Hypoxia is a phenomenon that happens because of a lack of oxygen in cells and tissue. This can happen in a number of ways, and ischemia is one of them. Ischemia – “isch” means restraint or suppression and “emia” refers to blood. So ischemia means some kind of suppression or reduction of blood flow to an organ or tissue.
Blood carries oxygen, so when there’s a reduction in blood flow to cells, that also means that there’s a reduction of oxygen to those cells. This is due to lowered blood flow in the blood vessels. This lowered flow could be from something blocking the blood from the inside, or it could be something compressing the blood vessel from the outside.
An example of something blocking the blood vessel from the inside is a thrombus also known as a blood clot. These are solid clumps of platelets and fibrin that obstruct blood flow. Ischemia resulting from something outside the blood vessel is a traumatic injury, which can cause inflammation and swelling. This physically applies external pressure to the blood vessel, compresses it, and restricts blood flow.
Let’s consider an artery, which carries blood to a specific organ. There the oxygen is given to the organ and the waste is picked up by the blood. Then the deoxygenated blood cells drain out through veins and go back toward the heart.
So, one way that this organ would become ischemic, if there’s some obstruction to arterial flow into the tissue. Then, only a few red blood cells get in at a time. The organ gets a lot less blood and a lot less oxygen and becomes ischemic.
A super important and well-known example of this arterial ischemia is atherosclerosis.
In atherosclerosis, where plaque builds up in the arteries going to your heart tissue, which blocks the arterial flow. This reduces the amount of blood and oxygen that make it to your heart tissue and causes ischemic heart disease.
Since you can have a blockage of red blood cells coming in, you can also have a blockage of red blood cells going out, leading to a decrease in blood drainage on the venous side.
If the oxygen supply is low enough for long enough, it can cause cell death. Cells die in a region of tissue and we could call that as tissue necrosis and infarction. In some cases, there are areas of tissue that are getting close to an infraction, as cells begin to die off. But they are still able to get saved if they receive blood. This high-risk area that’s teetering on the brink of death is called the ischemic penumbra.
A number of causes may produce ischemia. These causes are discussed below with regard to different levels of blood vessels.
1. Cause in the heart
Inadequate cardiac output resulting from heart block, ventricular arrest and fibrillation from various causes may cause hypoxic injury to the brain.
2. Causes in arteries
The most common and most important causes of ischaemia are due to obstruction in arterial blood supply.
- Thrombosis
- Embolism
- Arteriosclerosis
- Vasospasm
- Hypothermia
3. Causes in the veins
Blockage of venous drainage may lead to engorgement and obstruction to arterial blood supply resulting in ischemia. The examples include the following:
- Thrombosis of mesenteric veins
- Volvulus
- Cavernous sinus thrombosis
- Varicose veins of the legs
- Strangulated hernia
4. Causes in the microcirculation
Ischemia may result from occlusion of arterioles, capillaries, and venules. The causes are as under:
- By red cells- in sickle cell anemia, sludging of blood, acquired hemolytic anemia, red cells parasitizes by malaria
- By white cells- in chronic myeloid leukemia
- By fat embolism
- Frost-bite injuring the wall of small blood vessels
- By fibrin-defibrination syndrome
If perfusion (volume of blood flowing through a certain mass of tissue per unit time) to the area is re-established quickly, then it’s possible that the ischemia is reversible. Because the cells were dying out but not dead. If too much time passed and the cells have actually died, then the ischemia is irreversible since you can’t bring back dead cells.
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