Answer
Inspiration/inhalation is an active process by which air is taken into the lungs. But exhalation is a largely passive process in which air flows out, or is forced out of the lungs.
During inhalation, the diaphragm and other inspiratory muscles-external intercostals-- contract. The result is that the diaphragm is flattens and the chest wall moves up and outward. As a consequence, the cavity of the thorax is increased and the lungs are pulled outward. When the volume of the lung (alveolar volume) increases, this causes the pressure in alveolar air to decrease. At this point, atmospheric pressure is greater than alveolar pressure (Patm>Palv), and, following Boyle's Law, the air rushes into the lungs down its concentration gradient.
Expiration (unforced expiration) has been described as passive. This means that no effort is required to effect normal expiration: rather, in this process , the diaphragm and the internal intercostal muscles relax, the rib cage moves down and outward to its normal position, and the lungs recoil. The chest cage falls partly of its own weight and partly because of elastic recoil of some of its tissues. Elastic recoil is also responsible for return of the lungs to normal size and .volume.
In accordance with Boyle's law, when the volume of the thoracic cavity decreases, intrathoracic pressure increases. This pressure is transmitted to the alveoli. When the alveoli volume rises above atmospheric pressure (Palv>Patm), air is pushed out or flows out of the lung into the atmosphere.
Surface tension tends to close the lungs (alveoli), but surfactant on alveolar surfaces decreases surface tension, and keeps the alveoli open; therefore, there is a residual volume of air in the lungs at the end of expiration.
Work Step by Step
According to Boyle's Law, the pressure of a gas in a closed container is inversely proportional to its volume. Therefore, if the size of a closed container of gas is increased, the pressure of the gas inside the container will decrease. Also, if the volume of the container of gas is decreased, the pressure of the of the gas will increase. Therefore (P1V1=P2V2 or PiVi=PfVf), where "i" means initial and "f" means final.
In inhalation, the diaphragm contracts, the chest expands, and the lungs are pulled outward. This causes the alveolar pressure (Palv to fall to about 758 mm Hg. Since atmospheric pressure remains at 760 mm Hg, PalvPatm). Under these circumstances, the air flows down its concentration gradient --out of the lungs into the atmosphere.
In forced expiration, the internal intercostal muscles are involved, and in singing the abdominal muscles play a vital role in the forceful expiration necessary in that endeavour.
Note. The so-called epigastrium--juncture of diaphragm and abdominal muscles (rectus abdominis)-- is believed by professional singers to be of special importance in holding a high note. But there is no muscle named epigastrium. There is, however, an epigastric region of the upper abdomen-- situated between the left and right hypochondriac regions.
