The blood–air barrier (alveolar–capillary barrier or membrane) exists in the gas exchanging region of the lungs. It exists to prevent air bubbles from forming in the blood, and from blood entering the alveoli. ... The barrier is permeable to molecular oxygen, carbon dioxide, carbon monoxide and many other gases
its also in your lungs
There is not necessarily a sad event associated with it
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The word "distinguishing" as used in the given excerpt above is used as an adjective that means noticing or distinctive. It is the word used in order to identify the difference of the characteristics between the objects being compared in the sentence.
The DRI (Dietary Reference Intake) is 0.8 grams of protein per kilogram of body weight, or 0.36 grams per pound. This amounts to: 56 grams per day for the average sedentary man