hope this image help to understand it, the pumps don't work because lack of ATP
I teg atht eth reanws is rtorecc ofr a lrevbeersi yriunj eewhr tehre si llce silnwlge eabcseu fo eth irscaedne neuartcrallil aN+ dan C2a+ deu ot irmadpie /aKN nad omapcsarilcs irmectuul ayictivt ...
tBu if rethe aer ndserceai aaccird esymezn ni eht oobdl aiigtdnnic llce dtahe dan mmeebran aem,adg ’twuolnd eht eiurtallnarcl etolrlsyctee eb low nscie hyte are edlresea iton eht ?dloob
In cellular ischemia, the Na+/K+ ATPase pump stops working due to decreased ATP levels. Consequently, sodium is not pumped out and potassium is not pumped into the cell, leading to an accumulation of sodium in the cell and potassium outside the cell. Furthermore, the sarcoplasmic reticulum Ca-ATPase pump fails, which leads to an increase in calcium in the cell.
Bottom line, ischemic tissue: there is a buildup of sodium and calcium in the cell.
I thought this was a pretty good summary from wikipedia.
Steps 1-4 explain the question:
Another way to think about this is just that decreased O2 leads to dysfunction of the Na+/K+ ATPase as others have mentioned.
This is pretty much identical to the mechanism of digoxin, which blocks the Na/K ATPase and calcium accumulates in the cell because it cant be exchanged for extracellular Na+ (which is not intracellular due to defective Na/K ATPase)