Purring was once thought to be the sound of turbulent blood flow in the vena cava, the large vein that carries blood into the heart. More recent research, however, suggests that it is produced in the larynx (voice box), which connects the back of the throat with the trachea.
The vocal cords, two infoldings of membrane in the larynx, vibrate as exhaled air passes over them, to make vocalizations, such as meows and screeches. During purring, however, the muscles that control the vocal cords vibrate, causing the cords to bang into each other repeatedly. Air passing through the larynx as the cat breathes in and out produces bursts of noise, 25 times a second, known as purring.
Other cat species, such as the bobcat, cougar, and cheetah, can purr, too. Big cats of the genus Panthera, such as lions and tigers, roar rather than purr. They can do this because of their enlarged larynx. Folds in their vocal cords vibrate to produce sound, while the hyoid bone lowers the pitch and increases the resonance of the roar.