The last article described what decompression sickness is and also shed light on concepts essential to understand the underlying science. This article focuses on how Henry’s law and the partial pressures come to play in deep diving. And also on how increase in pressure due to deep diving cause such a fatal disorder.

The exchange of gases occurs at lungs in the human body. Oxygen inhaled is absorbed by blood in the lungs with a simultaneous release of carbon dioxide into lungs that is eventually exhaled. The oxygen absorbed is carried by blood vessels to all the parts of the body. Imagine a cylinder with a piston, let’s say that the cylinder is half filled with a solvent and the rest half is filled with two different types of gases. The gas molecules continuously collide with the surface of the liquid and some of them are absorbed by it.

As per Henry’s law the amount of a particular gas absorbed depends on its partial pressure. The partial pressure of a particular gas can be varied by adding extra molecules. The partial pressure of both the gases can be varied by external factors. Let’s the say the piston is moved down to decrease the volume of the gas. As we have learnt during our Physics tuition classes on kinetic model of matter, due the smaller volume the distance gas molecules travel in between collisions is reduced and hence the molecules collide with liquid surface more frequently. The chances of gas molecules getting absorbed increases. Hence the extent of absorption of gas molecules by a solvent depends on the partial pressure of the gases which can be varied by external factors like piston this case.

The blood present in our blood vessels contains different gases dissolved in it. The amount of the gas dissolved can be increased by increasing the pressure exerted on these blood vessels. One can assume the blood vessel as a cylinder and the exertion of extra pressure due to deep sea diving can be seen as decreasing the volume of cylinder by the compression induced by piston. So when one dives deep the amount of gas absorbed into the blood increases.

The absorption of extra gas doesn’t as such cause any issue. But when the same person surfaces rapidly, the sudden change in pressure associated with surfacing becomes the root cause of the problem. Think of what happens when the cylinder is decompressed suddenly by moving the piston upwards (which increases the volume) — To be continued…