Re: When we bruise, is this a chemical or physical change?

Hi Mia,

Thanks for submitting your question to the MadSci network. The issue of what constitutes a chemical or a physical change becomes complicated when we try to apply it to living organisms. For example, when a fertilized egg splits into two cells, is that a chemical or a physical change? The two cells contain all the same stuff that the original cell contained, so maybe it is a physical change. But, the two cells are now different from each other, and after a few rounds of division, they'll have changed from a few identical cells into an embryo made of many different types of cells. Is that a chemical or a physical change?

Anyway, this is my way of saying that for biological systems, the definition of chemical and physical changes are not as clear cut. But lets talk about bruising. A bruise occurs when physical damage is inflicted to part of the body; blood vessels under the skin break, and blood leaks out into the surrounding tissue where it is trapped. So, the start of the bruise results from a physical change -- breaking of blood vessels.

However, as we watch the bruise, we note that it changes color. Depending on your skin-color you may notice the bruise turn green, and then purple, and then maybe even yellow. What is happening here is that enzymes are breaking down the heme molecules that give red-blood cells their characteristic color. The heme ring is broken, forming biliverdin, which is green. Then biliverdin is converted to bilirubin, which is red at high concentrations, and yellow at low concentrations. So, the conversion of heme to bilirubin (and the color change of a bruise) is a chemical change.

Bilirubin can either be physically carried away to the liver, where it is excreted from the body, or it can be converted into a water-soluble form through exposure to blue light, and excreted. So the final stages of the fading of a bruise can be a physical change, when the bilirubin is physically carried to the liver, or a chemical change, when light converts it to a soluble form, but most likely it is a combination of both!