Oxygen from the Atmosphere to the Blood

The mechanical structure of the respiratory system is made up of a conducting zone and a respiratory zone. The conducting zone warms and humidifies the air we breathe in order for gas exchange within the respiratory zone where gasses are diffused from the atmosphere to the blood and vice versa.

The air is made up of Nitrogen (79%) and Oxegen (20.9%). Gas transfer between the external environment (outside the body) and the internal environment (the bloodstream) is governed by two things, the partial pressure of a gas in the atmosphere and the partial pressure of a gas within the blood stream. Gasses with higher partial pressures will always flow to a place where the same gas has a lower partial pressure. The difference between the pressure of a gas in the atmosphere and the blood will dictate which direction the gas will travel. For example, If the blood stream contains a lower partial pressure of Oxygen and a higher partial pressure of carbon dioxide (CO₂) than in the atmosphere, the oxygen from the atmosphere will diffuse through the lungs into the blood stream and the CO2 will transfer into the atmosphere. This transfer of gasses is why we breathe.

An example as we dive to depth the Pressure increases due to the increased weight of all that water and gravity. Therefore partial pressure of O2 and Nitrogen increases. Our bodies have a lower partial pressure of these gasses within it. Therefore we absorb more O2 and Nitrogen into our blood stream. During ascent the inverse happens. 

One way to explain this pressure gradient concept is to imagine a long sausage-like balloon. Pinching it loosely in the middle would symbolise the membrane in which gass transfer happens. Now squeeze one end. The increased pressure put on the squeezed end would force the air to move to the other end so that it would inflate more. If you let go of the squeezed end the air would move back to the end that had experienced lower pressure.

So what rates does gas diffuse from the atmosphere into the bloodstream?

This does depend on a few things. Ficks Law of diffusion states that a gas diffuses through a sheet of tissue at a rate directly proportional to the tissue area (A), diffusion co-efficient of gas (D), and the difference between the partial pressures of each gas on each side of this tissue (P₁ – P₂) and is inversely proportional to the tissue thickness (McArdle, Katch and Katch, 2010).

In exercise and physical performance we want this diffusion to be as efficient as possible. For aerobic sports that rely heavily on being able to utilise O2 such as swimming, running, cycling and other sports that involve the aerobic system, this would be really important. For example, if an asthmatic has a large build up of mucus over the inside of the lungs,This increases the tissue thickness (membrane) and decreases the area of the lungs. Therefore when applying Ficks law of diffusion you could say that a thickened membrane will reduce the diffusion of O2 into the blood stream and therefore the supply to the muscles that use this O2 would be limited, therefore reducing performance of those muscles.

So we would have to assume that O2 drives breathing?
Short answer... no CO2 does


Why?
A by-product of muscle contraction is CO2. CO2 when it gets into the blood stream it binds with water (another by product of muscle contraction). This lowers the PH level of the blood. This in turn makes the blood become acidic. As we exercise more CO2 is released into the Blood stream. Aha! you say but doesn't the tissues demand O2 also?

Although the body needs oxygen for energy metabolism, low oxygen levels normally do not stimulate breathing.


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Take for example the article 100M Man - William Trubridge (Scott, 2007). Trubridge is a breath hold diver who can hold his breath for over 8 minutes. There are many things that a breath hold diver must overcome to be able to perform breath hold for long periods.

Firstly he has to overcome the desire to breathe. This urge is caused by a build up of CO2 within the blood stream. This CO2 is produced by his tissues. Increased CO2 within the blood causes the blood to become more acidic. The urge to breath is a rise is acid within the lungs tissues. This is called respiratory acidosis. Molecules of CO2 is produced faster than O2 is consumed. CO2 builds up in the bloodstream faster than O2 is consumed. This builds up acid and triggers the response to breathe.


So how is O2 used - what happens once it gets into the blood stream?