## 4 Discussion  
According to the best of our knowledge, the presented study is the first to examine the direct delivery of biologically active vitamin D3 (calcitriol), as well as its precursor (calcidiol), to the respiratory tract via inhalation as a potential strategy to neutralize vitamin D deficiencies in lung tissue. The study was performed on murine strain C57 BL/6, which is the most common animal model for studying pulmonary disorder with signs of fibrosis [16]. The first challenge for the presented study was the lack of literature data on the physiological and pathological levels of vitamin D3 in C57 BL/6 mice. There were also no clear recommendations regarding the standard diet of rodents in the context of vitamin D content (cholesterol content in commercial feed for laboratory rodents ranges from 0.5 IU/g to 5 IU/g); even less is known about the method of achieving vitamin D deficiency in the body of small rodents [17,18]. In the presented study, the control mice received a diet with \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{
\begin{tabular}{} \end{tabular} }} & \multirow{2}{*}{**Group 1**} & \multirow{2}{*}{**Group 2**} & \multicolumn{2}{c}{**Group 5**} & \multicolumn{2}{c}{**Group 6**} \\ \cline{4-7}  & & & **A** & & **B** & **A** & **B** \\ \hline frequency of breathing & 266.72 \(\pm\) & 291.40 \(\pm\) & 296.47 \(\pm\) & 283.97 \(\pm\) & 289.53 \(\pm\) & 266.85 \(\pm\) \\
[breaths/min] & 22.97 & 10.79 & 22.63 & 21.88 & 15.28 & 15.07 \\ \hline tidal volume & 0.310 \(\pm\) & 0.330 \(\pm\) & 0.366 \(\pm\) & 0.350 \(\pm\) & 0.364 \(\pm\) & 0.316 \(\pm\) \\
[mL] & 0.061 & 0.027 & 0.039 & 0.032 & 0.024 & 0.041 \\ \hline minute volume & 82.94 \(\pm\) & 95.14 \(\pm\) & 109.27 \(\pm\) & 100.75 \(\pm\) & 106.19 \(\pm\) & 85.02 \(\pm\) \\
[mL/min] & 21.27 & 7.44 & 13.87 (*) & 16.50 & 10.77 & 15.43 \\ \hline time of inspiratory & 0.104 \(\pm\) & 0.090 \(\pm\) & 0.091 \(\pm\) & 0.094 \(\pm\) & 0.092 \(\pm\) & 0.101 \(\pm\) \\
[s] & 0.013 & 0.004 & 0.010 & 0.007 & 0.007 & 0.012 \\ \hline time of expiratory & 0.138 \(\pm\) & 0.129 \(\pm\) & 0.123 \(\pm\) & 0.136 \(\pm\) & 0.140 \(\pm\) & 0.138 \(\pm\) \\
[s] & 0.015 & 0.006 & 0.011 & 0.012 & 0.013 & 0.008 \\ \hline peak inspiratory flow & 4.726 \(\pm\) & 5.626 \(\pm\) & 6.066 \(\pm\) & 5.949 \(\pm\) & 6.377 \(\pm\) & 4.943 \(\pm\) \\
[mL/s] & 1.071 & 0.508 & 0.987 & 0.855 & 0.494 (*) & 0.942 \\ \hline peak expiratory flow & 3.900 \(\pm\) & 4.380 \(\pm\) & 4.957 \(\pm\) & 4.829 \(\pm\) & 5.307 \(\pm\) & 4.221 \(\pm\) \\
[mL/s] & 0.956 & 0.246 & 0.540 & 0.660 & 0.401 (**) & 0.625 \\ \hline mid-tidal expiratory flow & 3.536 \(\pm\) & 3.990 \(\pm\) & 4.375 \(\pm\) & 4.251 \(\pm\) & 4.621 \(\pm\) & 3.656 \(\pm\) \\
[mL/s] & 0.880 & 0.245 & 0.496 & 0.570 & 0.388 (**) & 0.666 \\ \hline inspiratory flow rate & 3.059 \(\pm\) & 3.660 \(\pm\) & 4.041 \(\pm\) & 3.778 \(\pm\) & 3.999 \(\pm\) & 3.176 \(\pm\) \\
[mL/s] & 0.786 & 0.309 & 0.552 & 0.650 & 0.470 & 0.645 \\ \hline expiratory flow rate & 2.302 \(\pm\) & 2.564 \(\pm\) & 2.989 \(\pm\) & 2.615 \(\pm\) & 2.632 \(\pm\) & 2.302 \(\pm\) \\
[mL/s] & 0.685 & 0.231 & 0.457 & 0.484 & 0.389 & 0.331 \\ \hline inspiratory duty cycle [\%] & \begin{tabular}{c} 42.91 \(\pm\) \\ 3.59 \\ \end{tabular} & \begin{tabular}{c} 41.19 \(\pm\) \\ 1.12 \\ \end{tabular} & \begin{tabular}{c} 42.53 \(\pm\) \\ 3.23 \\ \end{tabular} & \begin{tabular}{c} 40.89 \(\pm\) \\ 1.35 \\ \end{tabular} & \begin{tabular}{c} 39.63 \(\pm\) \\ 1.42 \\ \end{tabular} & \begin{tabular}{c} 42.33 \(\pm\) \\ 3.60 \\ \end{tabular} \\ \hline expiratory duty cycle [\%] & \begin{tabular}{c} 57.09 \(\pm\) \\ 3.59 \\ \end{tabular} & \begin{tabular}{c} 58.81 \(\pm\) \\ 1.12 \\ \end{tabular} & \begin{tabular}{c} 57.47 \(\pm\) \\ 3.23 \\ \end{tabular} & \begin{tabular}{c} 59.11 \(\pm\) \\ 1.35 \\ \end{tabular} & \
\begin{tabular}{c} 59.11 \(\pm\) \\ 1.42 \\ \end{tabular} &
\begin{tabular}{c} 60.37 \(\pm\) \\ 3.60 \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}