When we discussed hypoxia vs hypoxemia last week, we briefly mentioned ischemia. That word is probably worth taking an extra look at.
Ischemia is caused by restricted blood flow to an organ or even specific tissue. Restricted blood flow, of course, results in a shortage of oxygen and glucose. In this way, ischemia leads to hypoxia but it can’t really be caused by hypoxemia…sort of. Hypoxemia is strictly a shortage of oxygen, but not glucose, so it’s not quite the same. Again, it may look the same on a gross scale, but it’s important to understand these seemingly minute differences. The pathogenesis is quite different.
When we use the term ischemia, we normally think of tissue death. This is true in that prolonged ischemia can result in infarction, but we have to remember that ischemia refers specifically to the restricted blood flow. So, prior to this stage, there is such a thing as reversible ischemia. Any tissues or organs in your body that have a high metabolic demand are especially susceptible to infarction or tissue death caused by prolonged ischemia. Heart and brain are probably the first ones that come to mind, right? Don’t take that pun to heart. Or that one.
It is also interesting to note that the kidney can be especially at risk for ischemic damage. Obviously, lot of blood flowing through that area, but all of those pumps that allow us to resorb electrolytes and such? They use ATP. With a lack of oxygen and glucose supply, they can quickly run out.
Another interesting area is the colon. There is an important watershed area right where the colon heads south by the spleen. A watershed area is location that is, in a way, between blood supplies. So, the splenic flexure is fed blood by both the superior and inferior messenteric artery, but just barely from each. At that point, it is the very ends of their arterial branches that are working together to provide enough. If one of those goes down then that watershed area is particularly vulnerable. Also, since the splenic flexure is located so far from the blood sources, arterially speaking, we have plenty of options for disrupting that blood flow.
Quick side note, fixing ischemia can also be dangerous. Its called repurfusion injury. When the blood returns to quickly, that damaged tissue can undergo further stress due to the sudden influx of free radicals. It’s always kind of crazy to me that our interventions can do exactly what we’re hoping but still cause damage by their very nature.