Pulmonary blood flow, ventilation and perfusion

Question 1

What is hypoxic vasoconstriction?

Answer 1

Regional vasoconstriction in response to hypoxia. Myogenic in mechanism. Its purpose is to direct blood to areas of the lung with better oxygenation.

Question 2

Extra-alveolar/bronchial capillaries receive blood from where?

Answer 2

The left ventricle, high O2, low CO2; deliver oxygen and CO2 to tissue of lungs.

Return to the heart via the azygous v or the pulmonary veins (leads to venous admixture)

Question 3

What is the equation for Pulmonary Vascular Resistance? How does it compare to the systemic side?

Answer 3

PBP = CO x PVR

usually: PBP = 25/15 mm Hg
Systemic BP = 120/80

so, PVR is much lower than in the body

Question 4

What determines PVR?

Answer 4

o High number of capillaries
o How many are open at any given moment in time (it is
perfectly normal not to be using all your lungs at rest)
o Much less sympathetic tone/vasoconstricting forces

*can depend on lung volume as well

Question 5

How does regional blood flow work in the lungs?

Answer 5

•The regional blood flow is determined by how gravity is acting on the blood and the how much air is in that region of the lung.

•Short and sweet:
o Apex: high alveolar pressure, low blood pressure:
Little blood flow
o Middle regions: Normal pressures on both sides:
Normal blood flow
o Base: Relatively low alveolar pressure, relatively high
blood pressure: Lots of blood.

Question 6

How does NO modulate pulmonary blood flow?

Answer 6

o Made by the endothelium
o Causes smooth muscle relaxation and vasodilation
o Significant role in normal pulmonary blood flow

Question 7

How do endothelin 1 & thromboxane A2 modulate pulmonary blood flow?

Answer 7

o Also made in lungs
o Vasoconstrictor
o Seems to be a player under pathologic conditions – not under normal conditions

Question 8

How are alveoli kept “dry”?

Answer 8

• The interstitial hydrostatic pressure is believed to be negative and favors filtration of fluid from the capillary to the alveolus.
• The net Starling forces are believed to favor filtration along the length of the pulmonary capillary
• The lymphatics are crucial in removing the filtered fluid from the alveolus.

Question 9

What are the metabolic functions of the lung?

Answer 9

o The lungs have angiotensin converting enzyme (ACE),
the enzyme that converts angiotensin 1 to angiotensin
2.
o ACE breaks down bradykinin in the lungs.
o Other enzymes in the lungs break down the
leukotrienes and PGF2αand PGE

Question 10

Define the alveolar gas equation

Answer 10

PAO2 = PIO2 - (PaCO2/R)

PAO2 = ((760 - 47) x 0.21) - (PaCO2/R)

Question 11

What is a normal A-a gradient?

Answer 11

<20 mm Hg

Question 12

What does an increase in the A-a gradient indicate?

Answer 12

diffusion impairment

Question 13

Describe the distribution of air flow within the upright lung.

Answer 13

o Intrapleural pressure is acted on by gravity too.
o Apex: little fluid, pressure is more negative than other regions
o Base: more fluid, pressure is less negative than other regions.
o Because of this, the alveoli in the apex are larger than the alveoli in the base.

Question 14

What are the V/Q ratios from highest to lowest in the lungs?

Answer 14

1. Zone 1 - apex
2. Zone 2 - middle
3. Zone 3 - base