The tidal volume is defined as the volume of what during each breathing cycle?

The tidal volume refers specifically to the volume of air that is inhaled or exhaled during each complete breath cycle. This measurement is crucial in respiratory physiology as it helps assess how efficiently the lungs are functioning and the overall health of the respiratory system. By understanding tidal volume, one can gain insights into the breathing patterns and respiratory capabilities of an individual or an animal, such as a horse. The other options involve processes unrelated to the respiratory cycle: blood circulation pertains to cardiovascular function, digestion involves the gastrointestinal system, and urine production relates to the renal system. Thus, the correct association of tidal volume is with the air exchanged during breathing.