U01.16.021 Carbon dioxide transport

Carbon dioxide (CO₂) is a waste product of cellular respiration and needs to be efficiently transported from the tissues, where it is produced, to the lungs, where it is exhaled. This process is vital for maintaining the body’s acid-base balance and ensuring proper oxygen delivery to tissues.

Forms of CO₂ Transport

CO₂ is transported from the tissues to the lungs in three primary forms:

Bicarbonate (HCO₃⁻):
- The majority of CO₂ (approximately 70%) is transported as bicarbonate ions in the plasma. This occurs when CO₂ reacts with water under the influence of the enzyme carbonic anhydrase, forming carbonic acid (H₂CO₃), which quickly dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).
Carbaminohemoglobin (HbCO₂):
- About 21–25% of CO₂ is transported by binding to hemoglobin, forming carbaminohemoglobin. CO₂ binds to the N-terminus of the globin chains (not the heme group). This form of CO₂ transport is more efficient when hemoglobin is in its deoxygenated form (after O₂ has been unloaded in the tissues).
Dissolved CO₂:
- A small fraction (5–9%) of CO₂ is transported dissolved in the plasma.

Mechanisms in Lungs and Peripheral Tissues

In the Lungs:

Oxygenation of hemoglobin (Hb) promotes the dissociation of hydrogen ions (H⁺) from Hb. This change in the ionic environment shifts the equilibrium toward the formation of CO₂, facilitating its release from red blood cells. This process is known as the Haldane effect.

In Peripheral Tissues:

Hydrogen ions (H⁺) generated from tissue metabolism increase, which shifts the oxygen-hemoglobin dissociation curve to the right. This shift promotes the unloading of oxygen to the tissues, a process known as the Bohr effect.

Table: Forms of CO₂ Transport

Form of CO₂ Transport	Percentage of Total CO₂	Mechanism
Bicarbonate (HCO₃⁻)	70%	CO₂ converted to HCO₃⁻ in plasma via carbonic anhydrase.
Carbaminohemoglobin (HbCO₂)	21–25%	CO₂ binds to Hb at the N-terminus of globin chains.
Dissolved CO₂	5–9%	CO₂ is transported directly dissolved in plasma.

Table: Forms of CO₂ Transport

Points to Remember

The majority of CO₂ in the blood is transported as bicarbonate (HCO₃⁻).
The Haldane effect facilitates CO₂ release in the lungs, while the Bohr effect promotes oxygen unloading in peripheral tissues.
CO₂ transport is crucial for maintaining acid-base balance and ensuring efficient oxygen delivery.

References

Boron, W. F., & Boulpaep, E. L. (2016). Medical Physiology. Elsevier Health Sciences.
Hall, J. E. (2015). Guyton and Hall Textbook of Medical Physiology. Elsevier Health Sciences.

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USMLE Step 1 Review Series

Curriculum

U01.16.021 Carbon dioxide transport

Forms of CO₂ Transport

Mechanisms in Lungs and Peripheral Tissues

Table: Forms of CO₂ Transport

Points to Remember

References

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