Cardiac Output Question

[HH91]

From a BRS book, it's killing me because the answer isn't given:

If a patient's cardiac output is calculated at 7.0 L/min, what would be the blood flow to each lung?

1.) 7.0 L/min or
2.) 3.5 L/min

I believe that the answer is 7.0 L/min, correct?
My roommate is explaining that CO is calculated from either ventricle, and that the right vent pumps to a bifurcation to the pulmonary arteries (one to the left, one to the right) so that the answer should be 3.5L?

---

Members don't see this ad.

 

[thehundredthone]

I believe that the answer is 7.0 L/min, correct?
My roommate is explaining that CO is calculated from either ventricle, and that the right vent pumps to a bifurcation to the pulmonary arteries (one to the left, one to the right) so that the answer should be 3.5L?

The pulmonary arteries constitute a parallel circuit, so the flow will be divided between the two. Assuming equal resistance in both, the flow will be equally divided between the two pulmonary arteries. Your roommate is correct.

 

[Phloston]

CO from the LV = CO from the RV, as you've said.

Both lungs, together, must transfer the same total volume to the LV, so each lung must receive half of the RV's CO.

In terms of how this might be applied on the USMLE:

Removal of a lung would increase CO through the remaining lung, increasing perfusion and causing equilibration of gases further along the length of the pulmonary capillaries.

 

[Morsetlis]

Removal of a lung also decreases total lung compliance, doesn't it?

 

[Phloston]

Removal of a lung also decreases total lung compliance, doesn't it?

Interesting question.

Well the elastic properties wouldn't change for the remaining lung, so it would still expand to the same extent, but because compliance = change in volume per change in pressure, for the same change in pressure (i.e. for the same diaphragmatic movement), the vital capacity is only half as great, so compliance would have to decrease by half.

 

[thehundredthone]

Yes, since the thoracic compliance will not change, and neither will the intrapleural/intrathoracic pressure. Some compensatory emphysema also usually takes place, if I remember correctly, which could possibly offset the decrease in compliance slightly?