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. A similarly steep gradient affects the diffusion of oxygen from capillaries to body tissues. The partial pressures shown in the table below 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 is less molecules present (for example oxygen molecules). People often say the air is “thinner” at altitude, and the result is that you will need to breathe faster and deeper to get the same amount of oxygen, 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 3-3.5% in maximal oxygen consumption for every 1000 feet ascended above 5000 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 9000 feet it can take 7-10 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.
EFFECTS OF CHANGES IN PRESSURE
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 occurs when flying in a plane or driving up into the mountains; anything where the atmospheric pressure is raised. In general, the air in body cavities is normally an equal pressure to the air outside of the body. However, if atmospheric pressure changes fast, 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 has 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 their 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 the air spaces with the surrounding pressure (see below for more information). 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 and 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. This is explained in more depth later on in this lesson.
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