4.
Just outside the alveoli lies the blood capillary. Inside the capillary, blood flows from one end to the other and, with it, the erythrocytes (= red blood cells).

5.
You may know/remember that it is the red blood cells that transport oxygen through our body!

1.
Surfactant is a compound that decreases the surface tension of water in the alveoli. Why is that important??

2.
Well, did you ever notice, in the rain, or in the kitchen or the bathroom, how water has the tendency to shape itself in drops?

3.
This is because water molecules attract each other, due to their electrical polarity (H₂⁺⁺O^--).

4.
This also occurs in the thin film of water fluid lining the inner membrane of the alveolus. But that is actually a problem!

5.
This surface tension would actually make the alveoli smaller! That is not good.

6.
Therefore, it is necessary to decrease the surface tension of the water and this is the purpose of surfactant (=Surface Active Agent).

7.
You can see the importance of surfactant very well in premature babies. Sometimes, especially if they are born prematurely, there is not enough surfactant in the alveoli, their alveoli therefore collapse, and they have great difficulty in breathing! This is called Respiratory Distress Syndrome (=RDS)!

1.
To explain how an alveolus works, I have reduced the figure in the previous section into its basics as shown in this diagram.

2.
In this diagram, I have sketched two cell layers:
   a. the alveolar epithelium
   b. the endothelium of the blood vessel.
I also indicated the air (in the alveolus) and the erythrocytes (in the capillary).

3.
Between these two layers, there is a very thin interstitium. This interstitial layer, together with the two cell layers, forms the respiratory membrane. This is the boundary where all our respiratory gasses are exchanged!

4.
The transport of the blood gasses is performed by simple diffusion. If the concentration of oxygen in the blood is lower than in the alveolus (which is usually the case), then oxygen will diffuse from the alveolar air to the blood.

5.
The same thing is also true for the major exhaust gas of our body; carbon dioxide. We make plenty of carbon dioxide in the metabolism of our body and therefore its concentration in blood is high.

6.
Note that both the O₂ and the CO₂
   a. the alveolar epithelium
   b. the interstitium between the alveolar wall and the blood vessel wall.
   c. The endothelium of the capillary

7.
As you may remember from a previous webpage (link), diffusion is indirectly proportional to the distance of diffusion; the shorter the distance, the faster the diffusion. Fortunately, in the healthy lungs, the respiratory membrane is extremely thin; about 0.5 to 1.0 micron!

8.
Note that both O₂ and CO₂ have to pass several membrane layers before reaching their goal. In fact they have to pass five plasma membranes:
   1. two from the alveolar epithelium
   2. two from the blood vessel endothelium
   3. one through the erythrocyte.

1.

Very fast!

2.
From the previous section, it would seem that it takes a long time for the gas concentration to equalize across both sides of the respiratory membrane.

5.
One reason for this fast equilibration is the thinness of the respiratory membrane (0.5 – 1.0 micron) and the shorter the distance, the faster the diffusion.

6.
But another even more important reason is the total surface area of the membrane. (link)

7.
If you put all the millions of alveoli together from both lungs, you get an enormous surface area of about 50-100 m² (depending on body size and gender).

8.
Imagine! Your respiratory membrane is probably bigger than the size of your sitting room; about 5-10 x 5 meters (15-30 x 15 feet)!

9.
And the larger the diffusion area, the faster is the diffusion. This is how we breathe fresh air. Through a membrane the size of your room!

1.
How much of which gasses is there actually in blood and how much is exchanged?

2.
Well, that depends of course on how much is used in the body.

3.
As you know, oxygen-poor blood comes from the body to the right heart and is pumped in the pulmonary artery by the right ventricle. This blood has, on average, a pO₂ of 40 mmHg and a pCO₂ of 45 mmHg.

4.
The CO₂ also has to diffuse into the alveoli, just like oxygen has to diffuse into the blood.

7.
In the pulmonary veins, after passage through the lungs, the pO₂ in the blood has increased to 104 mmHg and the pCO₂ decreased to 40 mmHg (see table).

8.
This is exactly the same as the pO₂ and the pCO₂ in the alveolar air! (but they had ample time to equilibrate of course!).

1.
The table above is a continuation of the table we discussed in a previous page (“Partial Pressures“).

2.
There, we had discussed the effect of water (“humidification”) on the other gas pressures in the inspired air.

3.
Now, in the alveolar air, we can see that the oxygen pressure has further decreased as oxygen has now left the alveoli and diffused into the capillaries (from 149 to 104 mmHg).

4.
At the same time, the pCO₂ has increased (from 0.3 to 40 mmHg) as CO₂ diffused from the blood into the alveoli.

5.
Since the pressures in the alveoli have had more than enough time to equilibrate with that in the blood (see the two diagrams above), the partial pressures in the venous blood will be exactly the same as that in the alveoli.

6.
It is also interesting to see the changes in the pressures from the arterial blood to the venous blood.

7.
One final point. Look at the partial pressures in the table column “Expired air”. See anything strange?

8.
It would seem that the expired air has more oxygen than the alveolar air (and less CO₂). Is that really correct?

9.
Yes, it is. This is because the expired air is a mixture of air coming from the alveoli and that of air from the dead space (which is equal to the “humidified air”).

10.
The air in the dead space has not been exchanged with blood and therefore its oxygen concentration is still high (about 149 mmHg). When this is mixed with the air coming from the alveoli (which is approximately 104 mmHg), the mixture will be somewhere between these two values.

11.
In fact, if you understand the above material, then you can now think, when you exhale, which part of the air you exhale will have a high oxygen concentration and which part will have a low oxygen concentration!! (Hint: you exhale air from the dead space first while the alveolar air will be exhaled the last).