Introduction to pathology
USMLE® Step 1 style questions USMLE
A 38-year-old woman is brought to the emergency department following a motor vehicle accident. The patient’s estimated blood loss is around 2 liters. A day later, she began experiencing crampy abdominal pain and multiple bloody bowel movements. Which of the following regions of the gastrointestinal tract is at the most significant risk for ischemia?
Content Reviewers:Rishi Desai, MD, MPH, Tanner Marshall, MS
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 this organ-city could become ischemic, is if there’s some obstruction to arterial flow into the tissue. 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.