The Effects of Exercise on SCUBA Divers

Like any sport, an individual's health and fitness can impact their ability to safely engage in that activity. For a long time, SCUBA (self-contained underwater breathing apparatus) divers have been warned against exercising around the time of their dives, however, more recent research shows that exercise before, during and after may actually reduce a diver's risk for decompression sickness (DCS).

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Physiology of DCS

According to Dalton's Law (see box) the total pressure of air equals the combined pressure of all gases that make up air, in the case of air at sea level this is mostly Nitrogen (78 percent) and Oxygen (21 percent).

While the actual percentages do not change much with altitude, their relative partial pressures do.
Dalton also tells us that individual gas pressures of any mixture will have the same pressure at a given altitude whether alone or as part of a mixture as shown below for three representative altitudes.

Comparison of barometric and partial pressures at various altitudes/depths

Altitude

Barometric Pressure

O2 Partial Pressure

N2 Partial Pressure

10,000 ft

525 mmHg

410 mmHg

110 mmHg

Sea Level

760 mmHg

592 mmHg

160 mmHg

- 99 ft

3040 mmHg

2371 mmHg

638 mmHg


We can see from the above table that as altitude increases, pressure decreases and as altitude decreases pressure increases. These pressure changes are at the heart of both high-altitude decrements and decompression sickness.

The table also shows that small changes in depth below sea level have a pronounced effect on the partial pressures of gases, so going to 100 feet below sea level quadruples the gas pressures of each component, which affects the transport of gas in the lungs and to our tissues.

With each full breath in and out, oxygen (O2) (among other gases) enters the lungs and gets picked up by the blood passing through capillaries near the tiny air sacks in the lungs (alveoli), while carbon dioxide (CO2) carried by the blood gets dumped off in the alveoli to be exhaled.


Understanding the Gas Laws

Each of the gas laws mentioned plays a pivotal role in gas exchange in the body as we go high and low. Below is a brief synopsis of each law and how it applies to SCUBA.

  • Boyle's Law: Published in 1662, Boyle's Law states that at a fixed temperature the volume of a gas is inversely related to the pressure placed on it. Increasing pressure decreases volume, while a decrease in pressure will lead to an increase in volume.
  • Dalton's Law: Dalton noted that the total pressure of a mixture of gases (e.g., air) is equal to the sum total of the pressures of each individual gas in that mixture. Dalton's Law is important in understanding the effects of both SCUBA dives and endurance activity at altitude because individual gas pressures affect the ability to transport gases in and out of both the blood (at the lungs) and the working muscles.
  • Henry's Law: The Henry and Dalton Laws are inextricably tied together because at a constant temperature, partial pressure of a gas directly affects how much of that gas is dissolved in a liquid (e.g., blood). Of particular importance in SCUBA, as pressure increases, so does the amount of gas dissolved in a liquid.

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