Question

Explain how carbon dioxide is transported in the blood. (Note, this is asking for the HOW, not the WHY.)

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Answer 1

• Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion.

• 1. carbon dioxide is more soluble in blood than oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Carbon dioxide is 20 times more soluble than oxygen; it obeys Henry’s law, which states that the number of molecules in solution is proportional to the partial pressure at the liquid surface. The carbon dioxide solubility coefficient is 0.0308 mmol per litre per mm Hg or 0.231 mmol per litre per kPa at 37C. (Solubility increases as the temperature falls.) This corresponds to 0.5 ml per kPa carbon dioxide in 100 ml blood at 37C. The partial pressure of carbon dioxide is 5.3 pKa in arterial blood and 6.1 kPa in mixed venous blood; therefore, arterial blood will contain about 2.5 ml per 100 ml of dissolved carbon dioxide and venous blood 3 ml per 100 ml.

• 2. carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body.

• 3. the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H2CO3). Carbonic acid is an unstable intermediate molecule that immediately dissociates into bicarbonate ions (HCO−3)(HCO3−) and hydrogen (H+) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood down its concentration gradient. It also results in the production of H+ions. If too much H+ is produced, it can alter blood pH. However, hemoglobin binds to the free H+ ions and thus limits shifts in pH. The newly synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (Cl−); this is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H+ ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation.

CO2+H2O⟷H2CO3(carbonic acid)⟷HCO3+H+(bicarbonate)

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