Air pressure is the force that is exerted on you by air molecules, being the weight of tiny air particles. Atmospheric pressure is a measure of the force exerted by the atmosphere, so therefore at any point on the earth’s surface, there is a quantity of air sitting above your body. If that quantity of air is greater, there will be more pressure on the body, and if it is less, there will be less pressure on the body. This is traditionally measured in pounds per square inch (PSI). 1 PSI is the force of one pound applied to an area of one square inch.

At high altitudes the quantity of air is less, and the density of air is also less. As such, there is less air pressure and as a result, less oxygen in a given volume of air. To demonstrate this, if a person dives below the surface of water in scuba diving, their body has to contend with both the air exerting pressure on the surface of the water, and the water above that exerting further pressure, hence, the deeper you dive, the more pressure there is.

At sea level, we say atmospheric pressure is 1 atmosphere (this is equal to 14.7 psi). This arbitrary measurement provides a reference point from which we can determine air pressure at varying altitudes or depths.

For every 10 metres deep which you go in water, the pressure increases by 1 atmosphere. For example, at 10 metres it is 2 atmospheres; at 40 metres it is 5 atmospheres.

Partial Pressure Gradients
Partial pressure gradients follow Henry’s law. Henry's law states that at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. In terms of atmospheric pressure, because a large percentage of the body is water, as the pressure increases (i.e. as a scuba diver goes deeper) more gas will dissolve in the blood and body tissues. As long as the person remains at the same pressure, the gas will remain in solution.

The air we breathe is a mixture of gases. Nitrogen is the most abundant gas, and nitrogen molecules (N2) make up about 78% of our atmosphere. Oxygen molecules (O2) molecules make up about 21% of the air we breathe, water molecules 0.5%, and carbon dioxide 0.04%. Each of these gases contributes to the total pressure in the atmosphere proportional to its relative abundance.

Partial pressure of a gas = the pressure exerted by that one gas (e.g. oxygen) in a gas mixture (e.g. air).

The partial pressure of oxygen is much higher in alveoli than in capillaries. That is, there is a steep partial pressure gradient for oxygen. This partial pressure gradient causes oxygen to diffuse rapidly from alveoli to capillaries, affecting the diffusion of oxygen from capillaries to body tissues. The partial pressures are important in determining the movement of oxygen and carbon dioxide between the atmosphere and lungs, the lungs and blood, and the blood and body cells. When a mixture of gases diffuses across a permeable membrane, each gas diffuses from an area of greater partial pressure to an area of lower partial pressure (the gas moves down its concentration gradient). Each gas in a mixture of gases exerts its own pressure as if all other gases were not present.

At altitude the air pressure decreases, so in the same volume of air, there are less molecules present (for example, oxygen molecules). People often say the air is “thinner” at altitude, with the result being that you will need to breathe faster and deeper to get the same amount of oxygen from the air, and your heart will pump more blood to increase the supply of oxygen to the brain and muscles.

Physical performance is affected at altitudes over 500 feet (1524 metres) the higher the altitude, the more impaired the physical performance of the body. Physical or work performance is related to oxygen consumption, which decreases at high altitudes, due to less oxygen in a given volume of air.

Endurance capacity is commonly measured by a reduction of 3 to 3.5% in maximal oxygen consumption for every 1,000 feet ascended above 5,000 feet. At a height of around 25,000 feet, performance and oxygen consumption can be reduced by up to 60%.

If a person remains at high altitudes for long periods, they begin to acclimatise. At 9,000 feet it can take 7 to 10 days to acclimatise. At higher altitudes it can take longer. A minority of people will never acclimatise. With acclimatisation, a person’s performance at higher altitudes will approach normal levels but never quite reach their norm.

In contrast, for explosive athletic events, such as 100m sprint and long jump, reduced atmospheric pressure results in less atmospheric resistance, so the athlete’s performance is improved.

The skin which covers the human body will adjust to changes in pressure, however body cavities such as ears, sinuses & lungs, do not automatically adjust to such changes.

Therefore, this is the reason that changes in air pressure can have the effect of causing a popping in the ears. This can occur when flying in a plane or driving up into the mountains; in any situation where the atmospheric pressure is raised. In general, the air in body cavities is normally of equal pressure to the air outside of the body. However, if the atmospheric pressure changes quickly, or if there is any blockage between the outside of the body and the internal cavities, "equalising" of pressure might not occur properly.

A tangible example of how you may have experienced this is when you take a drink bottle on a flight. If you open an empty plastic bottle while you are in the air, then tightly close it, when you land, you will find the increase in air pressure has caused the air in the bottle to compress, as if it has been sucked out with a vacuum, and the bottle will have collapsed inwards.

When scuba diving, as the pressure increases the air spaces in a diver’s body and equipment will compress. As the pressure decreases, the air spaces will expand. The amount of compression follows Boyle’s law, which describes how the volume of gas varies, depending on the surrounding pressure.

Boyle’s law is: PV = c  (where P= pressure, V = volume of a gas, c = a constant).
This shows that when you multiply the surrounding pressure of a gas, by the volume of the gas, you will always have the same number. So if the amount of pressure is increased, the volume of gas must decrease, and vice versa.

The implications of Boyle’s law for scuba diving are that as a diver descends, the air spaces in the diver's ears, masks and lungs are decreased, creating a negative pressure and a vacuum-like effect. To avoid injury, the diver will need to equalise the pressure in their air spaces with the surrounding pressure. While they are diving, care must be taken to continue breathing – if a diver holds their breath and ascends to an area where less pressure is exerted, the air trapped in the lungs will expand and can stretch the lungs which can lead to injury. When ascending, the air in the diver’s ears and lungs will expand, creating a positive pressure. These air spaces can become overfull, so the diver will need to equalise, and breath out any excess air. Failure to do so can cause the eardrum and lungs to burst. The buoyancy compensator (BCD) will also expand due to decreased pressure, so the diver will need to release air from the BCD to control their ascent. On the ascent, consideration also needs to be taken for the affect of Boyle’s law on nitrogen gas in the diver’s body.

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