Hypoxia, or lack of oxygen in cells and tissue, can happen in a number of ways, and ischemia’s
one of them.
Ischo- means “restraint” or “suppression”, and -emia refers to the blood, so ischemia
must mean some kind of suppression or reduction of blood flow to an organ or tissue.
And blood carries oxygen right?
So when there’s a reduction in blood flow to cells, that also means there’s a reduction
of oxygen to those cells, and 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 traumatic injury, which
can cause inflammation and swelling that physically applies external pressure to the blood vessel,
compresses it, and restricts blood flow.
Alright, so let’s say this is your artery, and it’s like the one-way highway leading
all these red-blood-cells into the city, which is like a major organ in the middle here,
so maybe this is organ-apolis.
These red blood cells are super pumped for their day where they can drive around the
capillaries, like the smaller city streets, and supply the city with fresh oxygen and
pick up waste.
And this organ-apolis is made up of thousands of cells, like homes, that use up the oxygen
and create waste that needs to be picked up, the deoxygenated blood cells drain out through
different small streets which are the veins and go back towards the heart.
So one way that this organ-city could become ischemic, is if there’s some obstruction
to arterial flow into the tissue, so on this side, say we have a major obstruction.
Now only a few red-blood-cells can get in at a time.
You might imagine that organ-apolis sees 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,
where plaque builds up in the arteries going to your heart tissue, which blocks arterial
flow, 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 the 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.
So in this case, you’ve got a major obstruction in the venous highway leaving the city, so
say these are like the veins draining blood out of organ-apolis . All these workers are
getting into organ-apolis, but they can’t leave because there’s a blockage heading
out, and traffic gets majorly backed up, causing flow to slow down throughout the whole city!
This again leads to less oxygen to the tissues and ischemia.
Thinking about an organ, it might get so congested that pressure can rise causing fluid to get
forced out of the blood vessels and into the tissues generating edema.This whole example
is very similar to Budd-chiari syndrome, where the hepatic veins that drain blood out of
the liver are blocked by a thrombosis, or a clot, and now blood can’t flow through
the liver and the liver tissue becomes ischemic and can lead to liver edema and hepatomegaly,
or enlargement of the liver.
If the oxygen supply is low enough for long enough, it can cause cell death, and if enough
cells die in a region of tissue we would call that tissue necrosis and infarction.
In some cases, there are areas of tissue that are getting close to infarction, 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.
In some cases, there are two arteries serving a single area of tissue.
This goes against how we often think about tissue as being served by a single capillary
bed from a single artery, but when you think about it in three-dimensions, there can be
a lot of overlap.
If there are these secondary or collateral arteries going to the area then there might
be enough blood flow to keep the penumbra alive for a while.
Over the course of weeks to months, these secondary arteries can even grow in size to
bring through more blood to that area, a bit like disaster relief; this process is called
collateralization.
So if blood flow to an organ is blocked all of a sudden, a super important concept to
keep in mind is called time to reperfusion.
This refers to the importance of re-establishing perfusion to the affected organ before the
cells die and the ischemia becomes irreversible.
Remember that perfusion and blood flow are slightly different; blood flow is the volume
of blood flowing per unit time, like L / min.
Perfusion is like a measurement of how much blood is flowing to a chunk of tissue per
time, measured by weight, so it might be given in L per min per gram of tissue.
So as an example, if you compared the total blood flow to the kidneys with the total blood
flow to the liver, it’d be higher to the liver because the liver’s a lot larger.
But if you took a 10 g cube of liver tissue and compared it to a 10 g cube of kidney tissue,
the kidney actually has higher perfusion, since it has more blood vessels packed into
its cube.
Now, back to reperfusion, if perfusion to the area is re-established quickly, then it’s
possible that the ischemia is reversible because the cells were dying but not dead, if too
much time has passed and the cells have actually died, then the ischemia is irreversible since
you can’t bring back dead cells.
One example of irreversible ischemic damage is something like heart attack, or myocardial
infarction, where an artery supplying a specific part of the heart with blood becomes blocked,
causing death of that part.
Depending on the area that’s affected, this could cause serious heart dysfunction, like
if enough of the heart’s ventricle was damaged, it might not be able to pump blood as well
anymore.