Respiratory Volumes

are two muscle groups involved in the normal inspiration: the external intercostal muscles and the diaphragm. There are other muscles that can be involved if a larger breath is indicated. The diaphragm moves downward and the ribs move upward and outward. This expands the ribcage and, because the lungs are adherent to the rib cage, the lungs expand as well. This causes a decrease in intra-alveolar pressure and air rushes into the lungs.

Expiration is normally passive so that no energy is necessary to push air out of the lungs. The lungs are elastic so they recoil as the diaphragm and intercostal muscles relax. There is a subsequent increase in the intrapulmonary pressure so that air leaves the lungs.

There are different types of breathing. Quiet breathing is called eupnea. It occurs at rest and does not require cognitive thought. It does require contraction of the diaphragm and external intercostal muscles. Deep breathing or “diaphragmatic breathing” involves contraction of the diaphragm. Shallow breathing or “costal breathing” depends on the contraction of the intercostal muscles. Forced breathing or “hyperpnea” happens with things like singing or exercising. Muscle contraction is required for both inspiration and expiration. It requires the addition of the scalene muscles for inspiration and the oblique muscles of the abdomen and the internal intercostals of the ribcage in order for this type of respiration to occur.

RESPIRATORY VOLUMES

There are several respiratory volumes important in the respiratory cycle. The tidal volume is about 500 milliliters and is the volume inhaled or exhaled during quiet breathing. The expiratory reserve volume is about 1200 milliliters and is the amount of air that can be forcefully exhaled. The inspiratory reserve volume is the amount that can be inhaled deeply. The residual volume is the amount that must stay in the lungs after forced exhalation to keep the alveoli from collapsing. Figure 8 describes a normal breath:

Figure 8.

In addition to these volumes, there is the anatomical dead space. This is the air that is present in the airways that cannot get exchanged because it is kept within the bronchi. The alveolar dead space is that which is in the alveoli that are not functioning because of a ventilation/perfusion mismatch or because there is disease in the alveoli. This can happen because of a pulmonary embolism, for example. The total dead space is the anatomical dead space and the alveolar dead space together.