A rise in the blood level of carbon dioxide is called:

A. Bicarbonate is a buffer in the blood that helps maintain pH balance, but it is not a gas and does not directly regulate breathing.
B. Carbon dioxide (CO₂) is the primary regulator of respiration. Elevated CO₂ levels in the blood lower pH (making it more acidic), which stimulates chemoreceptors to increase the rate and depth of breathing to expel more CO₂ and restore balance.
C. Nitrogen makes up the majority of the air we breathe, but it is inert and plays no active role in regulating respiration.
D. Oxygen does influence breathing, especially at very low levels, but it is not the primary driver. In healthy individuals, CO₂ levels are the main factor controlling respiratory rate.

A. Apnea refers to a complete cessation of breathing, not just a decrease in depth and rate.
B. Hypoxemia is a condition characterized by low oxygen levels in the blood, but it does not specifically refer to breathing rate or depth.
C. Hypoxia is a broader term indicating low oxygen supply to tissues, which can result from various causes, including but not limited to respiratory issues.
D. Hypopnea specifically means shallow or slow breathing, indicating a reduction in both the rate and depth of respiration. It is often seen in sleep-related breathing disorders and can lead to decreased oxygenation.