Video narration
Respiratory physiology is how animals get oxygen into the body and remove carbon dioxide. Oxygen is needed to release energy from food in cells, and carbon dioxide is a waste product that must be carried away. First comes ventilation: moving air or water across a breathing surface, like lungs, gills, or even moist skin. Next is gas exchange: oxygen diffuses in, and carbon dioxide diffuses out, across a thin, wet membrane. Then the circulatory system transports these gases. Blood picks up oxygen at the exchange surface, delivers it to tissues, and returns carbon dioxide to be exhaled or released. Different animals solve the same problem in different ways, but the goal is always the same: keep oxygen coming to cells and keep carbon dioxide low, so metabolism can run efficiently—whether the animal is swimming, flying, or resting.