Question of the Week

Why do hot objects cause injury?

Sun, 8th Mar 2009

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Question

Adrian from Romania & Wales asked:

What happens when you get burned at the molecular level when you touch something hot?

Answer

We put this to Peter Djiewulski, Plastic and Reconstructive Surgeon, Director of Burns Centre at St Andrew's Centre for Burns.

What happens when somebody gets burned and their tissue is burned is that heat causes direct damage to cells. 2nd degree burns on handIt denatures proteins within and without cells. It's that injury and the breakup of cells and the contents with in the cells, particularly some of the enzymes within the cells that will initially cause pain but secondarily bits of cells that break down cause local irritation. The cell wall breaks down and that leads to a number of breakdown products which are involved in inflammation and the inflammatory response. Most people have burnt themselves and therefore would be well-aware of the local effects that the burn and the body's reaction to the burn will cause. That is usually swelling, redness and tenderness. At a molecular level these events are mediated by the inflammatory mediators which have effects on, particularly, the tiny ittle blood vessels that go up to and into the skin to make them leaky. This allows fluid which is usually in the blood vessels to leak out and this gives rise to swelling.

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It's a matter of energy transfer. Temperature can be thought of as a measure of the average kinetic energy of the molecules in a substance. When a hot object (like a stove eye) comes into contact with a cooler object (like a human hand), the faster moving molecules in the stove eye bump into (and tranfer some of their kinetic energy into) the molecules in your hand. This causes the molecules in your hand to move faster, creating a rise in the temperature of your hand. This is called thermal conduction.

If molecules in your hand received enough energy, they can be altered. In the case of proteins and enzymes, certain temperature ranges are required for them to retain their functional shape. Heat them up too much and they get "bent out of shape" and lose their function. That's obviously not a good thing for your body. This occurs at relatively lower temperatures than the following effects.

At higher temperatures, the water in your skin may boil. As water boils, it expands greatly in volume. Cells are mostly water, so if the water within them boils, they will burst.

Even higher up, certain substances within your body (such as sugars, fats, and proteins) will begin to burn. Burning a substance greatly alters its chemical structure. When you burn an organic substance, you typically get water vapor and carbon dioxide (among other gases) as a by-product. A human being obviously cannot survive in the form of gas.

So in conclusion, high temperatures cause injury by altering the chemicals in your body, either by changing their shape (like enzymes) changing their phase (like water), or changing their composition (like carbohydrates and lipids).

It's also possible that certain connective substances in your tissues like collagen might be melted by high temperatures, but I'm less sure of this. Supercryptid, Wed, 4th Mar 2009

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