Chp. 4: Blood Flow & Metabolism

Question 1

Composition of pulmonary artery and branches

Answer 1

Walls are remarkably thin, contain relatively little smooth muscle

Question 2

PA Pressure

Answer 2

Mean = 15

25/8

Question 3

Pulmonary capillary pressure

Answer 3

Arterial end: 12
Venous end: 8

Question 4

RV Pressure

Answer 4

25/0

Question 5

RA Pressure

Answer 5

2

Question 6

LV Pressure

Answer 6

120/0

Question 7

LA Pressure

Answer 7

5

Question 8

Aortic pressure

Answer 8

Mean = 100

120/80

Question 9

Systemic capillary pressure

Answer 9

Arterial end: 30
Mid: 20
Venous end: 10

Question 10

What is the pressure in the alveoli?

Answer 10

Usually close to atmospheric

Question 11

Transmural pressure

Answer 11

Difference in pressure between inside and outside of capillaries

Question 12

Why are pulmonary arteries and veins considerably less than alveolar?

Answer 12

As lung expands, these larger IVs are pulled open by radial traction of the elastic lung parenchyma

Both arteries and veins increase their caliber as lung expands

Question 13

What determines the caliber of alveolar vessels?

Answer 13

Relationship between alveolar pressure and pressure within them

Question 14

Extra-alveolar vessels

Answer 14

All arteries and veins running through lung parenchyma

Caliber greatly affected by lung volume because this determines the expanding pull, or radial traction, of the parenchyma on their walls

Question 15

PVR vs. SVR

Answer 15

PVR is 1/10th that of SVR

Question 16

What causes the high resistance of the systemic circulation?

Answer 16

Very muscular arterioles that allow the regulation of BF to various organs

Question 17

What decreases PVR?

Answer 17

Increase in either pulmonary arterial or venous pressure

Question 18

Why does increase in pulmonary arterial or venous pressure cause a decrease in PVR?

Answer 18

Recruitment is the chief mechanism as the PA pressure is raised from low levels.

Distension is the predominant mechanism at relatively high vascular pressures.

Question 19

Lung volume and PVR

Answer 19

Vascular resistance is low at large lung volumes due to radial traction

Caliber of capillaries decreased at large lung volumes due to stretching across wall

Question 20

Critical opening pressure

Answer 20

PA pressure has to be raised several centimeters of water above downstream pressure before flow occurs

Question 21

Substances that increase PVR

Answer 21

Those that cause smooth muscle contraction: serotonin, histamine, norepinephrine, endothelin

Question 22

Substances that decrease PVR

Answer 22

Those that relax smooth muscle in the pulmonary circulation: acetylcholine, calcium channel blockers, NO, phosphodiesterase-5 inhibitors, prostacyclin (PGI2)

Question 23

Blood flow in the lung

Answer 23

Decreases in an almost linear fashion from bottom to top, reaching very low values at apex

Question 24

What is the effect of exercise on regional differences in pulmonary blood flow?

Answer 24

Regional differences become less, as flow to both the upper and lower zones increase

Question 25

What explains the uneven distribution of blood flow to the lungs?

Answer 25

Hydrostatic pressure differences within BVs

Question 26

West Zone I

Answer 26

PA > Pa > Pv Does NOT occur under normal conditions because pulmonary arterial pressure is just sufficient to raise blood to top of lung, but may be present if the arterial pressure is reduced (eg. septic shock or hemorrhage) or if alveolar pressure is raised

Question 27

West Zone II

Answer 27

Pa > PA > Pv PA pressure increases because of the hydrostatic effect and now exceeds alveolar pressure. Blood flow is determined by difference between arterial and alveolar pressures "Waterfall effect"

Question 28

West Zone III

Answer 28

Pa > Pv > PA Venous pressure now exceeds alveolar pressure and flow is determined in the usual way: by arterial-venous pressure difference. Caused chiefly by distension of the capillaries

Question 29

West Zone IV

Answer 29

At low lung volumes, the resistance of the extra-alveolar vessels becomes important, and a reduction in regional blood flow is seen, starting first at the base of the lung, where the parenchyma is least expanded. Explained by narrowing of the extra-alveolar vessels, which occurs when the lung around them in poorly inflated

Question 30

Hypoxic pulmonary vasoconstriction

Answer 30

Local action of the hypoxia on the artery itself. The PO2 OF ALVEOLAR GAS, *NOT* the pulmonary arterial blood, determines the response When alveolar PO2 <70mmHg, vasoconstriction may occur. At very low PO2, local blood flow may be almost abolished.

Question 31

HPV MOA

Answer 31

Inhibition of voltage-gated potassium channels and membrane depolarization are involved, leading to increased cytoplasmic calcium ion concentrations

Question 32

Endothelium-derived vasoactive substances

Answer 32

NO is formed from L-arginine via catalysis by endothelial NO synthase. NO activates soluble guanylate cyclase and increased cGMP. cGMP inhibits calcium channels, preventing a rise in intracellular calcium. Vasoconstrictors: Endothelin-1, Thromboxane A2

Question 33

Other causes of pulmonary vascular tone changes

Answer 33

Low pH causes vasoconstriction, especially in the presence of alveolar hypoxia. Severe hypothermia attenuates this response.

Question 34

Colloid osmotic pressure within capillary

Answer 34

25-28mmHg

Question 35

Where is capillary hydrostatic pressure highest in the lung?

Answer 35

Bottom

Question 36

Pulmonary edema

Answer 36

Earliest form is characterized by engorgement of periobronchial and perivascular spaces --> INTERSTITIAL edema Fluid that reaches the alveolar spaces is actively pumped out by Na-K ATPase

Question 37

Additional functions of pulmonary blood

Answer 37

Reservoir for blood Filter blood

Question 38

Metabolic Functions of Lung

Answer 38

Converts angiotensin I to angiotensin II via ACE ACE inactivates up to 80% of bradykinin Inactivation of serotonin Inactivation of PGE1, PGE2, and PGF2alpha NE taken up by lung (30%) Plays a role in clotting mechanism of blood Synthesis of dipalmitoyl phosphatidylcholine (surfactant) Carbohydrate metabolism

Question 39

KEY CONCEPTS for blood flow and metabolism

Answer 39

1) Pressures within pulmonary circulation are much lower than systemic. Capillaries are exposed to alveolar pressure, whereas pressures around the extra-alveolar vessels are lower 2) PVR is low and falls even more when CO increases because of recruitment and distension. PVR increases at very low or high lung volumes. 3) BF is unevenly distributed in the upright lung. Higher flow at base than apex due to gravity. If capillary pressure is less than alveolar, capillaries collapse and there is no flow. Uneven BF at any given level due to random variations in blood vessels. 4) HPV reduces BF to poorly ventilated regions of lung. Release of this mechanism is largely responsible for a large increase in BF at birth. 5) Fluid movement across the capillary endothelium is governed by the Starling equilibrium. 6) Many metabolic functions, including angiotensin I to II by ACE.