Transport of Carbondioxide

Definition of CO2 Transport

Carbon dioxide (CO2) transport is a critical physiological process that ensures the waste product of cellular metabolism is efficiently removed from the body while maintaining the acid-base balance of blood. In the human body, approximately 200 mL of CO2 is produced every minute at rest through aerobic respiration in the mitochondria. This gas must travel from the tissue cells into the blood, then to the lungs, and finally be exhaled into the atmosphere.

I. Transport from Tissues to Blood

As cells undergo metabolism, they produce CO2, creating a high partial pressure in the tissues. This CO2 diffuses down its partial pressure gradient across the cell membranes and capillary walls into the systemic blood.

Pressure Variations:

• Intracellular/Tissue PCO2 : This is the highest, usually greater than 46 mmHg.

• Arterial Blood PCO2: Entering the systemic capillaries, it is approximately 40 mmHg.

• Gradient: A difference of at least 6 mmHg drives the diffusion of CO2 from tissues into the blood.

II. Forms of Carbon Dioxide Transport in Blood

Once in the blood, CO2 is carried in three distinct forms to the lungs:

1. Dissolved Form (~5-7%): Because CO2 is about 20 times more soluble in plasma than oxygen, a significant portion (roughly 5% to 7%) remains free in solution.

2. Carbamino Compounds (~20-23%): CO2 binds reversibly to the terminal amino groups of proteins. The most significant protein for this is hemoglobin (Hb), forming carbaminohemoglobin.

3. Bicarbonate Ions (HCO3−) (~70-90%): This is the most important form of transport. In the red blood cells (RBCs), CO2  combines with water (H2O) to form carbonic acid (H2CO3), a reaction catalyzed by the extremely fast enzyme Carbonic Anhydrase (CA).

III. The Chloride Shift (Hamburger Phenomenon)

Inside the RBC, the H2CO3 quickly dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3−). To prevent the accumulation of products, the HCO3−is moved out of the RBC into the plasma through a specific Anion Exchanger (Band 3 protein) in exchange for a chloride ion (Cl−). This movement of Cl− into the RBC is called the Chloride Shift. The H+produced is buffered by deoxyhemoglobin, which prevents a drastic drop in intracellular pH.

IV. Transport to the Alveoli and the Reverse Chloride Shift

The venous blood, now rich in CO2, returns to the right heart and is pumped into the pulmonary capillaries. As blood reaches the alveoli, the entire process reverses to allow CO2 elimination.

1. Reverse Chloride Shift: Bicarbonate (HCO3−) moves back into the RBC from the plasma, and chloride (Cl−) moves out.

2. Regeneration of CO2: Inside the RBC, HCO3−​combines with H+ released from hemoglobin to form H2CO3​ which Carbonic Anhydrase then dehydrates back into CO2 and H2O.

3. Diffusion into Alveoli: The regenerated CO2 diffuses out of the RBC, through the respiratory membrane, and into the alveolar air space.

Pressure Variations:

• Mixed Venous/Pulmonary Capillary PCO2: 46 mmHg.

• Alveolar PCO2 (PACO2): 40 mmHg.

• Gradient: A 6 mmHg gradient drives the diffusion into the alveoli.

V. Elimination to the Atmosphere The CO2 in the alveoli is moved out of the lungs during expiration. The rate of CO2 elimination is determined by Alveolar Ventilation; if ventilation increases, more CO2 is "blown off".

Pressure Variations:

• Inspired Air PCO2 : Approximately 0 mmHg (or 0.3 mmHg), which provides the final steep gradient for elimination to the environment.

VI. Factors Affecting Carbon Dioxide Transport

1. Haldane Effect: 

The binding of oxygen to hemoglobin reduces its affinity for CO2​ Conversely, when hemoglobin is deoxygenated in the tissues, its affinity for CO2

and its buffering capacity for H+ increase, facilitating more CO2 uptake.

2. Carbonic Anhydrase Activity: 

The speed of the hydration/dehydration reactions depends entirely on this enzyme.

3. Cardiac Output and Perfusion: 

Since CO2 transport is perfusion-limited, an increase in blood flow increases the amount of CO2 delivered to the lungs.

4. Alveolar Ventilation: 

The inverse relationship between ventilation and PACO2 means that inadequate breathing leads to CO2 retention.

VII. Clinical Applications

• Respiratory Acidosis: 

Caused by hypoventilation (as in COPD or drug overdose), where CO2 is retained in the body, leading to an increase in PaCO2 and a decrease in blood pH.

• Respiratory Alkalosis: 

Caused by hyperventilation (due to anxiety or high altitude), where excessive CO2 is removed, increasing blood pH.

• Arterial Blood Gas (ABG) Analysis: 

Measuring PaCO2 (Normal: 40 mmHg) is a fundamental diagnostic tool to assess a patient's respiratory status and acid-base balance.

• Chloride Shift in Edema: 

Variations in these transport mechanisms can be seen in patients with severe lung pathologies where the respiratory membrane thickness increases, slowing diffusion.

--------------------------------------------------------------------------------

Probable Questions for First-Year MBBS

1. Short Note (5 Marks): Describe the different forms in which carbon dioxide is transported in the blood. Explain the role of Carbonic Anhydrase in this process.

2. Long Answer Question (10 Marks): Explain the mechanism of the Chloride Shift (Hamburger Phenomenon) in the tissues and its reversal in the lungs. Add a note on the Haldane effect.

3. Applied Physiology (3 Marks): A patient is suffering from chronic obstructive pulmonary disease (COPD). Explain the likely changes in their arterial PCO2 and blood pH.