Sentence:  Molten glare singed their eyelids an angry crimson ; suffocating air sapped their strength and strained their nerves to snapping ; dust choked their throats and lay like acid in their <WSD>lungs</WSD> . 
Meaning of the word marked with <WSD> tags:  either of two saclike respiratory organs in the chest of vertebrates; serves to remove carbon dioxide and provide oxygen to the blood