓度也下降。这是因为有一部分的H+离子和Ca2+离子可以和血浆白蛋白结合，二者是竞争性的。当白蛋白上结合的H+离子多了，能结合的Ca2+离子就相应的减少。当血液变成碱性的时候，与白蛋白结合的H+离子从白蛋白上脱离，使这部分白蛋白可以和Ca2+离子结合，从而使血浆中的游离钙离子减少，因此表现出低钙血症的症状。

As blood plasma hydrogen ion concentration decreases, caused by respiratory or metabolic alkalosis, the concentration of freely ionized calcium, the biologically active component of blood calcium, decreases. Because a portion of both hydrogen ions and calcium are bound to serum albumin, when blood becomes alkalotic, the bound hydrogen ions dissociate from albumin, freeing up the albumin to bind with more calcium and thereby decreasing the freely ionized portion of total serum calcium. For every 0.1 increase in pH, ionized calcium decreases by about 0.05 mmol/L. This hypocalcaemia related to alkalosis is partially responsible for the cerebral vasoconstriction that causes the lightheadedness, fainting, and paraesthesia often seen with hyperventilation.

https://en.wikipedia.org/wiki/Hypocalcaemia