The term that is used for the pressure of each gas in a mixture of gases is:

A. Exhalation is the passive phase of normal breathing and occurs without effort during relaxed respiration. It involves the diaphragm and intercostal muscles relaxing, allowing air to flow out of the lungs.
B. Apnea refers to a temporary cessation of breathing and is not a phase of normal breathing.
C. Inhalation is the active phase of normal, relaxed breathing. It requires energy as the diaphragm contracts and moves downward and the intercostal muscles contract to expand the chest cavity, allowing air to enter the lungs.
D. Expiration is another term for exhalation, which is passive during normal relaxed breathing.

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.