Gas exchange(air sac)

1.The pleural fluid is secreted by the outer pleural membrane and inner pleural membrane.The fluid acts as a lubricant (潤滑劑) to reduce friction between the lungs and the ribs during breathing.
2.The water film acts as a medium for oxygen and carbon dioxide to dissolve.
It is secreted by the epithelium of the air sacs.
For oxygen,
First,oxygen is inhaled.
Next,oxygen is dissolved in the water film
Finally,oxygen diffused into the red blood cells and being transported to other parts of our body.

For carbon dioxide,
First carbon dioxide is diffused out of the capillaries.
Next,carbon dioxide is dissolved in the water film.
Finally,carbon dioxide diffused into air sac.

Why oxygen and carbon dioxide have to dissolve in the water film first?
Because the diffusion rate in liquid is higher than that in gas,as the number of particles per unit of area is larger.
Thus,the presence of water film increases the diffusion rate between the capillaries and air sacs.

2012-02-29 01:32:53 補充：
仲有你錯區,你條係bio黎

2012-03-10 00:49:43 補充：
唔一定嫁.
如果個area係一樣既話
gas diffuse得快過liquid
因為particles in gas state 郁快過particles in liquid state
但係個air sac既area 大過water film咁多.
如果無左個water film,個diffusion rate一定會慢d.
water film就係可以令d oxygen同carbon dioxide dissolve 係d水到,
咁d dissolved gas集中係一個咁細既area,diffusion rate就會快D.