Sentence:  The <WSD>natural</WSD> anatomical and physiological defensive features of the upper respiratory tract , such as the turbinates of the nose and the cilia of the trachea and larger bronchi , are capable of impinging out the larger particles to which we are ordinarily exposed in our daily existence . 
Meaning of the word marked with <WSD> tags:  existing in or produced by nature; not artificial or imitation