Phys: Test 3: Lecture 5. Pulmonary Circulation

Front 1

1. List and describe the components and organization of the pulmonary circulation

Back 1

a. Pulmonary artery
i. Branch with airways
1. Mesh of capillaries at respiratory bronchioles
a. Pulmonary venules
i. Pulmonary veins

Front 2

Describe the hemodynamic characteristics of the pulmonary circulation, including
typical mean internal pressures, blood flow and vascular resistance.

Back 2

• Pulmonary circulation is high flow, the pressure is very low (in contrast to the system pressure; 15 vs. 100)
• This means the right ventricle doesn’t have to work as hard
• Additionally, there is less smooth muscle in pulmonary circulation
o You don’t need to direct the blood to different organs, just need to have a thin sheet of blood across alveoli, so you don’t need to redirect the blood

Front 3

. Explain why pulmonary capillaries are considered ‘alveolar vessels’. Explain why
these vessels can collapse or distend depending on the values of internal capillary
pressure and alveolar pressure.

Back 3

• Capillaries are sensitive to alveolar pressure
o If it is great enough, the capillaries can be squeezed shut
o If the internal capillary pressure is greater than the alveolar pressure, the capillary will be distended
o Thus the capillary is sensitive to the amount of alveolar gas
 This is why they are referred to as alveolar vessels

Front 4

List the pulmonary vessels that are considered to be ‘extra-alveolar’ vessels.

Back 4

• The veins are not subject to alveolar pressure
o Making them extra-alveolar
 These are the pulmonary arteries and veins
• Instead, they are subject to radial traction (as the lung expands, it pulls open the veins to increase their caliber)

Front 5

Describe the dependence of the caliber of the extra-alveolar vessels on lung volume.

Back 5

• Alveolar vessels (capillaries)
o Caliber determined by balance between internal and alveolar pressures
• Extra-alveolar vessels (pulmonary artery and vein)
o Caliber determined by lung volume

Front 6

How does resistance in pulmonic vasculature differ from systemic?

Back 6

• Pulmonary vascular resistance is 1/10th of the system vascular resistance

Front 7

Describe how pulmonary vascular resistance changes with increases in pulmonary
arterial or venous pressure and cardiac output.

Back 7

• As you increase pulmonary arterial/venous pressure you decrease pulmonary vascular resistance
o This is due to increased cardiac output
o THUS: Increased cardiac output or pulmonary arterial/venous pressure will DECREASE pulmonary vascular resistance

Front 8

• What are the two things that underlie the reduced pulmonary vascular resistance?

Back 8

o Recruitment (open more capillaries)
o Distension (distend the already open capillaries)

Front 9

Describe and explain the variation in pulmonary vascular resistance with changes in
lung volume.

Back 9

o At low lung volume
 The extra alveolar vessels aren’t well expanded (the lung isn’t so no traction)
 Thus they are the main site of vascular resistance
o At high lung volume
 The extra alveolar vessels are open due to radial traction (thus their resistance is NOT limiting)
 But now the alveoli are expanded, which is compressing (it’s actually a stretching, but w/e it acts to pinch them) the capillaries
• Now this is the site of resistance

Front 10

Describe and explain the variation in blood flow from lung apex to lung base in an
upright lung at rest.

Back 10

• Blood flow at the base of the lung is much greater than at the apex

Front 11

discuss the blood flow, alveolar pressure, venous pressure and arteriole pressure in ZONE 1

Back 11

• At the top of the lung, ZONE 1
o The blood flow is very low
o The alveoli pressure is greater than arteriole pressure, this collapses the capillaries

PA>Pa>Pv
VESSELS COLLAPSED

Front 12

discuss the blood flow, alveolar pressure, venous pressure and arteriole pressure in ZONE 2

Back 12

• In the middle of the lung, ZONE 2
o Some of the capillaries are open, arteriole pressure is greater than the alveoli
o So the blood flow is determined by the pressure difference between the alveolar pressure and the capillaries
o Venous pressure plays NO ROLE HERE

Pa>PA>no role of venous pressure
SOME VESSELS OPEN

Front 13

discuss the blood flow, alveolar pressure, venous pressure and arteriole pressure in ZONE 3

Back 13

o Arteriole and venous pressure are greatest here
o You will see distension of the capillaries
o Venous pressure now (as opposed to alveolar) is what is determining the rate of flow

Pa>Pv>PA
a-v difference important

Front 14

Explain the
mechanism by which hypoxic vasoconstriction is thought to occur.

Back 14

• Pulmonary vasoconstriction occurs in hypoxia
• As alveolar PO2 is reduced, blood flow decreases due to vasoconstriction
o This is hypoxic vasoconstriction
• You have K+ channels that are sensitive to O2, when it drops, the channels close
o You get depolarization of vascular smooth muscle
 Leads to contraction, and thus vasoconstriction
• Not sensitive to arteriole, only alveolar

Front 15

Explain why
hypoxic vasoconstriction is useful

Back 15

o Normally this mechanism is used to direct blood away from under-ventilated areas (such as in obstruction)
o This limits the amount of pulmonary shunt there is
 It redirects the blood to better areas of the lung

Front 16

Explain why
hypoxic vasoconstriction is
problematic

Back 16

o High altitude
 Less O2 in atmosphere
 Alveolar O2 in lung is much lower
• Get more generalized hypoxic vasoconstriction
o Thus the blood has nowhere to go
 It backs up into the pulmonary artery
• Get pulmonary hypertension and edema
o HAPE: high altitude pulmonary edema
o COPD
 Same idea that there is an inequality in gas exchange generalized in the lung
• Mechanism basically works the same as above

Front 17

What is HAPE?

Back 17

high altitude pulmonary edema
-due to hypoxic vasoconstriction
-get back up of fluid (b/c whole lung is constricted)
-will back up into pulmonary artery and leak out

Front 18

Explain why keeping the alveoli dry is essential for efficient gas exchange

Back 18

• If you have a wet alveoli, you will decrease gas exchange (increase surface tension-->decrease compliance)

Front 19

List the pressures which determine the extent of fluid leakage from the pulmonary capillaries (that will play a role in increasing/decreasing surface tension)

Back 19

• Pressure pushing fluid out of capillaries
o Hydrostatic pressure and surface tension (trying to suck water out of capillary)
• Pressure pushing on the capillary to keep it out of the alveoli
o Alveolar pressure (trys to push fluid back into the capillaries)
o Colloid osmotic pressure (COP!)

Front 20

. Describe how fluid that leaks out of pulmonary capillaries is removed from the
interstitial space. Describe what happens to leaked fluid in the early and late stages
of pulmonary edema.

Back 20

• Lymphatics are in the interstitial space
o They drain the fluid away
• If the lymphatics are blocked
o Interstitium will engorge (early edema)
 Fluid can then cross into the alveoli and mess with gas exchange (late edema)