Lecture 22: Pulmonary Circulation

Question 1

Compare the 2 circulations of the lung as described in this lecture

Answer 1

High pressure, low flow: thoraci aorta-> bronchial arteries->vessels of trachea and bronchial tree…systemic blood flow (basically)

Low pressure, high flow: carried to the lung by pulmonary artery from r ventricle to the alveoli. 1/3 wall thickness compared to aorta = more compliant

Question 2

Know the pulmonary arterial pressure, the mean pulmonary arterial pressure and the left atrium pressure (diastole):

Answer 2

Pulmonary arterial pressure (systolic): 25 mm Hg

Mean pulmonary arterial pressure: 15 mm Hg

Left atrium pressure(diastole): 8 mm Hg

Question 3

List agents that constrict and dilate pulmonary arterioles.

Answer 3

Constrict pulmonary arterioles: norepinephrine,epinephrine, angiotensin II, some prostaglandins

Dilate pulmonary arterioles: isoproterenol, acetylcholine

Question 4

List agents that constrict pulmonary venules

Answer 4

serotonin, histamine, e.coli endotoxin

–when O2 [ ] in alveoli is 70% or more below normal constrict adjacent blood vessels

Question 5

What effect does heavy exercise have on blood flow through the lungs and why does this cause minimal rise in pulmonary arterial pressure?

Answer 5

Exercise: blood flow increases 4x-7x. Increases # of capillaires open to 3x. Distended all capillaries and increases flow rate up to 2x. Increases pulmonary arterial pressure.

Minimal rise in pulmonary arterial pressure: d/t increase in number of open capillaries and distension of all capillaires and increases flow rate up to 2x.
-conserves energy of R side of heart. Prevents rise in pulmonary capillary pressure.

Question 6

Describe the 3 blood flow zones, how theyre distributed throughout the lungs normally, and what effect exercise has on this distribution:

Answer 6

3 blood flow zones:
Zone1=no blood flow; alveolar capillary pressure never higher than alveolar air pressure.
Zone2=intermittent blood flow-only during systole
Zone3=continuous blood flow

Distribution: apices have zone 2 flow. Lower areas of lung have zone 3 flow.

Exercise effect: exercise can convert apices from zone 2 to zone 3 flow.

Question 7

Describe the hydrostatic and osmotic forces involved in lung capillary fluid exchange

Answer 7

Pulmonary capillary pressure: 7 mm Hg

Peripheral tissue capillary pressure: 17 mm Hg

Interstital fluid pressure in lung: slightly more - than that in peripheral subQ tissue. AND pulmonary capillaires are more leaky to protein molecules.
Colloid osmotic pressure in lung interstium is about 14 compared to less than 7 mm Hg in peripheral tissues.

Capillaires->pulmonary interstitium:
Hydrostatic pressure: +7
Interstial fluid osmotic pressure: -14
Interstitial fluid hydrostatic pressure: -8
Total outward force: 29

Interstium->capillary:
Capillary osmotic pressure: 28=total INWARD force
Mean filtration pressure:(29-28) 1 mm Hg

Question 8

What effect does left-sided heart failure have on left atrial pressure

Answer 8

Increases blood volume as much as 100%

Increases BP

Mild systemic. Effect b/c systemic blood volume is 9 times that of the pulmonary system.

• blood begins to dam up in L atrium.
• L atrial pressure rises fro 1-5 mm Hg to 40-50 mm Hg. Increases above 8 mm Hg cause = increases in pulmonary arterial pressure.
• above 30 mm Hg, pulmonary edema is likely to develop.

Question 9

What are the most common causes of pulmonary edema?

At what capillary pressure level does it occur?

Answer 9

L sided Heart failure and mitral valve Dz

If pressure becomes more positive (-7->-4 mm Hg)

Above 30 mm Hg L atrial pressure, edema is likely to develop

Question 10

What are the results of bronchial obstruction and hypoxia in r/t blood flow

Answer 10

constriction of vessels supplying the poorly ventilated alveoli:

• locally low alveolar PO2 effect on vessel
• drop in pH to accumulation of CO2
• decline in pH produces vasoconstriction in pulmonary vessels.
• decline in pH produces vasodilation in other tissues

Increase pressure in the pulmonary artery-possibly d.t release of prostaglandin.

Question 11

What are the factors that control O2 [ ] in the alveoli?

Answer 11

• solubility of gas in the fluid
• cross-sectional area of the fluid
• distance through which the gas must diffuse
• molecular weight of gas
• temperature of fluid (remains reasonably constant)

Question 12

Explain why alveolar ventilation cannot increase PO2 above 149 mm Hg under normal conditions

Answer 12

Oxygen [ ] in alveoli (and partial pressure)is controlled by: rate of absorption of O2 into the blood and rate of new entry of O2 into the lungs.

cant increase PO2 above 149 mm Hg b/c it cant rise above the atmospheric pressure.

Question 13

What are the factors that control carbon dioxide [ ] in the alveoli?

Answer 13

Rate of CO2 excretion: alveolar PCO2 increases in direct proportion to rate of excretion.

Alveolar ventilation:
alveolar PCO2 decreases in inverse proportion to alveolar ventilation.

Question 14

Be familiar w/ the structure of the resp. Membrane.

Answer 14

p. 521-522

Fig 40-7,8,9

Question 15

List factors that determine how rapidly a gas will pass through the resp. Membrane.

Answer 15

• Membrane thickness
• Membrane SA
• Diffusion coefficient of gas in the substance of the membrane
• Partial pressure difference of gas btw the 2 sides of the membrane

Question 16

Explain what the Va/Q ratio refers to

Answer 16

Ventilation-perfusion ratio =

alveolar ventilation/blood flow

Question 17

Compare and explain the differences of Va/Q ratios under normal conditions, and when there is vascular obstruction

Answer 17

Norm: 0.8 = (pulmonary:5L/min;ventilation=4L/min)
PO2=104 mm Hg
PCO2= 40 mm Hg

Vascular obstruction: creates a physiological shunt. Air comes in but no blood flow. O2 and CO2=0
PO2=149 mm Hg
PCO2=0 mm Hg

Question 18

Define shunted blood, physiological shunt, and physiological dead space

Answer 18

Shunted blood: Va/Q below normal= portion of capillary doesn’t become oxygenated.

physiological shunt: normal 98% O2 2% CO2.

physiological dead space: w/o adequate blood flow, there is more O2 available than can be transported… thus the O2 is wasted and not able to perform gas exchange.