During respiration, when is oxygen taken in by the horse?

The process of respiration in horses, as in many animals, involves the intake of oxygen during inhalation. During inhalation, the diaphragm and intercostal muscles contract, which expands the thoracic cavity, creating negative pressure that draws air into the lungs. This air contains oxygen, which is essential for cellular metabolism and energy production.

During exhalation, the opposite occurs; the muscles relax, and the thoracic cavity decreases in size, pushing air out of the lungs. At this point, carbon dioxide, a byproduct of metabolism, is expelled, not oxygen. Therefore, oxygen is specifically taken in during the inhalation phase of respiration.

While oxygen uptake can increase during exercise due to a higher metabolic demand, it is not exclusive to that condition. Horses also take in oxygen while resting, as respiration is a continuous process necessary for life. However, the key moment in which oxygen is specifically taken in occurs when the horse inhales, making this the correct answer.