Our metabolism forms a cycle with our human environment. This also applies to breathing and the air we breathe. The air we breathe consists of two main components: around 78 percent is nitrogen and around 21 percent oxygen. Only 1 percent of other gases remain, with the inert gas argon accounting for the largest share of the air volume at a good 0.9 percent. Carbon dioxide makes up 0.04 percent of the total air we breathe. Even at high altitudes, the composition hardly changes. However, the density of the air decreases, i.e. the number of gas molecules per volume of air. For example, only a third of the oxygen can be inhaled in one breath on Mount Everst (at over 8000 meters above sea level) compared to sea level. In poorly ventilated rooms where many people are present, people often have the impression that the air is “bad” because the oxygen has been “used up”. This is a fallacy. If you find the air in closed rooms stale, it is not due to the supposedly low oxygen content. The oxygen content in indoor air only decreases minimally in closed rooms. What changes noticeably for people, however, is the CO2 concentration in the room. The CO2 concentration quickly doubles or triples. Even slight increases are noticeable. The air we breathe out contains around 4 percent CO2 and 17 percent oxygen. If the proportion of CO2 in the room air exceeds 0.15 percent, the air is considered “bad”, which is around four times the average value. However, the percentage oxygen content then hardly decreases at all at 20.9 percent. Even a slight increase in the CO2 content in enclosed spaces, and therefore in the air we breathe in, causes us discomfort. This can result in nausea, headaches and even unconsciousness. It is therefore advisable to sleep with the window open at night to counteract the accumulation of exhaled CO2. But why do we exhale more CO2 than we inhale? And why does our body need the oxygen we breathe in?
What happens to oxygen and carbon dioxide in our body?
Oxygen enters the blood in our lungs. Almost all of the oxygen is absorbed by our red blood cells (erythrocytes). Strictly speaking, the hemoglobin (Hb) of the red blood cells binds the oxygen to itself. The red pigment haemoglobin in our red blood cells can absorb up to four oxygen molecules and transport them around the body. Under normal conditions, oxygen saturation in the blood is 97 percent.
As the partial pressure of oxygen in the blood increases, the tendency of haemoglobin to bind oxygen to itself also increases. A haemoglobin molecule consists of four subunits, each of which can attach an oxygen molecule to bivalent iron (called haem). If no oxygen is bound to the haemoglobin, it is present as deoxyhaemoglobin in the so-called T-form (T for “tensed”) with low oxygen affinity.
Affinity represents the tendency with which oxygen is bound. The more of these four subunits have bound to an oxygen molecule, the more likely it is that the remaining subunits will also bind oxygen. The more oxygen is already bound, the more likely it is that further oxygen will be bound. Similar to a magnet, the “attractive force” increases the larger the magnet is. The T-form of the haemoglobin becomes an R-form (R for “relaxed”) and the haemoglobin is present as oxyhaemoglobin. This enables rapid oxygen saturation of the red blood cells in the lungs. Conversely, the release of oxygen molecules is accelerated the more subunits are free of oxygen. This facilitates the release of oxygen into the blood and consequently into the tissue.
Oxygen is needed to generate energy in our cells. Energy is mainly produced in our body by using the energy sources fat and carbohydrates are metabolized. These energy sources consist largely of carbon. The energy sources are broken down and utilized with the help of oxygen in what is known as aerobic energy production. What remains after the energy production process is “superfluous” carbon, which is bound in carbon dioxide. This carbon C is therefore removed from the body together with oxygen O2 via the lungs in the form of carbon dioxide CO2. To do this, the resulting CO2 is released from the cells into the blood. With the help of an enzyme (α-carboanhydrase), carbonic acid (H2CO3) is produced in the blood from carbon dioxide and water. Under normal conditions (alkaline conditions, see below), most of the carbonic acid decomposes further in the blood to form bicarbonate and an H+ hydrogen ion. This is how the majority of carbon dioxide becomes bicarbonate. A small proportion of the carbon dioxide binds to proteins in the blood plasma and to hemoglobin. Only a very small proportion of the carbon dioxide remains in the blood as dissolved gas.
Acidosis through sport
Whether a fluid such as blood is acidic or hyperacidic is indicated by the pH value. The pH value is defined as the potential of hydrogen. The more hydrogen ions (H+) are present, the more acidic our blood is. The pH scale ranges from 0 to 14, with 0 being acidic and 14 being basic (alkaline). A pH of 7 is considered neutral. Our blood is normally slightly alkaline with a pH value of around 7.4 (7.35 to 7.45). If the pH value is higher in the alkaline range, carbonic acid decomposes (dissociates), releasing a positively charged hydrogen particle H+particle and producing (negatively charged) bicarbonate. The two parts carbonic acid and bicarbonate form a buffer, called the carbonic acid-bicarbonate buffer system, to keep the pH value of the blood constant. If the blood is too acidic, the bicarbonate binds a positively charged hydrogen particle and becomes carbonic acid. The carbonic acid decomposes further into water and carbon dioxide CO2.
If the CO2 concentration in the blood rises and/or the pH value falls, the haemoglobin in the red blood cells binds less oxygen to itself. This is referred to as a decrease in the affinity of haemoglobin for oxygen, which is known as the Bohr effect. In addition to the CO2 concentration and the pH value, two other factors influence the binding behavior of haemoglobin and oxygen. The temperature and the 2,3-bisphosphoglyceric acid. The warmer it is, the lower the oxygen affinity of the red blood pigment. 2,3-bisphosphoglyceric acid reduces oxygen uptake in the lungs and facilitates oxygen release into the tissue. 2,3-bisphosphoglyceric acid is produced during anaerobic energy production by red blood cells (erythrocytes) as a result of glycolysis (sugar cleavage).
The concentration of 2,3-bisphosphoglyceric acid increases when there is a lack of oxygen due to anemia or at high altitudes. In this context, the effects of altitude training are also increasingly being discussed. Training camps at high altitudes are known to increase athletic performance. However, a certain placebo effect has now also been proven. There are numerous influencing factors, some of which are the subject of heated debate among scientists, such as the mean corpuscular volume (MCV). It is often said that the increase in performance through altitude training is due to the fact that the body produces more red blood cells as a result of staying and training at altitude. This is not correct, as the process of new erythrocyte formation takes several weeks and a noticeable increase in performance often only occurs after weeks or months of additional erythrocyte formation. However, two performance-enhancing effects can be observed with regard to erythrocytes and hemoglobin. The blood plasma thickens as a result of the altitude stay, regardless of the fluid intake, which increases the relative (not the absolute) number of erythrocytes. One advantage, however, is the increased production of 2,3-bisphosphoglyceric acid due to the lack of oxygen at altitude, which increases the release of oxygen into the tissue and thus the oxygen supply. This effect can be detected after just a few hours at high altitude and continues after returning from altitude.
Lactate is produced in our bodies, especially during intense physical exertion. Lactate itself is not responsible for acidosis, but the formation of lactate is. To understand this, you need to know that the terms lactic acid and lactate are not differentiated in German. Lactate is often referred to as lactic acid and is held responsible for over-acidification of the muscle and its fatigue. Strictly speaking, however, a distinction must be made between lactate and lactic acid. In English-speaking countries, the terms “lactic acid” and “lactate” are used. In chemical terms, lactate is the salt of lactic acid. In the body, the resulting lactic acid breaks down almost completely into lactate and a positively charged hydrogen particle H+. This is why the term “lactate” is used in German to describe acidosis. However, what is actually meant is lactic acid or the positively charged hydrogen particle H+ (while lactate is actually negatively charged). This means that a positively charged hydrogen particle H+ is always produced together with lactate during exercise. If our cells cannot utilize the lactate sufficiently, it enters the blood together with the hydrogen particle H+ (as both are always transported into the blood together (see lactate part 2). Too many H+ ions would lead to increasing acidification of the blood.
The carbonic acid-bicarbonate buffer system is required to prevent acidosis. Bicarbonate binds additional hydrogen ions and is then converted to water and carbon dioxide via carbonic acid (see above). The CO2 is transported to the lungs and exhaled. This buffering is necessary during intensive sport and the associated lactate production. To simplify matters, it is usually said that lactate is buffered, whereby the hydrogen ions produced are actually meant. During physical exertion and when lactate is produced, the concentration of bicarbonate in the blood drops measurably and additional CO2 is exhaled. So when lactate forms, the body’s own buffer system increases the bicarbonate content of the blood. This process is known as “bicarbonate loading” (2). Increased carbon dioxide is then exhaled through increased respiration. The (time) point at which an increase in the CO2 concentration in the breath can be detected with increasing physical exertion is called the respiratory compensation point (RCP). In the case of sustained higher exertion with constant or increasing lactate production, the acidosis due to lactate production is almost completely buffered by the bicarbonate buffer system (1).
Taking substances that increase the bicarbonate content in the blood shifts the pH value of the blood into the alkaline/alkaline range and increases the acid buffer. As a result, higher lactate or lactic acid concentrations in the blood can be tolerated. To increase performance in the short term, sodium bicarbonate (sodium hydrogen carbonate) is used as an aid. However, only anaerobic energy production (glycolysis) can be optimized (3). Supplementation also carries the risk of metabolic alkalosis with side effects such as diarrhea, abdominal pain or vomiting. Alkalosis means that the blood pH value is too high and is the opposite of acidosis, i.e. hyperacidity.
A lack of oxygen or excitement can lead to excessive breathing (hyperventilation), causing the pH value to rise. This process is called respiratory alkalosis. As the oxygen saturation of the blood is already very high at 97 percent under “normal” conditions and “normal” breathing, hardly any additional oxygen can be absorbed. However, more CO2 is exhaled. Even after the end of high physical exertion, we initially continue to exhale more CO2.
Spiroergometry
During spiroergometry, respiratory gases are analyzed under physical stress. It is used to test the resilience of the lungs and cardiovascular system in the medical field and, in the sports field, primarily for performance diagnostics. Spiroergometry determines the volume ratio RER (respiratory exchange rate) of exhaled carbon dioxide (VCO2) to inhaled oxygen (VO2) in the respiratory air: RER = (VCO2) / (VO2). This ratio is often also called the respiratory quotient RQ and is intended to provide an insight into cellular respiration, as the conversion of oxygen to carbon dioxide ultimately takes place at the cellular level during energy production. However, this only applies to a limited extent, as will be shown below. The respiratory quotient [RQ = (VCO2) / (VO2)] is used in spiroergometry to make a statement about the energy substrates used by the body. When energy is produced by fatty acids (fat oxidation), proportionally more oxygen molecules are required for energy production (production of adenosine triphosphate ATP) than carbon dioxide molecules are exhaled. The (molar) ratio of exhaled carbon dioxide to oxygen molecules is therefore <1 and results in a value of RQ = 0.7 (respiratory quotient). During the oxidation of glucose (glucose oxidation), the molecular proportions of exhaled carbon dioxide and inhaled oxygen molecules are equal and the respiratory quotient is: RQ = 1. In practice, there is usually a mixed ratio of fat and glucose oxidation and the value of the respiratory quotient is in between (0.7 < RQ < 1) and is usually between 0.82 and 0.85. With extreme diets in which the new production of glucose via the liver is increased, such as a ketogenic diet, values of RQ = 0.69 can also be measured (5). The third macronutrient, protein, is neglected in the analysis. Under normal conditions, it accounts for 5 percent of the energy supply and can increase to 10 percent or more during prolonged exercise (4). Energy production from stored carbohydrates, glycogen, leads to a slightly different RQ compared to energy production from non-stored carbohydrates, but this is also neglected. Whether it makes sense to neglect this or not will not be discussed here, but it shows the complexity of the issue of wanting to determine the substrates of energy production in a simplified way using the respiratory quotient. The respiratory quotient for protein is on average: RQ = 0.81 (depending on the structure of the proteins with different amino acids). Nutrition also has an influence on the respiratory quotient. It has been shown in animal fattening that a diet consisting exclusively of carbohydrates achieves a respiratory quotient of over 1 even without physical exertion (RQ > 1). Athletes who ate a low-carb diet shortly before spiroergometry showed a reduction in RQ (xxx). In people who consume an enormous amount of carbohydrates, the RQ can already reach values close to 1 during the time of digestion. As explained above, the CO2 content in the exhaled air increases due to increased exhalation and/or intensive exertion, as lactate in the blood is buffered by bicarbonate. The additional CO2 produced in the process is exhaled and increases the carbon dioxide content (VCO2). The value of the respiratory quotient can thus continue to rise and reach values of RQ > 1.
Increasing lactate production ultimately increases the value of the respiratory quotient due to additional CO2 in the air we breathe. This increase therefore has nothing to do with an increased use of carbohydrates or glucose! Only when no additional lactate is produced and the production rate is stable can the respiratory quotient be used to make a statement about the ratio of carbohydrate to fat oxidation in the context of energy production. With increasing physical exertion, the rate of lactate production also increases initially. After a few minutes, the rate of lactate formation stabilizes again at a higher level. This continues until the so-called threshold is exceeded, at which point the body is no longer able to stabilize lactate production at a level so that more and more lactate continues to accumulate in the blood. Above this threshold, the increasing lactate production means that the respiratory quotient can no longer be used to make a statement about the energy substrates used. In order to also use the respiratory quotient RQ above the threshold, the additional CO2 – which is produced by lactate buffering through bicarbonate – would have to be calculated out of the exhaled air (VO2). This is not usually done in practice and the buffering of the bicarbonate is neglected.
In short: Stale air in closed rooms is not caused by a supposedly low oxygen content, but by an increased CO2 concentration. Indoor training is therefore better done with a supply of fresh air. During altitude training, the performance-enhancing adaptation occurs less through the formation of red blood cells than through the increased formation of bisphosphoglyceric acid, which increases the oxygen supply. This effect can be demonstrated after just a few hours at high altitude and continues after returning from altitude. In addition to lactate, (intensive) sport always produces hydrogen ions H+, which make our blood more acidic. To protect our blood and thus our body from acidification, bicarbonate binds hydrogen ions in the blood and is then converted into CO2. The additional CO2 is exhaled and does not come from energy production from carbohydrates. In general, it should be noted that, according to current scientific knowledge, even just above the threshold, energy production mainly occurs through aerobic energy production and only around 2 percent through anaerobic energy production (6). For aerobic energy production in the mitochondria, fats and carbohydrates are the main sources of energy. As a lot of energy has to be produced in a short time, it is assumed that energy production in stressed muscles is mainly via carbohydrates. More detailed scientific studies are still pending.
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(5) Bob Murray, Christine Rosenbloom, Fundamentals of glycogen metabolism for coaches and athletes, Nutrition Reviews, Volume 76, Issue 4, April 2018, Pages 243–259, https://doi.org/10.1093/nutrit/nuy001
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