We need air (clean air!)
During evolution, as animals grew bigger and more active, there was a need to transport more oxygen to the tissues. How did nature fill this need? A simple solution would have been to increase the number of hemoglobin molecules in our blood. But that would have presented a problem, which has to do with “osmotic pressure.”
When two solutions are separated by a semi-permeable membrane (one that lets some substances pass through, like the solvent, but not others, like the solute), there could exist an osmotic pressure difference between the two sides of the membrane. The substance that could get through would move to the side that has less osmotic pressure until the pressures on the two sides are equalized. If the osmotic pressure difference is great, the movement could be large and there could be a large change in the volumes of the compartments on the two sides of the membrane. Osmotic pressure depends on the number, but not the size, of the particles that cannot pass through the membrane.
The walls of our blood vessels separate our blood from our tissues and cells; those walls are permeable to water, lipids, and other small substances, but not to proteins and other big molecules. If nature had tried to increase the amount of oxygen that our blood could transport by simply increasing the number of hemoglobin molecules dissolved in our blood, it could cause a big change in the osmotic pressure difference between our blood and our tissues. The result could be catastrophic. Instead, nature did increase the number of hemoglobin molecules in our blood, but encased them in red blood cells. Incredibly, one red blood cell (one particle as far as osmotic pressure is concerned) contains approximately 280 million hemoglobin molecules! And one ml of our blood contains about five billion red blood cells! So, our blood can transport a huge amount of oxygen and deliver it to our tissues, and that allows us to be very active.
(An example of the harm that a drastic change in osmotic pressure can do is what results from giving the wrong type blood to a patient during transfusion. Because we have antibodies to substances on the surface of red blood cells of incompatible type, lysis of those cells will occur and their contents will spill out. The hemoglobin molecules that are released will change the osmotic pressure difference between the blood and the surrounding tissues. Because of the huge number of hemoglobin molecules in each red blood cell, 30 ml of the wrong type blood (a tiny amount since we have a total of about a gallon of blood in our body) are all it takes to make enough of a change in osmotic pressure to cause “osmotic shock” and death.
Nature had devised yet another way to overcome the “osmotic pressure” problem. It created the giant hemoglobins (and chlorocruorins). But that solution obviously cannot compare with simply encasing the hemoglobin molecules in cells. This is probably why the organisms, which have those giant molecules to transport their oxygen, cannot grow very big, or be very active — except in movies.
To ensure that we always have an adequate supply of oxygen, we have evolved so that our oxygen intake and its transport to all parts of our body are done without conscious effort. We breathe even while we sleep and our heart beats rhythmically without needing commands from our brain. While this was a survival advantage in the early days and still is in many parts of the world, it has become detrimental to health in major cities and other places where there is a lot of air pollution. In those polluted places, we inhale not only oxygen, but also noxious gases and particles. And we know that particulate pollution could trigger heart attacks and that substances in polluted air could cause respiratory diseases, including cancer. Could nature have failed us in this regard? Maybe, not. Since air pollution is largely our doing, maybe nature is just making us pay for our crime. We need air — but clean air!
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