Complex sentence: However, in most mammals, the lungs cannot be emptied completely and in an adult human there is always still at least 1 litre of residual air left in the lungs after maximum exhalation.
Split into simpler sentences: However, in a normal mammal, the lungs cannot be emptied completely. In an adult human there is always still at least 1 liter of residual air left in the lungs after maximum exhalation.