The partial pressure of carbon dioxide in venous blood is approximately__________
The correct answer and explanation is :
The partial pressure of carbon dioxide (pCO₂) in venous blood is approximately 45 mmHg (millimeters of mercury).
Explanation:
The partial pressure of carbon dioxide (pCO₂) in the blood refers to the pressure exerted by CO₂ gas when it is dissolved in the blood. This value is different in arterial and venous blood due to the exchange of gases occurring in the tissues and lungs.
1. Difference Between Arterial and Venous pCO₂:
- Arterial Blood pCO₂: Around 40 mmHg
- Venous Blood pCO₂: Around 45 mmHg
Venous blood has a higher pCO₂ than arterial blood because it carries CO₂ from tissues back to the lungs for exhalation. As tissues metabolize oxygen for energy, they produce CO₂ as a waste product, which diffuses into the blood, increasing its partial pressure.
2. Transport of CO₂ in Blood:
Carbon dioxide is transported in venous blood in three main forms:
- Dissolved CO₂ (5-10%): Directly dissolves in plasma.
- Carbamino Compounds (20-30%): CO₂ binds to hemoglobin to form carbaminohemoglobin.
- Bicarbonate Ions (60-70%): CO₂ reacts with water in red blood cells to form carbonic acid, which dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).
3. Physiological Significance:
Maintaining the correct pCO₂ levels is critical for acid-base balance and proper respiratory function. The body regulates pCO₂ through respiration—when levels rise, breathing rate increases to expel CO₂, preventing respiratory acidosis.
