  Exhaled air contains aerosol of submicron droplets of the alveolar lining
fluid (ALF), which are generated in the small airways of a human lung. Since
the exhaled particles are micro-samples of the ALF, their trapping opens up an
opportunity to collect non-invasively a native material from respiratory tract.
Recent studies of the particle characteristics (such as size distribution,
concentration and composition) in healthy and diseased subjects performed under
various conditions have demonstrated a high potential of the analysis of
exhaled aerosol droplets for identifying and monitoring pathological processes
in the ALF. In this paper we present a new method for sampling of aerosol
particles during the exhaled breath barbotage (EBB) through liquid. The
barbotage procedure results in accumulation of the pulmonary surfactant, being
the main component of ALF, on the liquid surface, which makes possible the
study its surface properties. We also propose a data processing algorithm to
evaluate the surface pressure ($\pi$) -- surface concentration ($\Gamma$)
isotherm from the raw data measured in a Langmuir trough. Finally, we analyze
the $(\pi-\Gamma)$ isotherms obtained for the samples collected in the groups
of healthy volunteers and patients with pulmonary tuberculosis and compare them
with the isotherm measured for the artificial pulmonary surfactant.
