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The measurement of HCO3 in arterial blood is specifically used to quantify the amount of bicarbonate present in the blood. Bicarbonate is a vital component of the body's buffering system, helping maintain a stable pH in the blood and tissues. It plays an essential role in the regulation of acid-base balance.

In the context of blood gas analysis, bicarbonate levels provide insight into the metabolic component of acid-base status. Elevated levels can indicate metabolic alkalosis, while low levels can reveal metabolic acidosis. Understanding bicarbonate levels is critical for practitioners when assessing a patient's respiratory or metabolic condition, guiding necessary interventions and therapies.

While the other choices relate to important parameters in arterial blood gas analysis, they do not pertain to HCO3 directly. The partial pressures of oxygen and carbon dioxide, as well as oxygen saturation, reflect different aspects of respiratory function, but they do not measure bicarbonate levels, which specifically informs practitioners about the metabolic aspect of the patient's acid-base balance.