When you bump your head for example.
In: Biology
Going with your example of an injury when you say swelling, and I’ll completely exclude things like tumours and similar causing lumps.
It’s all related to the body dealing with damaged tissues via inflammation. Inflammation isn’t just for infections and allergic reactions.
When an area gets injured blood vessels and other tissues are injured and as a result they release chemicals that trigger an inflammatory response. It aim is to get white blood cells to the injury to kill of any pathogens that have gotten into the injury if it’s an open injury, but they will also clear out any damaged and dead cells/tissues. This sounds dramatic, even a minor injury will have some inflammation as it is all part of the body dealing with damaged tissue, clearing it out and healing being able to happen.
So in inflammation, blood vessels open up in the area, allowing more blood to the site (one of the reasons such areas go red and hot, more blood). The damaged vessels are often a bit leaky, so fluid can seep out of the blood, even if it’s not blood (think of it as the water being able to get out without the blood cells). Obviously any broken blood vessels will have blood coming out of them too which will add to any fluid in the tissues (although hopefully not for long as blood clotting kicks in). Within the area of inflammation in damaged blood vessels also get leaky as part of the inflammatory process as this allows the white blood cells to get out of the circulation system and into the surrounding tissues.
As the body deals with the injury and clears away damaged tissues, the things perpetuating the inflammation start to disappear and the inflammation starts to die down. Blood vessels return to normal. Excess fluid in the area is absorbed back into the blood stream, and the swelling, redness, pain subsides.
Obviously if bigger blood vessels, or enough smaller ones are broken, a haematoma may also collect, increasing any swelling that can be seen and felt on top of the swelling from the inflammation.
There are other factors but it’s easiest to see it as a reflection of your body trying to get enough repairing and protecting equipment eg white blood cells) to the area as fast as possible and allowing them to stay there to do their job
(Btw I’m rather sleep deprived while writing this so please excuse spelling and rambling!)
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