Pulmonary Diffusion and Pulmonary Circulation

Question 1

how does steady state alveolar air for CO2 and O2 compare to steady state venous blood and arterial blood pressure?

Answer 1

PAO2 = 100 mmHg
PaO2 = 95 mmHg
PvO2 = 40 mmHg

PACO2 = 40 mmHg
PaCO2 = 40 mmHg
Pv = 46 mmHg

Question 2

Henry’s Law of Solubility

-what does alpha mean?

Answer 2

Ci = alphai * Pi where C = gas concentration in solution, alpha = solubility coefficient, and P = partial pressure of i

• this is only when liquid and gas phase are in equilibrium at a given temperature
• liquid pressure only from dissolved gases

Question 3

Rick’s Law of Diffusion

-what do D and Dl mean?

Answer 3

gas flow = (area/thickness) * D * (P1 - P2) where D = diffusion coefficient

• single most important is P gradient
• since Dl = (DA)/T, flow = Dl * (P1 - P2) where Dl = diffusion capacity of the lung

Question 4

how does D relate to solubility and molecular weight?

Answer 4

diffusion coefficient = solubility / (square root MW)

Question 5

is there hypoxemia and hypercapnea in hypoventilation and diffusion problems?

Answer 5

there is hypoxemia in both (O2 = 78 mmHg), but hypercapnea only in hypoventilation (CO2 = 55); diffusion problems have CO2 = 35 b/c CO2 is 20X more diffusable than O2

Question 6

what is the regular transit time through a pulmonary capillary?

Answer 6

0.75 - 1.2 seconds, since t = V/Q = 75 mL / 100 mL/sec

Question 7

how is the diffusion of O2, N2O, and CO along length of pulmonary capillary?

Answer 7

N2O has higher perfusion than O2, but both are quickly equilibrized within 0.1 sec
-transfer of N2O and O2 is perfusion-limited (more gas flow = more transfer)
CO has very low partial pressure and perfusion, b/c transfer is diffusion-limited
-doesn’t reach equilibrium even after 0.75 seconds b/c affinity for Hb is 210 X that of O2

Question 8

what happens if diffusion of O2 is abnormal?

Answer 8

O2 will not reach equilibrium until after the full 0.75 seconds of flow through capillaries

Question 9

CO2 diffusion along pulmonary capillary

Answer 9

it seems like the inverse of O2 (b/c diffusing out of body)

• it equilibriates quickly through fibrotic tissue and fluids present in pulmonary edema
• slowed down by CO2 chemical reactions

Question 10

what happens to PO2 during exercise-induced hypoxemia?

Answer 10

transit time is reduced to 0.25 seconds, since flow can increase 3x
-a normal person would still equilibrate capillary blood with alveolar gas, but if there is a diffusion problem, they would get hypoxemia during exercise (longer to equilibrate)

Question 11

what happens to PO2 and DlCO at high altitudes?

Answer 11

inspiratory hypoxia and vasoconstriction reduces DlCO and slows the rate of equilibration

Question 12

what is relation of pressure gradient (between alveolar and capillaries) and equilibration time?

Answer 12

the smaller the difference, the shorter time to equilibrate

Question 13

what happens if you increase the BP in the pulmonary artery?

Answer 13

there is edema, which increases the thickness of the lung, which decreases flow and rate of diffusion, taking longer to equilibrate

Question 14

equations to measure diffusion capacity

Answer 14

flow of gas in blood (comparable to diffusion capacity) = inspiratory flow - expiratory flow
-usually assume inspiratory flow = expiratory flow

Question 15

how does DlO2 compare to DlCO?

Answer 15

DlO2 = 1.23 DlCO

Question 16

how does body position influence DlCO?

Answer 16

DlCO is greater when supine than upright, b/c when you lie down, there is more blood to the lung

Question 17

how does exercise influence DlCO?

Answer 17

increased blood to lung will increase blood flow and DlCO

Question 18

how do lung diseases and dysfunction influence DlCO?

Answer 18

they decrease it

• loss of lung tissue from surgery will decrease area
• mismatch of ventilation to perfusion (such as airway obstruction, shut, alveolar dead space that increase thickness)
• pulmonary HTN with edema

Question 19

differences between pulmonary and systemic circulations

Answer 19

• pulmonary is only vascular bed to receive entire CO, w/ low pressure
• lung volume changes during breathing affects pulmonary vascular resistance
• ischemic damage is rare in lung due to multiple supplies of O2
• minimal basal tone in pulmonary vessels (passive distension w/ increased P or Q w/o significant autoregulation)
• hypoxic vasoconstriction in lung

Question 20

what are 3 O2 supplies to lung?

Answer 20

• bronchial circulation
• pulmonary circulation
• alveolar gas O2 supply

Question 21

pulmonary circulation and pressures

Answer 21

pulmonary circulation is smaller than systemic circulation (thus “lesser” circulation)

• pulmonary BP are lower b/c pulmonary vascular resistance is 10x lower than TPR
• afterload of RV is less than LV, so right heart does less work than left

Question 22

where is pulse pressure largest?

Answer 22

in the left ventricle

Question 23

anatomic shunts

Answer 23

normal, left to left that’s 1-2% of CO (up to 20% if obstruction)

• bronchial circulation starts at the base of the aorta, perfuses large airways, vessels, and nerves, then drains into bronchial vein and then pulmonary vein, thus bypassing the lungs
• responsible for the slight drop from 100 mmHg in alveoli to 95 mmHg in aorta

Question 24

what do lymphatic vessels return excess fluid to? what are they regulated by?

Answer 24

returns it to circulation via caudal mediastinal lymph node and thoracic duct

• regulated by intrinsic propulsion, mechanical pumping during breathing, and sympathetic activity
• -intrinsic propulsion can generate up to 20 mmHg if flow occluded

Question 25

physiological shunt

Answer 25

sum of normal anatomic shunt (left to left) plus any pathological intrapulmonary right to left shut that occurs when airways are blocked, resulting in hypoxemia

Question 26

pathologic shunt

Answer 26

right to left shunting of blood - causes hypoxemia w/o hypercapnea - if Qs = Qt, that means all blood is shunted, and there's no O2ation

Question 27

features of pulmonary vessels - wall thickness - what kind of vessels they are - what capillaries are surrounded by

Answer 27

- pulmonary arteries and veins are both thin-walled and highly distensible - graduation of muscularity from muscular to partially muscular to nonmuscular with no distinct arterioles, and can be gas-exchanging (less than 1 mm) or non-exchanging (all larger) - capillaries are surrounded by alveolar air, so external pressure is alveolar pressure that oscillates during breathing - alveolar and extra-alveolar vessels have different mechanical properties, and are affected differently by changes in lung volume, but don't differ anatomically - changes in lung volume during breathing also affect pulmonary vascular resistance

Question 28

pulmonary blood pressure along vasculature | -what is average PBP, and what values make it pulmonary HTN or pulmonary edema

Answer 28

PBP is low, and dissipates gradually along vasculature - PBP is 11-12 mmHg, compared to 93 mmHg of aorta - over 20 mmHg is pulmonary HTN - over 25 mmHg is pulmonary edema, resulting in diffusion problem b/c hydrostatic pressure is higher, forcing filtration

Question 29

how is pulmonary arterial pressure measured?

Answer 29

a cardiac catheter called Swan Ganz is inserted in jugular, brachial, or femoral vein and advanced into pulmonary artery

Question 30

what is pressure (LA)?

Answer 30

pulmonary wedge pressure obtained when catheter in pulmonary vasculature is inflated to occlude flow downstream

Question 31

how does CO relate to pulmonary vascular resistance?

Answer 31

CO = (P1 - P2) / PVR = (P PA - P LA) / PVR so PVR = (P PA - P LA) / CO

Question 32

are changes in PVR passive or active?

Answer 32

changes in PVR are mostly passive -pulmonary blood volume is about 200-300 mL, and can increase 2-3x during exercise, but compliance of vessels is high so pressure doesn't increase much

Question 33

what is the relationship between radius, resistance and pressure if vasodilate?

Answer 33

increased radius will decrease resistance and increase pressure -this means that vessels don't contract under constant pressure

Question 34

how do alveolar and extra-alveolar blood vessels respond to decreased intrapleural pressure during inspiration?

Answer 34

- alveolar blood vessels are stretched and become narrower as capillaries increase in length, which increases resistance, due to changes in capillary pressure relative to alveolar pressure - extra-alveolar blood vessels expand due to the more negative intrapleural pressure

Question 35

when can alveolar vessels collapse completely?

Answer 35

if pressure outside (alveolar pressure) exceeds capillary pressure - flow will stop - larger vessels never collapse b/c protected by surrounding parenchyma

Question 36

what is lung volume's relationship to PVR?

Answer 36

the lung has an optimum volume that minimizes PVR - alveolar and extra-alveolar vessels are in series, so resistances are additive - thus, PVR is at a minimum at FRC (equilibrium position of system) - long volume changes during breathing, which affects PVR (PVR is highest at RV and TLC extremes)

Question 37

what are "corner vessels"?

Answer 37

small vessels in alveolar septum that are capillaries at junctional "corners" of alveoli -when the lung inflates, they expand to lower resistance as extra-alveolar vessels that never close

Question 38

what does gravity have to do with blood flow?

Answer 38

blood flow is 6X greater at the base of lung than at the apex, due to gravity and passive distension - pressure outside capillaries is constant throughout lung, but inside it increases from top to bottom b/c hydrostatic pressure is exerted by intrapleural pressure - thus, at base there is high transmural pressure and capillaries are wide open, more distended, lower resistance, and increased blood flow

Question 39

what is the three zone model for pulmonary blood flow

Answer 39

capillaries become more distended as you go down, increasing flow 1. usually not present in healthy lung (capillaries collapse and flow stops, so ventilated but not perfused, b/c top lung pressure falls below alveolar pressure); alveolar > arterial > venous pressure 2. flow depends on alveolar pressure (hydrostatic pressure increases and intravascular pressure exceeds alveolar pressure causing recruitment and opening of capillaries w/ increased blood flow); arterial > alveolar > venous pressure 3. flow increases due to gravity and passive distension (capillaries become wide open, and flow determined by difference in P pa - P pv; driving pressure constant, but resistance decreases and flow increases); arterial > venous > alveolar

Question 40

what clinical problems can cause increased capillary permeability?

Answer 40

- adult respiratory distress syndrome (ARDS) - O2 toxicity (via ROS) - inhaled or circulating toxins

Question 41

what clinical problems can cause increased capillary hydrostatic pressure?

Answer 41

- increased left atrial pressure from LV infarction or mitral stenosis (will lead to pulmonary edema) - overadministration of intravenous fluids

Question 42

what clinical problems can cause decreased interstitial hydrostatic pressure

Answer 42

too rapid evacuation of pneumothorax or hemothorax

Question 43

what clinical problems can cause decreased colloid osmotic pressure

Answer 43

- protein starvation - dilution of blood proteins by intravenous solutions - renal problems causing urinary protein loss

Question 44

what clinical problems can cause insufficient pulmonary lymphatic drainage?

Answer 44

- tumors | - interstitial fibrosing diseases

Question 45

how is the O2 and CO2 concentration in the right to left pathologic shunt? can these problems be solved with O2-enriched gas?

Answer 45

a small change in O2 content will decrease PO2 very much (steep slope), but not so much in CO2 (flatter slope) -no, unlike with diffusion problems, breathing gas enriched with O2 will not help patients with these abnormal shunts