Carbonic acid-bicarbonate system
The carbonic acid-bicarbonate buffer system is the most important blood buffer for absorbing pH fluctuations in the human bloodstream. It consists of carbonic acid (H 2 CO 3 ) as acid and bicarbonate (actually called hydrogen carbonate, HCO 3 - ) as base . If the blood is not acidic enough, a proton (H + ) is released from the carbonic acid, which then becomes bicarbonate. If, on the other hand, the blood contains too many protons, i.e. it is too acidic, the bicarbonate binds a proton and becomes carbonic acid. This breaks down into water (H 2 O) and carbon dioxide (CO 2 ). Increased breathing activity then increases the amount of carbon dioxide exhaled. Conversely , if there is too little acid in the blood, lung activity is reduced. Not only about the respiration, using the kidneys can acid-base balance can be influenced. Because they are able to selectively excrete protons and bicarbonate ions or retain them in the body. Certain blood proteins are also involved in regulating the acid-base balance. They bind or release protons as needed.
functionality
The buffer system is based on the following chemical relationship:
In simplified terms, carbonic acid (H 2 CO 3 ) can be omitted because its concentration in the blood is 400 times less than that of carbon dioxide (CO 2 ). This results in this equation:
The Henderson-Hasselbalch equation applies to the buffer system :
The pKa value for equilibrium is 6.1 at 37 ° C.
The partial pressure of carbon dioxide in the blood is approximately 40 mmHg (40 Torr ), which corresponds to a concentration of 1.2 mM . At a concentration of the hydrogen carbonate ions of 24 mM, the pH is calculated to be 7.4.
The blood buffer is an open system. Carbon dioxide is added to or removed from the blood. The partial pressure of carbon dioxide is kept at an approximately constant value under normal conditions ( respiratory regulation ). In addition, there is a metabolic regulation of the buffer system so that the pH value is roughly in the same range. The blood pH value can be quickly regulated by increasing or decreasing breathing activity. With increased breathing activity ( hyperventilation ), the proportion of CO 2 in the air we breathe decreases, which in turn reduces the partial pressure of CO 2 in the lungs and thus also the proportion in the blood. Since the law of mass action applies, CO 2 is modeled from HCO 3 - and protons. The number of protons decreases, thereby increasing the pH value in the blood, it becomes more basic ( respiratory alkalosis ). Conversely, reduced breathing activity ( hypoventilation ) leads to an increased CO 2 partial pressure, now the reactions run in the other direction, the blood pH drops, it becomes more acidic ( respiratory acidosis ). Rather, acidosis is caused by a shift of H + ions from the intracellular to the extracellular space. This process is a result of the increased PCO2 in the blood and has to do with the above. Buffer equation only limited to do something.
However, the carbonic acid-bicarbonate system is not suitable for permanent regulation, as the bicarbonate ions also disappear with the formation of carbon dioxide. The kidneys are therefore involved in the excretion of protons and above all in the recovery of hydrogen carbonate ions, but can only respond effectively to a sudden change in blood pH after 3–4 days.
literature
- Christoph Maulbetsch: The hydrogen carbonate buffer system in human blood, Praxis der Naturwissenschaften Chemie 2/61, March 2012, pp. 26–30, Aulis Verlag
Web links
Individual evidence
- ↑ R. Klinke, HC. Pape, St. Silbernagl: Physiologie Georg Thieme Verlag, Stuttgart 2005, ISBN 3-13-796005-3 , p. 314.