Rather than in hypoxia, a more general term denoting a shortage of oxygen, ischemia is an absolute or relative shortage of the blood supply to an organ. Relative shortage means the mismatch of blood supply (oxygen delivery) and blood request for adequate oxygenation of tissue.
This can be due to:
- atherosclerosis (lipid-laden placques obstructing the lumen of arteries)
- hypotension (low blood pressure, e.g. in sepsis, heart failure)
- thromboembolism (blood clots)
- outside compression of a blood vessel, e.g. by a tumor
- foreign bodies in the circulation (e.g. amniotic fluid in amniotic fluid embolism)
As the carrier of oxygen (oxygen is mainly bound to hemoglobin) insufficient blood supply leads to hypoxic tissue (anoxic in case of no oxygen supply at all) with the consequence of necrosis which determines the celldeath.