Hypoxemia is:

A. Rhythmic variation in breathing refers to breathing patterns such as Cheyne-Stokes respiration, not orthopnea.
B. Orthopnea is defined as difficulty breathing when lying flat, which is relieved by sitting or standing upright. It is often seen in patients with heart failure or other conditions causing fluid buildup in the lungs.
C. Difficult or labored breathing is called dyspnea, not orthopnea. While orthopnea is a type of dyspnea, the key feature of orthopnea is its positional nature.
D. Cessation of breathing is known as apnea, not orthopnea.

A. Tachypnea refers to rapid breathing. While it can affect the balance of gases in the blood, it typically leads to decreased CO₂ levels, which would actually reduce blood acidity (increase pH).
B. Dyspnea refers to difficult or labored breathing. It can be associated with respiratory distress, but it does not directly cause significant changes in blood acidity.
C. Hyperpnea is characterized by increased depth and rate of breathing, often to meet metabolic demands, like during exercise. This typically decreases CO₂ levels, which can lead to a reduction in blood acidity (increase in pH).
D. Apnea, which is the cessation of breathing, would lead to a buildup of CO₂ in the blood because the body is not expelling it through normal respiration. This increase in CO₂ would combine with water to form carbonic acid, thereby lowering blood pH and increasing acidity.