The Dynamics of Lung Aeration
Every living human needs proper oxygenation. We all breathe, most of the time unconsciously. The normal respiratory rate of a healthy adult is anywhere from 12-20 breaths per minute. Many people think of air as oxygen, but in reality the air we all inhale is approximately 79 percent nitrogen gas (N2) and 21 percent oxygen gas (O2) with trace amounts of negligible gases. When we exhale, we return the same 79 percent N2 to our surroundings with 16 percent O2, and about 5 percent carbon dioxide (CO2), so there’s a clear exchange between O2 and CO2 as we breathe in and out. The dynamics of how this occurs is indeed a fascinating scientific mechanism.
One of the fundamental questions, aside from the obvious, should be in regards to oxygen’s role in the maintenance of homeostasis within the body. In cellular respiration, oxygen is the ultimate electron acceptor in the mitochondrion of every cell, as you may remember from high school biology mitochondria is the powerhouse of the cell, it generates energy in the form of ATP (adenine triphosphate) to carry out biological processes. The fate of oxygen at the end of this process involves binding two hydrogen molecules to become water. Two byproducts of cellular respiration are water (H2O) and CO2, and yes, this is the same CO2 your lungs breathe out.
Two CO2 molecules are made in two separate decarboxylations in what we call the TCA cycle. When CO2 is generated the following takes place:
CO2 + H2O HCO3- + H+
In other words, when carbon dioxide is generated it mixes with water in a reversible reaction to make bicarbonate and a hydrogen ion. It is in this form that CO2 is transported through the circulation, bicarbonate. The conversion happens inside red blood cells, which have an enzyme known as carbonic anhydrase that converts CO2 into bicarbonate. As bicarbonate builds up inside red blood cells, it leaks out into the plasma, though electrical neutrality is maintained by a process called the chloride shift which, in short, neutralizes the hydrogen ions, thus preventing the acidification of the blood and maintains normal blood pH (pH of 7.4). Once the blood reaches the lungs, bicarbonate diffuses back into the red blood cells and is converted back to CO2, it is finally exchanged for oxygen at the alveoli, all the nutrient and gas exchanges happen at the capillary level.
When we discuss the respiratory system, it is important to understand the relation it shares with the circulatory system. Inside each red blood cell there are approximately 280 million molecules of hemoglobin, each hemoglobin houses four heme molecules. Heme is where oxygen binds. More precisely heme is composed of a porphyrin ring with an iron molecule at its center; it is at this iron molecule that oxygen binds for transport. Interestingly CO2 and O2 do not bind at the same site, but carbon monoxide (CO) does compete with O2 at the binding site, in fact CO is 200 times more likely to bind than oxygen as it has been observed to have a much greater affinity. CO is found in automobile emissions and cigarette smoke. It is well known that cigarette smokers can suffer from anemia and not be known to have it, this is because CO binds more readily than O2 and the body attempts to compensate by increasing hemoglobin, thus in a basic complete blood count (CBC) hemoglobin would appear normal, and anemia could go undiagnosed. But that’s not all, the oxygen that is able to bind does not unload at the tissues as it should, causing a decrease in oxygenation. In cases of CO poisoning, pure oxygen can be administered to displace the CO from hemoglobin by saturation.
Another important piece of understanding involves partial pressure. To generalize these concepts just know that gases will diffuse to areas of lower pressure. In the lungs, the partial pressure of O2 is said to be around 110 mm Hg while CO2 is approximately 40 mm Hg; in tissue where cellular respiration is actively happening the partial pressure of CO2, a waste product of cellular respiration, is higher than that of blood while the oxygen partial pressure is lower than that found in the lungs because it is used in cellular respiration, and thus the gases can diffuse to the areas where the pressure is lower. Remember that small molecule like oxygen and carbon dioxide can easily diffuse through the membrane of cells.
Some athletes have been known to use blood doping as a means of enhancement for competitions. Blood doping is the act of removing one’s own red blood cells, storing them, and injecting them back the day of the competition. This increases the amount of red blood cells available for oxygen binding in the body, at least for the duration of the competition and until the body equilibrates, thus enhancing endurance and decreasing muscle fatigue. The problem with blood doping is that it increases the body’s hematocrit (percentage of red blood cells), which makes blood more viscous and more likely to clot, leading to a potential heart attack or stroke. Another enhancement measure is the use of erythropoietin, a hormone secreted by the kidneys that increases the rate of red blood cell production in the bone marrow; this hormone is activated when oxygen levels are low in the tissues.
If you are a good reader, you might be wondering about nitrogen gas, the larger percentage of what we breathe. Well, nitrogen gas has a strong triple bond that links both nitrogen molecules together. This triple bond is almost unbreakable rendering nitrogen unreactive with other chemical inside the blood; it literally leaves the same way it comes in. In fact, nitrogen only becomes a problem when the body is subjected to higher pressures as in scuba diving. When underwater the pressure shoves more nitrogen into the tissues, if the diver were to rapidly come out of the water, that is, without gradually allowing nitrogen to leave the tissue as the pressure lessens, he or she could suffer something call decompression sickness, colloquially known as the bends. Here, as the diver comes back up the pressure decreases and the volume increases, if not enough time is allowed nitrogen forms bubbles that may occlude a vessel, causing muscle pain, joint pain and even paralysis.
To close this blog, I want to make a side note about smoke. Smoking is the leading cause of lung cancer in America. Cigarette smoke releases nicotine, carbon monoxide, and polycyclic aromatic hydrocarbons into the lung tissue; these toxic fumes can lead to emphysema and chronic obstructive pulmonary disease among other conditions. It also damages the vocal cords, which causes voice hoarseness also known as “raspy” voice. But the effects of smoking are not just limited to the lungs, in the arteries of the heart and brain smoke causes endothelial dysfunction which can lead to heart attacks and strokes, another noted effect of the heart is that smoke leads to irregular cardiac rhythms like atrial flutters and fibrillations. These irregular heart rhythms can cause blood pooling in the atria or the ventricles, which has the potential to form clots that could lead to obstructions known as embolisms. So this is the one important recommendation: if you smoke, STOP you will be glad you did in the future. Also, you can always measure your lung capacity with a spirometer just ask your primary care doctor next time you’re at doctor’s office.
We have reached the end of this blog post; I hope you found this information as enjoyable as I did. As always, leave you kudos and remember you can always email me your thoughts by pressing the “say hello” button at the bottom of this page. I apologize for not being able to write much lately due to the rigors of MCAT studying, hopefully after test day I can get back in track. Thank you all who have emailed me asking for another blog, I appreciate the time you take to read my material, I do this to spread information with you all.
Until next time,
MC