Chapter 4: Blood Flow and Metabolism Flashcards
Where do the pulmonary ateries branch off into the capillary bed?
pulmonary arteries accompany airways to the terminal bronchioles and then branch off to supply pulmonary capillaries
What are the systolic, diastolic and mean pressures of the pulmonary artery?
- 25, 8, and 15 mm Hg, respectively
what makes up the pressure gradient for pulmonary capillary perfusion?
pulmonary arterial mean pressure (15 mm Hg) minus left atrial pressure (5 mm Hg)
Pressure gradient of only 10 mm Hg
Fill in the pressures in the blanks
What is the transmural presure of the pulmonary capillaries?
pressure difference between the inside and outside of the capillaries
i.e., outside effective presssure is the alveolar pressure –> if alveolar pressure rises and is above capillary pressure –> capillaries collapse
explain how the transmural pressure of pulmonary arteries and veins differs from the capillaries?
when lungs expand the large vessels are pulled open by radial traction - less pressure around the vessels during increased lung volume/alveolar pressure
capillaries - with increased lung volume - transmural pressure decreases –> collapse
Explain how three different sizes of pulmonary vessels undergo different pressur influence from their surrounding structures
alveolar vessels - capillaries which caliber is determined by the surrounding alveolar pressure
extra-alveolar vessels- arteries and veins running through the lung parenchyma, their caliber is determined by the the radial pull
very large vessels near the hilum - outside of the lung substance - intrapleural pressure exposure
What is the equation for vascular resistance?
Vascular resistance = (input pressure - output pressure) / blood flow
blood flow is the same as systemic circulation but pressure gradient is small (see above, 10 mm Hg), so vascular resistance must be very small
How does pulmonary vascular resistance respond to an increase in pulmonary arterial or venous pressure?
What are the two mechanisms responsible for this process?
pulmonary vascular resistance will decrease
mechanisms:
1. Recruitment: at normal condition some pulmonary capillaries are either closed or open with no blood flow - as pressure increases they conduct blood - increased surface area –> decreases resistance
2. Distension: increased pressure widens individual capillary segments - increases their caliber - possible due to thin capillary membranes
How does lung volume affect the caliber of extra-alveolar vessels?
increased lung volume –> expands/pulls open the vessels - low vascular resistance
collapsed lungs –> elastic tissue and smooth muscles pull vessels together –> increased vascular resistance
* if the lungs are completely collapsed - smooth muscle is very strong - pulmonary artery pressure must be raised significantly to allow blood flow - critical opening pressure
What is the critical opening pressure?
the air pressure needed to allow blood flow through the extra-alveolar vessels
Name 4 susbtances that can cause smooth muscle contraction and increase pulmonary vascular resistance
- serotonin
- histamine
- norepinephrine
- endothelin
Name 5 substances that can relax smooth muscles in the pulmonary circulation and decrease vascular resistance
- acetylcholine
- Ca-channel blockers
- Phosphodiesterase-5 inhibitors
- nitric oxide
- prostacyclin (PGI2)
Explain with this graph why pulmonary vascular resistance is the lowest at normal lung volumes
At low lung volumes, the extra-alveolar vessels collapse due to their elastic/smooth muscle wall
At high lung volumes the alveolar vessels will be compressed/collapse
How can you measure pulmonary blood flow?
- Fick’s principle
- indicator or thermo dilution techniques
What is the Fick’s principle?
states that the O2 consumption/min measured with mouth-piece (VO2) is equal to the O2 taken up by the lungs per minute (Q x (CaO2 = CvO2)
so, VO2 = Q x (CaO2 - CvO2)
so, Q = VO2 / (CaO2 - CvO2)
Q is blood passing through lungs per minute
In the upright lung, how does blood flow change?
blood flow is the highest in the most dependet/lowst region and decreases almost linearly from bottom to top
What force explains blood flow differences between lung levels?
gravity affects hydrostatic pressure - e.g., if the top of the lung is 30 cm above the bottom, there will be about 30 cm H2O or 23 mm Hg difference in hydrostatic pressure iwthin the blood vessels –> large difference for the low-pressure pulmonary circulation
Describe Zone 1 of lung blood flow
Within Zone 1, the pulmonary arterial pressure is so low, it falls below the alveolar pressure –> capillaries collapse and no blood flow occurs
* Zone 1 is not present under normal physiologic conditions
Describe Zone 2 of lung blood flow
In Zone 2, the pulmonary arterial pressure is above alveolar pressure, but the pulmonary venous pressure is lower than alveolar pressure»_space; venous pressure has no influence on blood flow, which is now depndent on the gradient between arterial and alveolar pressure
called:
* starling resistor effect
* sluice effect
* waterfall effect
Describe Zone 3 of lung blood flow
Venous pressure now exceeds alveolar pressure»_space; blood flow depends on arterial to venous pressure gradients
Why does zone 3 have the most blood flow?
due to the increase of vascular transmural pressure –> capillaries will be distended
this is because capillaries lie between arterial and venous vessels which now have a higher pressure (lying lower with higher hydrostatic pressure), but alveolar pressure remains constant –> capillaries will distent more
What determines hypoxic pulmonary vasoconstriction
PO2 of alveolar gas - not the pulmoanry arterial PO2
Describe the nonlinear stimulus-response curve of hypoxic pulmonary vascular constriction
with changes at higher PAO2 around 100 mm Hg, only little vasoconstriction occurs
PAO2 < 70 mm Hg, marked vasoconstriction occurs
severely low PAO2 may almost abolish blood flow
