Work of Breathing

The rate and depth at which one breathes under normal circumstances, is managed to minimize the amount of work that is done. If you try to breathe rapidly and shallowly for an extended period of time, you can transfer the necessary oxygen, but will rapidly grow tired from the effort. Work for a system that executes a cyclic process with only expansion and compression can be modeled by Eq. (3.6) (Wark and Richards, 1999).

In Eq. (3.6), Wrepresents the work done between points a and b. P represents the pressure inside the system (in this case the lung). The volume of the system is represented by V. Figure 3.4 shows a typical P-V curve during breathing. The work term can be thought of as an area under the P-V curve.

For example, one part of the work done by the diaphragm on the lungs due to inspiration can be thought of as the work needed to overcome the elastic resisting forces of the chest wall and diaphragm. This work done to overcome elasticity can be represented by the area under the line AB as shown in Fig. 3.5.

The total work done by the diaphragm on the lungs due to inspiration can be thought of as the work needed to overcome the elastic resisting forces of the chest wall and diaphragm, plus the work done to b

Wasb

P dV

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