We breathe every moment of our lives. We continuously take the air into our lungs and let it out. We do it so much that we might think of it as normal. In fact, respiration is quite a complex process.

Our bodily systems are so perfectly designed that we don't need to think about breathing. Our body estimates how much oxygen it needs and arranges for the delivery of the right amount whether we're walking, running, reading a book, or sleeping. The reason breathing is so important to us is that the millions of reactions that must constantly take place in our bodies to keep us alive all require oxygen.

Your ability to read this article is thanks to the millions of cells in the retina of your eye constantly being supplied with oxygen-derived energy. Similarly, all the tissues of our bodies and the cells forming them get their energy from the "burning" of carbon compounds in oxygen. The product of this burning–carbon dioxide–must be discharged from the body. If the level of oxygen in your bloodstream drops to low, the result is fainting; and if the absence of oxygen persists for more than a few minutes, the result is death.

And that's why we breathe. When we inhale, oxygen floods into about 300 million tiny chambers in our lungs. Capillary veins attached to these chambers absorb the oxygen in a twinkling and convey it first to heart and then to every other part of our body. The cells of our body use this oxygen and release carbon dioxide into the blood, which conveys it back to the lungs where it is expelled. The whole thing takes less than half a second: "clean" oxygen comes in and "dirty" carbon dioxide goes out. You might be wondering why there are so many (300 million) of those little chambers in the lungs. They're there to maximize the surface area that is exposed to the air. They're carefully folded up to occupy as little space as possible; if they were unfolded, the result would be enough to cover a tennis court.

There is another point here that we need to keep in mind. The chambers of the lungs and the capillaries connecting to them are designed so small and perfectly in order to increase the rate at which oxygen and carbon dioxide are exchanged. But that perfect design depends on other factors: the density, viscosity, and pressure of air must all be right in order for the air to move properly in and out of our lungs.

At sea level, air pressure is 760 mm of mercury and its density is about 1 gram/liter. Again at sea level, its viscosity is nearly 50 times that of water. You might think these numbers unimportant but they are vital for our lives because, as Michael Denton notes: The overall composition and general character of the atmosphere–its density, viscosity, and pressure, etc-–must be very similar to what it is, particularly for air-breathing organisms.

When we breathe, our lungs use energy to overcome a force called "airway resistance". This force is the result of the resistance of air to movement. Owing to the physical properties of the atmosphere however, this resistance is weak enough that our lungs can take air in and let it out with a minimum expenditure of energy. If air resistance were higher, our lungs would be forced to work harder to enable us to breathe. This can be explained by an example. It easy to draw water into the needle of an injector but drawing honey in is much more difficult. The reason is that honey is denser than water and also more viscous.

If the density, viscosity, and pressure of air were higher, breathing would be as difficult as drawing honey into a needle. Someone might say "That's easy to fix. We'll just make the hole of the needle larger to increase the rate of flow." But if we did that in the case of the capillaries in the lungs, the result would be to reduce the size of the area in contact with air, with the result that less oxygen and carbon dioxide would be exchanged in the same amount of time and the respiratory needs of the body would not be satisfied. In other words, the individual values of air's density, viscosity and pressure must all fall within certain limits in order for it to be breathable and those of the air we breathe do exactly that.

The numerical values of the atmosphere are not only necessary for us to breathe but are also essential for our Blue Planet to stay blue. If sea-level atmospheric pressure were much lower than its present value, the rate of water vaporization would be much higher. Increased water in the atmosphere would have a "greenhouse effect" trapping more heat and raising the average temperature of the planet. On the other hand, if the pressure were much higher, the rate of water vaporization would be less, turning large parts of the planet into desert.

All these finely-tuned equilibriums indicate that our atmosphere has been deliberately designed precisely so that life on Earth can exist. This is the reality discovered by science and it shows us again that the universe is not just an accidental jumble of matter.