Carbon Dioxide Transport in Blood

• Carbon dioxide (CO2) is a major waste product of aerobic respiration.
• Too much or too little CO2 in the blood can lead to serious consequences.
• CO2's primary role is to regulate blood pH, more critical than transporting it to the lungs for exhalation.

CO2 Dissolving in Blood

• CO2 dissolves in the blood and converts to carbonic acid (H2CO3).
• Carbonic acid almost instantaneously converts to hydrogen and bicarbonate ions (H+ + HCO3–).
• Even a small increase in dissolved CO2 can significantly alter the blood pH due to the resulting increase in hydrogen ions.
• The ratio of CO2 to HCO3– is roughly 1:20 so to maintain a stable pH, any rise in CO2 must be matched by a 20-fold rise in HCO3–.
• Since each CO2 molecule only generates one HCO3–, the blood becomes more acidic due to the excess hydrogen ions.

Methods of Transport

• CO2 is transported in the blood in 3 ways: as carbamino compounds, hydrogen carbonate (HCO3–), and dissolved CO2 itself.

Carbamino Compounds

• Approximately 30% of CO2 is transported as carbamino compounds.
• CO2 binds directly to amino acids and the amine groups of haemoglobin to form carbaminohaemoglobin, especially in areas of high CO2 concentration.
• In the lungs, where O2 concentration is high, haemoglobin preferentially binds to O2, promoting the release of CO2 (Haldane effect).

HCO3– Ions

• 60% of CO2 is transported through the production of HCO3– ions in red blood cells.
• Carbonic anhydrase in red blood cells converts CO2 to H+ and HCO3–.
• HCO3– is then transported back into the blood via a chloride-bicarbonate exchanger.
• The HCO3– acts as a buffer against hydrogen in the blood plasma.
• The H+ created by the carbonic anhydrase reaction binds to haemoglobin, forming deoxyhaemoglobin, contributing to the Bohr effect.
• Oxygen binding to haemoglobin in the lungs releases H+ ions, allowing them to react with bicarbonate ions to produce CO2 and H2O, which is then exhaled.

Dissolved in Plasma

• About 10% of CO2 is transported dissolved in plasma.
• The amount of gas dissolved in a liquid is influenced by its solubility and partial pressure.
• CO2 is very soluble in water, approximately 23 times more soluble than O2.
• The partial pressure of inspired CO2 is ~40mmHg.
• Tissues at the periphery have a higher partial pressure, and the alveoli have a lower partial pressure.

Clinical Relevance - Metabolic Acidosis

• Acidosis occurs when blood pH falls below 7.35, classified as either metabolic or respiratory.
• Metabolic acidosis results from excess H+ production or a reduction in the HCO3– buffer.
• Conditions, such as diabetic ketoacidosis, can increase acid production.
• Disorders of the kidneys themselves such as chronic kidney disease may reduce HCO3– production.
• The respiratory system compensates by increasing respiration rate (hyperventilating) to "blow off" CO2.
• Kidneys help correct the issue by increasing hydrogen excretion and bicarbonate reabsorption.
• Symptoms of acidosis include rapid breathing, confusion, fatigue, and headache.