Boyle's law states that the volume of a gas and pressure are inversely proportional at a given temperature. In other words, when the pressure increases, the volume decreases, and vice versa. Boyle's law is important because it helps us understand the relation between pressure and volume in the lungs when breathing. During inspiration, the volume of the lungs increases, which results in lower pressure. This allows air at atmospheric pressure to rush in and fill the lung.