The process by which a continuous exchange of gases is maintained across respiratory surfaces. Often called external respiration, this is achieved by respiratory movements; in air-breathing vertebrates it is movement of air into and out of the lungs (see also air sac; expiration; inspiration; trachea; ventilation centre). The minute ventilation rate (or minute respiration rate) of an animal is the volume of air breathed per minute, i.e. tidal volume × number of breaths per minute. It can be measured with the aid of a respirometer.