Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Why don't I burst cells in my rear when I sit down?

Category: Biology      Published: October 28, 2013

diagram of skin layers
The outer skin is made up of millions of tough keratinized cells that distribute and withstand your weight. Public Domain Image, source: NIH.

You do burst cells when you sit down on broken glass or a pile of pins. The experience is painful and messy. But let's assume you were referring to sitting on a flat chair, in which case you do indeed burst very few of the cells in your body. Why is this? The answer lies in the difference between sitting on broken glass and sitting on a flat chair.  When you sit on a flat surface, your entire weight is spread over the large surface area of your rear. There are millions of cells per square inch of skin, so the weight that each individual cell experiences when you sit down is actually quite small. This effect is one of the great advantages to having a body made out of very small, self-contained subunits. In contrast, when you sit on an upright pin, much of your weight gets focused to just a handful of cells and they do indeed burst. Sharp objects are so good at cutting because they deliver a moderate total force over a very small area.

Also, the outer layer of your skin is not made of fragile, squishy cells. Instead, most of your outer skin consists of tough dead cells that have been almost completely filled with the strong protein keratin. As part of the cornification process, typical skin cells destroy all of their interior parts, including the nucleus holding the DNA, and fill up with keratin. The destiny of typical skin cells is to die and become hard chunks of protein in order to maintain the tough outer skin layer that protects all the other, more fragile cells deeper inside. Furthermore, skin contains networks of filaments supporting the cells. When you sit down, your weight is felt first by this front line of tough skin cells surrounded by a network of filaments. From there, your weight is distributed to your whole frame by the cushioning effect of the fat and muscle in your rear.

Topics: cell, pressure, sitting, surface area