C.4.6. Ventilation and Perfusion


A. The Pulmonary Circulation:
1.
As you know, blood is being pumped into the lungs from the right ventricle, where it is oxygenated, and flows back to the left atrium. (link to CVS)
2.
So, the blood in the pulmonary artery is O2-poor and CO2-rich while the opposite is true for the venous blood going from the lungs to the left atrium.
Diagram of the Pulmonary Circulation
3.
It is very important to realize and appreciate that the blood pressure in the pulmonary circulation is much lower than in the systemic circulation: 25/8 mmHg (instead of the usual 120/80 mmHg in the systemic circulation).
4.
Why? In the first place, it is not necessary to have such a high blood pressure in the pulmonary circulation.


5.
The lungs are direct neighbors of the heart and it is therefore not so far away or so difficult to perfuse than the brain, the arms or the legs.
6.
But there is also another reason; the low blood pressure keeps the alveoli dry! How is that achieved?

7.
Well, do you remember the Starling exchange system in the capillaries?

B. Quick repeat of the Capillary Exchange System in the Systemic Circulation: top?
1.
This system is the interplay between the hydrostatic pressure (=blood pressure in the capillaries) and the oncotic pressure.
2.
In the systemic circulation, the hydrostatic pressure in the capillaries varies between 30 and 20 mmHg. This pressures ‘pushes’ water out of the capillaries.

diagram of the pressures involved in Systemic Capillaries
3.
The oncotic pressure is the pressure developed by all the blood components that cannot pass through the capillary membrane (especially albumen) and therefore develop an osmotic pressure that ‘sucks’ water back into the capillary. The oncotic pressure is about 25 mmHg.
4.
The difference between the two (5 mmHg) is the actual filtration pressure. (and, if you still remember, this became negative at the end of the capillary, so that the fluid flowed back into the capillaries).


5.
In the lungs, the same principles apply, but the values are slightly different.

6.
In the first place, the hydrostatic pressure is much lower; about 15-10 mmHg (compared to 30 mmHg in the systemic circulation).
7.
The oncotic pressure is actually also lower because the lung capillaries are more leaky to blood proteins than in other capillaries; it is about 15 mmHg.
8.
The net filtration pressure is therefore about -5 mmHg and is negative, meaning that no (or little) fluid will leave the capillary.

diagram of the pressures involved in Pulmonary Capillaries
9.
In fact, if there is some fluid in the alveolus, it will quickly be absorbed by the alveolar capillaries.
10.
So, this is how the alveoli are kept dry!



C. Ventilation-Perfusion coupling: top?
1.
As you know very well by now (!), the function of the lungs is to oxygenate the blood (and get rid of the CO2).
2.
Therefore, it works best if adequate amounts of air come in contact with adequate amounts of blood.
3.
This gas exchange is not as automatic as described above and there are several local regulation systems that help.


4.
The most important local regulation inside the lungs is to match the amount of blood that perfuses one part of the lung with the ventilation of that same part; this is called the ventilation-perfusion coupling.
5.
Suppose that for some reason there is much more blood then air going to one part of the lung. Then not all the gases will be equilibrated and too much CO2 and not enough O2 will flow back to the veins and to the systemic circulation.
6.
Or suppose that for some other reason a group of alveoli are not ventilated at all (maybe be due to an obstruction of a local bronchus). In that case, the blood perfusing this area will not be exchanged at all!

7.
To solve these local problems, there are two systems that work all the time:
   a. constriction or dilatation of the arterioles by O2
   b. construction or dilatation of the bronchioles by CO2
8.
For the arterioles, if the pO2 is low then the arterioles will constrict and if the pO2 is high then the arterioles will dilate (this is, by the way, the exact opposite reaction to what arterioles do in the systemic circulation!!).

9.
So, for example, if in a region in the lungs there is not a lot of oxygen, then the arterioles leading to this area will constrict and less blood will go to that area.

10.
If however there is a lot of oxygen, then a lot of blood will flow to this area. In this way, the lungs work more efficiently by shunting the blood from areas with poor ventilation to areas with rich ventilation.
11.
The bronchioles can also dilate or constrict (like the capillaries) but this is determined by the pCO2 in the alveolar air.

12.
If the pCO2 is high, then the bronchioles will dilate to allow more CO2 to escape in the expired air while those whose alveoli have little CO2 will not be ventilated as strongly.

D. Bronchial Circulation: top?
1.
In the above, we have discussed the pulmonary circulation. This is the circulation that conducts blood, at a low blood pressure, from the right ventricle, through the lungs, to the left atrium.
2.
This circulation has typically a low blood pressure (25/8 mmHg) and a high flow (same as the cardiac output of the left heart = 5 l/min at rest).

3.
But the lungs also have a (small) bronchial circulation. This is oxygen rich (and CO2 poor) blood that flows from the systemic circulation (so at a high blood pressure of 120/80 mmHg).
4.
This circulation is necessary to perfuse all the supporting tissues of the lungs; the trachea, the bronchial tree, and the pleurae.


5.
This oxygen rich blood comes from the aorta and drains (mostly) in the pulmonary veins.

6.
Ah! Interesting! This bronchial venous blood (now poor in oxygen) drains into and mixes with the oxygen rich blood from the lungs.
7.
This mixing is quite little (maybe 100 ml bronchial blood compared to 5 liters pulmonary blood) but still …

8.
In fact, this mixing with the bronchial blood decreases somewhat the pO2 from 104 (as blood goes out of the alveoli) to about 98 mmHg as it enters the left atrium.

E. The perfusion zones of the lung: top?
1.
There is another interesting problem that some teachers like to confuse their students with!


2.
It deals with the fact that the blood pressure, pumped from the right heart (25/8 mmHg), as we have seen, is much lower than in the systemic circulation (where it is 120/80 mmHg).
3.
This pulmonary blood pressure is actually quite low and has consequences for the perfusion of the top of the lungs.
4.
As you can see in the diagram, the heart is located close to the bottom (= the base) of the lungs (= 0 mmHg).
5.
From the right heart, the right ventricle pumps blood to the lungs at a pressure that varies between 8 mmHg (diastole) and 25 mmHg (systole).
6.
In fact, 25 mmHg is not very much and amounts to a height of about 33 cm.


7.
In a standing person, especially in a tall and thin person, this height might not be enough to reach the top of the lungs!!

8.
This means that the alveoli in the very top of the lungs may not get pulmonary blood (= are not perfused). This zone, above 25 mmHg, is called the zone of non-perfusion or Zone 1.
9.
But the blood pressure is not all the time 25 mmHg. It actually varies, with the heart beat, between 25 and 8 mmHg (8 mmHg = 11 cm height).

10.
So, there is a second zone in which the vessels are sometimes perfused, during systole, but not perfused during diastole. This is called intermittent perfusion and is labeled Zone 2.
11.
Below this second zone, below 11 cm (8 mmHg), at the base of the lungs, the blood pressure is always high enough to continuously perfuse the lungs; this is zone 3; the zone of continuous perfusion.

12.
Of course, this whole story only applies if a person is standing up. If the person is lying flat, as on a bed, then the heart is at practically the same height as all parts of the lungs and there will then be continuous perfusion; the whole lung is then in zone 3.
Tricky question: If the top of the lungs are not perfused in a standing person, does that mean that that tissues becomes ischemic and will die?

No. All the lung tissue, also in the top of the lungs, is perfused by the bronchial circulation, which has a high blood pressure (120/80 mmHg) and not by the pulmonary circulation. Only the alveoli in the top of the lungs will not get perfused and this has consequences for the gas ventilation, not for the survival of the tissue.

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C.4.6. Ventilation and Perfusion

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