Wednesday, 24 February 2010

Why Your Ribcage Is Like A Car Door In An Action Movie

A post about diving, this one. I haven't posted on diving for a while, mainly because I haven't been doing any. I intend to do a refresher session in Fenham pool first thing next month, and after that I hope to be back in the sea very soon.

Usual disclaimer: I'm not a trained diving instructor, I'm posting this because I like to talk about diving, DO NOT rely on stuff I post here for your safety whilst diving.

So why is your ribcage like a car door in an action movie? This relates back to a much earlier post where I commented that the foundation stone of diving knowledge is that gas compresses under increased pressure, and expands under decreased pressure. To build on this, we need to know that when Cousteau and Gagnan developed the "aqualung", what they developed was a way to deliver breathing gas on demand to the diver from a portable tank on the diver's back at ambient pressure. In other words, the gas the diver breathes in is at precisely the same pressure as the air (if on the surface) or water (if submerged) surrounding her.

Why is this important? Going back to the car door. If you have ever seen an action movie where the hero and his girlfriend are trapped in a sinking car in the river, you will know that it is not possible (assuming the movie is obeying the laws of physics) for him to open the door until the water has flooded into the car. Why? Because the water on the other side of the door is pressing against the door with more pressure than the air on the inside of the door (water being heavier than air), holding it closed. Not until there are equal amounts of water on both sides of the door can he open the door and swim to safety.

So too with the human ribcage. To expand and contract, the pressure within the lungs must be equal to the pressure outside the ribcage. Normally, at sea level, we breathe air at one atmosphere (i.e. the air pressure is the pressure of the Earth's atmosphere pressing down on us). To submerge beneath the sea, we must breathe air at the same pressure as the pressure of the weight of the water pressing against us. Otherwise, our lungs would simply collapse - only if you were Superman would you be able to expand your ribcage to get air into your lungs.

Water is much heavier than air. As anyone who has done their Open Water basic diving certificate knows, at ten metres deep, the water pressure is equivalent to two atmospheres of air. (10 metres of water. 122,000 metres of air. Mindbending.) Thus, the regulator delivers air at the equivalent of two atmospheres of pressure, meaning that the ribcage can expand and contract with the same ease it does at sea level.

Impressive stuff, is it not?

Now, watch this space for my next post on this topic: Why You Don't Want Your Lungs To Be Like A Bursting Balloon.

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