Geronimo! Splat!

Dear Matthew:

The other day my wife was recounting her childhood story of how she leaped from the garage roof with her self-made parachute. She survived the crash landing but not without getting "the wind knocked out of her." What is this phenomenon? Why is it soooo hard to take that restoring breath?

-- Fred and Ethel, San Diego

Without going into yawn-inducing detail about respiratory physiology, suffice it to say that our lungs are just passive air bags that would like nothing better than to shrivel up and collapse. They're kept inflated by positive air pressure inside them and by a fluid layer that holds the lungs against a membrane that lines our rib cages. To breathe in, you have to constrict your diaphragm (a big flat muscle under your lungs) that draws the lungs down and expands your rib cage to draw your lungs out. This creates a momentary low-pressure zone, and the higher-pressure air rushes in to fill partial vacuum. If you can't move your diaphragm and rib cage, you can't breathe.

So when our would-be Wonder Woman launched herself from the roof, the heavy fall momentarily paralyzed her diaphragm and expelled some residual air that usually helps keep her lungs inflated. Once things are moving again and the panic is over, of course, then she was left to worry about getting the tar knocked out of her by Mom for pulling that stunt in the first place.