system
Given the following post, respond as if you're commenting on Reddit:
human
What does a bruise look like underneath the skin?

So a bruise is caused by bleeding underneath the skin right? What would that look like if you could see through the skin? If your skin just disappeared would it look like a little puddle of blood on top of your muscles?
gpt
Bruises are indeed caused by blood, however, if the bruise is visible it is inside the skin and not underneath it. The skin consists out of various layers and these various layers consist out of different cell-types and tissues. The shape and colour of the bruise on the outside varies depending on how deep inside the skin the bruise is, how big the hemmoraging and how old the bruise. As such, how they look if you were to make a mental image of them would differ wildly from bruise to bruise. 

Generally though, bruises are a collection of many small burst capillaries (the smallest type of bloodvessel) which each lose a very small amount of blood which then pushes slightly into the surrounding tissues and coagulates. As such, it is rather difficult to picture how they would look. 

It is also possible to have 'bruises' under the skin (for example in fatty tissue or inside muscles) but these are generally invisible from the outside. After all, if we could really look that deep into our bodies we should also be able to see the colour of the underlying muscles, which are actually a very deep red. For that reason, these are more often referred to as hemmorages rather than actual bruises.