Can someone please explain why do injuries such as sprains swell, while imacts just bruise the skin?
In: Biology
Depends on the injury. There are two general causes of swelling: Inflammation and fluid accumulation.
Inflammation is an immune response we’ve evolved to keep infections at bay. Open wounds are places where bad things, particularly bacteria, can get inside the body. Because of this, when the body detects an open wound, it creates an inflammation response, in which blood flow to the wound is increased and it’s pumped full of white blood cells who can fight off any invaders. Inflammation also has a secondary purpose, which is to clear up any cells damaged in the injury so as to prevent necrosis. Inflammation is triggered by cells being damaged, not specifically by an open wound (because cells being damaged is often a sign of an open wound) so inflammation can occur even when all damage is internal. Most injuries lead to inflammation to a degree, but some are sufficiently small that the inflammation isn’t noticeable.
In some types of injuries, swelling is caused by fluid accumulation. Normally, all the water in the body is effectively accounted for – it’s all either in the blood stream, inside cells, inside something called the lymph system, or in the small gaps between cells. This is where water is supposed to be. Sometimes, injuries can cause tears in tissues that open up large gaps that aren’t supposed to be there. These gaps end up filling up with water too (usually also other components that make the liquid more like pus), and this can be seen as swelling.
Bruises are a minor injury where only a few tiny blood vessels called capillaries in the upper layer of the skin have been damaged. This allows blood to flow out of the vessels and kinda hang around in the skin, which builds up as a red spot. Impacts with enough force to do more than surface level damage don’t just cause bruises, and the wounds inflicted by those will typically swell up.
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