The heart and lungs make sure that oxygen is delivered—via airways and blood—to every cell in the body. Oxygen, which makes up about 21 percent of the air, reacts with nutrient molecules, such as the sugar glucose, in body cells to release energy.
This energy is then used to power biochemical activities in cells. As air travels to and from the lungs, it passes through the larynx, the source of a cat’s vocalizations, including its purr.
The airways and lungs comprise the respiratory system. Air inhaled through a cat’s nose is humidified in the nasal passages and drawn along the trachea (windpipe), which branches into two air passages called bronchi, one for each lung.
Within the lungs, the bronchi divide into smaller tubes called bronchioles, which end in tiny air sacs called alveoli. Gas exchange occurs inside the alveoli. Oxygen diffuses across the thin walls of the millions of alveoli into tiny blood vessels called capillaries, where it is picked up by red blood cells. The waste gas carbon dioxide travels in the opposite direction, out of the blood and into the alveoli, to be exhaled.
At rest, cats breathe in and out about 20–30 times a minute. During exercise, when the muscles need more oxygen, the rate increases. Muscles between the ribs and a sheet of muscle below the rib cage, called the diaphragm, power breathing.