system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does an air bubble in your blood kill you?
gpt
First let’s cover some basics.  Almost every part of your body needs oxygen to survive. Blood is what delivers that oxygen, and here’s how:

When blood comes through our lungs it loads up with oxygen and flows immediately to your heart. To get all that oxygen to your body, the heart pumps the blood through vessels that get smaller and smaller until the blood delivers its oxygen. Once the blood gets through the really tiny vessels, it has delivered all the oxygen and the vessels start getting bigger again and heading to the heart to load back up on oxygen. 

This system normally works great, but you can have major issues if something gets into the blood that isn’t supposed to. For instance let’s say a piece of fat come off of the inside of a large vessel near the heart.  The piece will keep flowing in the blood until it gets into a vessel that gets too small for it to get through. It gets stuck in the vessel and slows or completely blocks the blood flow behind it. Now the part of the body on the other side of that blockage can’t get oxygen and starts to die. Depending on where the block is, and how big of an area is blocked, this can be REALLY bad. For instance If the blockage happens somewhere in the brain, it’s called a stroke.  Parts of your brain are literally dying. 

So how does can an air bubble kill you? Believe it or not it does the same thing as the scab in the paragraph above. Our blood carries oxygen from our lungs inside special cells. It won’t form a bubble.  But if a bunch air enters our lungs in another way, the air can form a bubble big enough to block a vessel. Depending on what part of are bodies the bubble is keeping oxygen from, and how big the area is, the bubble could potentially kill us.