The bends – decompression sickness

One of my friends got the bends recently after SCUBA diving to 40 metres.  He’s been told he can’t dive any longer (which really sucks for him) because he now has a much higher risk of getting the bends a second time.  But what exactly is the bends (apart from an awesome Radiohead song)?

The bends or decompression sickness happens when nitrogen gas which that becomes dissolved in the blood (the transport system of the body) forms bubbles.  This is sort of like the bubbles that appear when you open a bottle of soft drink (or soda for all those playing along from the America’s). 

These bubbles can form anywhere in the body and so the bends can have a lots of different symptoms.  Bubbles in the joints cause aches and pains, in the skin they can cause rashes and itching, in the lungs they can cause coughs and in the brain they can cause memory loss, unconsciousness and even seizures.  In severe cases decompression sickness can be fatal.  It is treated by putting the patient in a hypobaric chamber.

But nitrogen isn’t normally in the bloodstream (just carbon dioxide and oxygen) you say….. well, humans aren’t usually 40 metres under the water.   At the surface of the water there is one atmosphere of air pressure (same as at sea level on land).    This pressure has the same push as 1Kg per square cm or like a 1L carton of milk sitting on your little finger nail.

I’m 58 Kg and 163 cm tall.  My body surface area is 1.62 m2 .  The push of air pressure is so strong its like I’m holding up 157 x 1L cartons of milk (~1.03 Kg each) or being sandwiched between 2 sumo wrestlers!  But I can’t feel it…I’m used to it……it’s normal.

Sarah can't feel 1atm of pressure

When you dive 10 metres (33 ft) underwater the pressure doubles (i.e. two atmospheres – 4 sumo wrestlers).  In freedivers with no special equipment the lungs halve in volume (good example of Boyle’s law).  SCUBA diving equipment is pretty cool because it allows you to keep breathing with the same lung volumes you would on land.  But this means that the deeper you dive, the higher the pressure (or push) in your lungs.

Air is 80% nitrogen.  Usually we breathe nitrogen in and out and it has no effect on us.  During a deep dive the high pressure nitrogen gas in the lungs dissolves into the blood similar to the way carbonated soft drinks are made.  While deep underwater (and at higher pressure) the nitrogen stays dissolved and causes no harm.  If the diver surfaces slowly then the nitrogen is able to slowly return to the lungs where it can be breathed out.

The bends

The bends

The bends most commonly happens when a diver rises to the surface too quickly.  The decrease in pressure is too fast and causes bubbles to form.   This is just like opening the bottle of soft drink.  They’re invisible (in solution) when the diver is at lower pressure at depth and then form when the pressure suddenly decreases just like when bubbles form when you twist open the cap and reduce the pressure inside the soft drink bottle.   Divers prevent this from happening by rising slowly and making decompression stops on the way up.  These stops allow the gas to slowly come out of the blood without forming bubbles.

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