Final answer:
Capillary blood gas values, especially for carbon dioxide (pCO2), may not correlate well with arterial samples due to the differences in physiology and gas solubility. Oxygen has a much larger partial pressure gradient across the respiratory membrane, which can lead to variability in capillary and arterial blood gas comparisons. Mercury manometers, due to their higher density, are preferred over water manometers for measuring arterial blood pressure.
Step-by-step explanation:
When comparing values from capillary blood gas (CBG) to those of an arterial sample, partial pressures are often used as indicators of gas exchange effectiveness in the respiratory system. Particularly, the partial pressure of carbon dioxide (pCO2) may not correlate well between capillary and arterial samples due to distinct physiological factors affecting carbon dioxide transport and blood gas dynamics.
While pCO2 in capillary blood is approximately 45 mm Hg and 40 mm Hg in alveolar air, representing a small gradient of about 5 mm Hg, the considerable solubility of carbon dioxide, which is about 20 times greater than that of oxygen, largely compensates for this small gradient. This allows similar relative concentrations of oxygen and carbon dioxide to diffuse across the respiratory membrane despite the differences in partial pressure.
Conversely, the partial pressure of oxygen (pO2) shows a much larger gradient. In the alveoli, the pO2 is about 104 mm Hg, while it is about 40 mm Hg in the capillary blood, creating a strong pressure gradient facilitating rapid oxygen diffusion into the blood. This gradient is crucial for efficient oxygen transfer from the lungs to the blood, but it also results in greater variability in pO2 readings between arterial and capillary blood gases.
Furthermore, mercury manometers are typically used over water manometers to measure arterial blood pressure due to mercury's higher density, requiring a much shorter column to measure the high pressures found in arterial circulation, as compared to a water column. This practical consideration ensures accurate and convenient readings in clinical settings.