Bruises are the bluish or brownish patches that mostly develop after trauma in living organisms with warm blood. They are the result of extravasation of blood outside the lumen of blood vessels, especially platelets within an area of the body -mostly skin and muscles-. Almost all humans including you and I have experienced bruises, whether from being hit by a punch, after injections or even one discovered without a memory of trauma.
Where do bruises come from?
Bruises occur due to the flow of blood from blood vessels after their rupture. Such rupture is almost always due to trauma, even minor trauma that is experienced in every day life and their medical term is “contusions.” Most contusions form on exposed surfaces of our bodies especially our arms and legs and vary from small almost unnoticeable spots to large patches several centimeters across. When the blood escapes from the injured blood vessels, it causes inflammation, and one of the so-called cardinal signs of inflammation is “Rubor” or red colour. Therefore, immediately after the injury, the area is reddish and swollen. After a few hours, the bruise starts to be well defined and bluish. The reason for the bluish discolouration is the chemical change of haemoglobin found in red blood cells, where it loses its oxygen and becomes “reduced” or “deoxygenated’. After a few days, the bruise becomes green then yellow or light brown and then eventually disappears. This is also due to the degradation of haemoglobin into biliverdin -which is green- then to bilirubin -which is yellow-, such changes can help doctors roughly estimate the age of a bruise. The primary source of extravasation of blood is stopped within minutes by the action of our blood vessels, clotting mechanisms and platelets, and the bruise is eventually absorbed into the surrounding tissue.