How is carbon dioxide (CO2) mostly transported in the blood during high-intensity interval training?

Carbon Dioxide Transport During High-Intensity Exercise

During high-intensity interval training in a 24-year-old male athlete, carbon dioxide is predominantly transported in the blood as bicarbonate in serum/plasma (70-85% of total CO2 transport). 1, 2

Mechanisms of CO2 Transport in the Blood

Carbon dioxide produced during intense exercise is carried in the bloodstream in three main forms:

1. Bicarbonate in serum/plasma (70-85%) 2, 1

   • This is the primary and most significant form of CO2 transport
   • CO2 combines with water to form carbonic acid (H2CO3), which rapidly dissociates into hydrogen ions (H+) and bicarbonate (HCO3-)
   • This reaction is catalyzed by carbonic anhydrase, primarily within red blood cells

2. Bound to hemoglobin (10-20%) 2, 1

   • CO2 binds to amino groups on hemoglobin to form carbaminohemoglobin
   • This represents a smaller but still significant portion of CO2 transport

3. Dissolved CO2 in plasma (5-10%) 2, 1

   • The smallest fraction is physically dissolved in plasma
   • CO2 is approximately 20 times more soluble in blood than oxygen

CO2 Production During High-Intensity Exercise

During high-intensity interval training, several key physiological processes occur:

• The athlete produces significantly increased amounts of CO2 through aerobic metabolism 2
• As exercise intensity increases beyond the anaerobic threshold, lactic acid production increases 2
• Hydrogen ions from lactic acid combine with bicarbonate, driving the reaction: H+ + HCO3- → H2CO3 → CO2 + H2O 2
• This generates additional CO2 above that produced aerobically, further increasing the CO2 load 2

The Chloride Shift and CO2 Transport

The efficient transport of CO2 as bicarbonate relies on a process called the chloride shift:

• CO2 diffuses into red blood cells where carbonic anhydrase catalyzes its conversion to bicarbonate 3
• Bicarbonate then exchanges with chloride ions across the red cell membrane 3
• This allows most bicarbonate to be carried in the plasma rather than remaining in the red blood cells 3
• During high-intensity exercise, this exchange process may become rate-limiting when capillary transit times are very short (less than 0.5 seconds) 3

Clinical Implications

Understanding CO2 transport has important implications:

• During high-intensity exercise, the respiratory exchange ratio (RER) often exceeds 1.0, indicating CO2 production exceeds O2 consumption 2
• This reflects both the increased metabolic production of CO2 and the additional CO2 generated from buffering of lactic acid 2
• The body's ability to efficiently transport and eliminate CO2 is crucial for maintaining acid-base balance during intense exercise 4
• Impaired CO2 elimination can contribute to exercise limitation through effects on muscle contractility and metabolism 4

In conclusion, while CO2 is transported in multiple forms in the blood, the bicarbonate form in serum/plasma represents the predominant method of transport during high-intensity exercise in this young athlete.