Take a deep breath. Feel that rush of new air into your lungs? Ever wonder why you have two of them?
There are certain organs we humans (and most animals) have two of. It is not just a case of redundancy. These organs are designed to function as a unit. Eyes, ears, nostrils, lungs, kidneys – these organs are designed to work as pairs. While you can survive with only one of them, your entire body system revolves around having two of them.
A dual organ is extremely efficient, more efficient than one organ twice the size. Take for example the dual lung. By having two lungs to absorb oxygen from the air and remove carbon dioxide from the blood, your lungs make it possible to keep the entire body supplied with the oxygen necessary for human activity. Your cells require an enormous amount of oxygen.
The human lungs, on average, can expand enough to hold 6 liters of air; but we rarely breath that deeply. The average breath is probably closer to 2 liters of air. This tremendous reserve capacity (the ability to triple air intake!) is an exceptional design, allowing the human body to transition from relative inertness to strenuous movement quickly.
Since about 21% of that is breathable oxygen, you take in less than half a liter of oxygen with every breath, but the lungs do not absorb all of that oxygen. In fact only about .1 liters (100 milliliters) is actually absorbed in an average breath. Partially, this is because your blood stream can only release so much carbon dioxide at a shot, so there is no way to absorb a volume of oxygen greater than the volume of carbon dioxide you remove.
So, if you take an average of 10 breaths every minute, then you are absorbing about 1 liter of oxygen into your bloodstream. During the course of a day (24 hours), you will exchange nearly 1,500 liters of oxygen for 1,500 liters of carbon dioxide.
(Just a side note. An average liter of air has a mass of 1.2 grams. In the course of a day, your lungs will move nearly 83 kilograms of air, even if you’re not exercising!)
The 100 ml of oxygen you breath in every time you inhale is diffused into your bloodstream through these tiny sacs of veins and arteries called alveoli. Each alveolus is a hive of capillaries, carrying CO2 to the surface and releasing it, then capturing oxygen molecules in. The alveoli are wonderfully folded and wrinkled for maximum surface area; and the 70 million or so in your lungs you have over 70 square meters of surface area. This is actually 35 TIMES more surface area than your entire skin covers through which you release carbon dioxide and absorb oxygen. (The average human being has less than 2 square meters of body surface.)
By having two lungs instead of one large lung, there is a perfect balance of surface area for the alveoli to be spread out upon. It is simply geometry.
Imagine the single large lung as a cylinder with a radius of 10 cm and a height of 20 cm. That cylinder would have a surface area of around 1,890 square centimeters. You could get the same volume from two cylinders of 7 cm radius and the same height. Their total surface area, however would be nearly 2,400 square centimeters. That’s over 25% more surface area.
Without this additional surface area, we could only have 75% of the number alveoli. This would not be a problem when the body is at rest, since we are using only a third of our lung capacity anyway. But when the body requires greater gas exchange, such as in high stress situations, the body literally would not be able to exchange the CO2 for oxygen fast enough to sustain any form of activity.
You might ask, well then what about three lungs or four? It is true that three or more lungs would have more surface area than two, but the compromise would be the loss of internal volume. The alveoli would crowd the space necessary for the air to be taken in. Also, the smaller surface area would require greater effort to inflate, and the system would become exponentially inefficient.
Now you know. We have two lungs because it provides the maximum surface area for gas exchange. It is better than one or three lungs.
Like Apple computers, God designed things that “just work.” Could it have happened by chance? By spontaneous generation? Certainly. But the probability is substantially low. There must be a designer.