Re: what is a path of a oxygen molecule,once inhaled then being exhaled ?

Sarah- Oxygen is inhaled through the mouth and/or nose (can you think of 3 
reasons that it is better to do it through the nose?) It then enters the 
naso pharynx, then the oro pharynx and then the laryngeo pharynx.  It then 
enters the larynx and trachea thru the glottis on its way to the 
respiratory tree.  This consists of primary, secondary ,tertiary 
bronchii, bronchioles of various sizes and finally alveoli which are where 
the O2 actually diffuses into the blood.  It has to diffuse across the 
alveolar membrane(single layer of squamous epithelium), a small & moist 
space, another single layer of the same kind of epithelium which makes up 
the wall of a capillary, diffuse into the plasma of the blood, diffuse thru 
the red blood cell's plasma membrane (a bilayer of phospholipid) and attach 
physically to the iron of the hemoglobin in the RBC.  It then travels 
through the body where it will leave the RBC and diffuse to a cell where O2 
is depleted.  CO2 leaves the metabolizing cell and enters the blood just as 
the O2 had done and "rides" around in the plasma or on the RBC (on a 
different site than the O2 did) and gets to the alveoli of the lungs and 
leaves the body taking the same pathway that the O2 took coming in.  I hope 
that this answers your question.  The answer about breathing thru the nose 
rather than the mouth is: moiturizes the air, cleans the air and warms the 
air.  Hope you got it correct!!  J. Bridger