Carbon Dioxide Transport
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Carbon dioxide is transported from the tissues to the blood, to the lungs, and out into the environment.
Three key forms of carbon dioxide present in the blood:
- Dissolved CO2 constitutes about 5% of the total carbon dioxide content and contributes to the partial pressure; recall that the partial pressure of a gas is a major determinant of its diffusion.
- Carbon dioxide bound to hemoglobin, constitutes about 3%, and is referred to as carbaminohemoglobin.
- The Haldane effect predicts that when blood oxygen increases, the affinity of hemoglobin for carbon dioxide decreases; in other words, when hemoglobin binds with oxygen, it more readily releases carbon dioxide.
- Notice that this is the opposite of the Bohr effect, in which increased carbon dioxide reduces hemoglobin's affinity for oxygen.
- Bicarbonate is the chemically modified form of carbon dioxide that comprises the majority of carbon dioxide in the blood.
Be aware that the percentages of each type of carbon dioxide given here are approximations, because chemical reactions within the blood are ongoing.
Three sites of carbon dioxide:
- Peripheral body tissues as byproduct of aerobic respiration.
- Red blood cells in the vessels.
- Lung.
Steps to remove carbon dioxide from the body:
1. Aerobic metabolism in the tissues produces carbon dioxide.
2. Partial pressure gradients between the tissues and the blood stream and also between the blood stream and the red blood cells, drives carbon dioxide diffusion into the red blood cells, where it mixes with H2O.
3. Within the red blood cell, show that carbonic anhydrase reversibly converts water and carbon dioxide to carbonic acid.
4. Carbonic acid dissociates to form hydrogen ions, which are buffered by hemoglobin, and bicarbonate, which can then exit the cell via chloride exchangers and enter the blood stream.
5. Bicarbonate enters the lungs, again via chloride exchangers; there, the reactions reverse to produce carbon dioxide, which is then expired during ventilation.
Hypercapnia
- Occurs when ventilation is inhibited and carbon dioxide accumulates.
- Hypercapnia can cause acidosis (low blood pH), which depresses the central nervous system.
- This can lead to a suite of symptoms, including headache and confusion; if extreme, hypercapnia can lead to coma.