mesenteric definition

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 kindof suppression or reduction of blood flow to an organ or tissue. and blood carries oxygenright? so when there’s a reduction in blood flow to cells, that also means there’s areduction of oxygen to those cells, and this is due to lowered blood flow in the bloodvessels. this lowered flow could be from something blocking the blood from the inside, or itcould be something compressing the blood vessel from the outside. an example of somethingblocking 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 resultingfrom something outside the blood vessel is traumatic injury, which can cause inflammationand 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, whichis like a major organ in the middle here, so maybe this is organ-apolis. these red bloodcells are super pumped for their day where they can drive around the capillaries, likethe smaller city streets, and supply the city with fresh oxygen and pick up waste. and thisorgan-apolis is made up of thousands of cells, like homes, that use up the oxygen and createwaste that needs to be picked up, the deoxygenated

blood cells drain out through different smallstreets which are the veins and go back towards the heart. so one way that this organ-citycould 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 inat 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 ischemiais 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 hearttissue, and causes ischemic heart disease. since you can have a blockage of the red bloodcells coming in, you can also have a blockage

of red blood cells going out, leading to adecrease in blood drainage on the venous side. so in this case, you’ve got a major obstructionin the venous highway leaving the city, so say these are like the veins draining bloodout of organ-apolis . all these workers are getting into organ-apolis, but they can’tleave because there’s a blockage heading out, and traffic gets majorly backed up, causingflow to slow down throughout the whole city! this again leads to less oxygen to the tissuesand ischemia. thinking about an organ, it might get so congested that pressure can risecausing fluid to get forced out of the blood vessels and into the tissues generating edema.thiswhole example is very similar to budd-chiari syndrome, where the hepatic veins that drainblood out of the liver are blocked by a thrombosis,

or a clot, and now blood can’t flow throughthe 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 longenough, it can cause cell death, and if enough cells die in a region of tissue we would callthat tissue necrosis and infarction. in some cases, there are areas of tissue that aregetting close to infarction, as cells begin to die off, but they are still able to getsaved if they receive blood. this high-risk area that’s teetering on the brink of deathis called the ischemic penumbra. in some cases, there are two arteries serving a single areaof tissue. this goes against how we often think about tissue as being served by a singlecapillary bed from a single artery, but when

you think about it in three-dimensions, therecan be a lot of overlap. if there are these secondary or collateral arteries going tothe 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 tobring through more blood to that area, a bit like disaster relief; this process is calledcollateralization. so if blood flow to an organ is blocked allof 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 thecells die and the ischemia becomes irreversible. remember that perfusion and blood flow areslightly 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 pertime, measured by weight, so it might be given in l per min per gram of tissue. so as anexample, if you compared the total blood flow to the kidneys with the total blood flow tothe liver, it’d be higher to the liver because the liver’s a lot larger. but if you tooka 10 g cube of liver tissue and compared it to a 10 g cube of kidney tissue, the kidneyactually 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 possiblethat the ischemia is reversible because the cells were dying but not dead, if too muchtime has passed and the cells have actually

died, then the ischemia is irreversible sinceyou can’t bring back dead cells. one example of irreversible ischemic damage is somethinglike heart attack, or myocardial infarction, where an artery supplying a specific partof the heart with blood becomes blocked, causing death of that part. depending on the areathat’s affected, this could cause serious heart dysfunction, like if enough of the heart’sventricle was damaged, it might not be able to pump blood as well anymore.