Pulmonary Circulation and Gas Diffusion, UAG School of Medicine Presentation

Pulmonary Circulation Overview

Pulmonary Circulation
Dra. Carla Romo
Anesthesiology / Critical Care
2025-1
UAG
SCHOOL OF MEDICINE
Right Lung
Superior
Vena
Cava
orta
Pulmonary
trunk
Left
APEX

• Contrast systemic and pulmonary circulations in terms of pressures, resistance to blood flow,
  and response to hypoxia.
• Recall Ohm's law and apply it to understand pulmonary circulation.
• Complement Fick's principle by calculating the content of oxygen.
• Describe the roles of distention and recruitment of pulmonary vessels in altering pulmonary
  blood flow and pulmonary vascular resistance.
• Define zones I, II, and III in the lung with respect to pulmonary vascular pressure and alveolar
  pressure.
• Explain how alveolar pressure, blood flow, and gravity interact to influence the function of each
  zone.
• Describe the consequence of hypoxic pulmonary vasoconstriction on the distribution of
  pulmonary blood flow.
• Explain the development of pulmonary edema through:
  
  • Increased hydrostatic pressure
  • Increased permeability
  • Impaired lymphatic outflow or increased central venous pressure.

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OBJECTIVES

Pulmonary and Systemic Pressures

Mean = 15
Mean = 100
25
120
8
80
Artery
= 12
Pulmonary
Systemic
25
0
120
0
RV
LV
Cap
Cap
20
RA
LA
2
5
=8
10
Vein
Vein
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West. Respiratory Physiology.2013.
Artery
30

Pulmonary Circulation Characteristics

PULMONARY CIRCULATION
LOW - RESISTANCE
NETWORK
HIGH - DISTENSIBLE
VESSELS
PULMONARY BLOOD
FLOW = 100% OF CO
LOW PRESSURE
SYSTEM
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Mean Pressures in Circulation

MEAN PRESSURES

• Pulmonary artery
  15
  mmHg
• Aortic pressure
  100
  mmHg
• Right atrium
  2
  mmHg
• Left atrium
  5
  mmHg
• Right ventricle
  25
  mmHg
• Left ventricle
  120
  mmHg
  Interventricular
  Apex
  Septum
  Mitral Valve
  LV
  RV
  Tricuspid
  Valve
  RA
  LA
  Interatrial
  Septum
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Ohm's Law and Pulmonary Vascular Resistance

What states Ohm's law and how can we use it to
calculate the Pulmonary vascular resistances?
Q = AP/R
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Calculating Pulmonary Vascular Resistance (PVR)

PULMONARY VASCULAR RESISTANCE (VR)
R =4P
Q
R = input pressure (Pi) - output pressure (Po)
Blood flow
PVR = (15 - 5)
6
PVR = 1.7 mmHg / L / min
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Pi = 15 mmHg (MAP)
Po = 5 mmHg (LAP)
BF = 6 L / min ( CO)

Pulmonary Artery Characteristics

Pulmonary artery CHARACTERISTICS
IS SHORTER THAN
AORTA
WALLS ARE THINNER
THAN AORTA
WALLS HAVE LESS
SMOOTH MUSCLE
PULMONARY VEINS
ARE THINNER
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Alveolar Vessels and Transmural Pressure

ALVEOLAR VESSELS

• Transmural pressure: pressure difference between
  the inside and outside of vessels.
• Alveolar vessels:
• Depend on the alveolar pressure.
• When alveolar pressure rises above the capillary
  pressure > capillary will collapse.
  -
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Alveolar Vessels Resistance

ALVEOLAR VESSELS
Alveolus
Alveolar vessels
Resistance
Extra-alveolar
vessels
A
A
_
Lung volume
+
West. Respiratory Physiology.2013.
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Extra-Alveolar Vessels

EXTRA-ALVEOLAR VESSELS

• Arteries
• Veins
  Their caliber is affected by lung volume
  As the lung expands > caliber increase
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  Resistance
  _
  Lung volume
  +
  West. Respiratory Physiology.2013.

Total Pulmonary Vascular Resistance

A
B
C
Resistance
_
Lung volume
West. Respiratory Physiology.2013.
+
Alveolar vessels
resistance
B
A
Extraalveolar vessels
resistance
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C
Total pulmonary vascular resistance

Vascular Recruitment and Distension

What is the difference between vascular
recruitment and distention?
Recruitment
Distension
-
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Pulmonary Blood Flow and Fick's Principle

PULMONARY BLOOD FLOW

• Fick's principle
  Q: Volume of blood passing throw the lungs each minute
  VO2: O2 consumed per minute
  CaO2: Oxigen concentration in the blood leaving the lungs
  CV02: Oxigen concentration in the blood entering the lungs
  VO2 = Q (Ca02 - CV02)
  Q =
  VO2
  _
  (Ca02-CvO2)
  Arteriovenus oxygen difference
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Oxygen Content Calculation

Oxygen content
Sum of oxygen bound to hemoglobin and
dissolved in plasma within arterial blood

• Ca02 = [1.34 x Hb x (SaO2/100)] + (0.003x PaO2)
  Arterial oxygen content is directly
  proportional to the Hb, SaO2, and PaO2
• Hb (g/dL) = hemoglobin
  concentration
• SaO2 (%) = arterial oxygen
  saturation in hemoglobin
• PaO2 (mm Hg) = partial pressure of
  oxygen
• 1.34 (mL) = maximum oxygen binding
  capacity of 1 g of hemoglobin.
• 0.003 = solubility coefficient of
  oxygen in plasma
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Distribution of Blood Flow in the Lung

DISTRIBUTION OF BLOOD FLOW

• Blood flow decreases
  from bottom to the top
• Bottom -> resistance v
  (more recruitment or
  distention).
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  150
  Radiation
  counters
  Blood flow/unit volume
  100
  50
  Bottom
  Top
  0
  0
  5
  10
  15
  20
  25
  Distance up lung (cm)
  West. Respiratory Physiology.2008.

West Zones of the Lung

Zone 1
PA>Pa>Pv
Pa= pressure
in the artery
PA= pressure
in the alveoli
Alveolar
Zone 2
Pa>PA>Pv
PA
Pv= pressure
in the vein
Pa
1
Pv
1
Arterial
Venous
Distance
E
Zone 3
Pa>Pv>PA
Blood flow
-
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WEST ZONES

Active Control of Circulation

ACTIVE CONTROL OF CIRCULATION
100
I
1
1
80
60
40
20
0
50
100
150
200
300
500
ALVEOLAR PO2

• Changes in alveolar PO2
• "Hypoxic pulmonary vasoconstriction"
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  BLOOD FLOW (% CONTROL)

Hypoxic Pulmonary Vasoconstriction

HYPOXIC
PULMONARY
VASOCONSTRICTION
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• Is independent of CNS.
• Some chemical mediators interacting in
  this process:
• Catecholamines.
• Histamine.
• Angiotensin.
• Prostaglandins.
• Decrease in vasodilators as Nitric
  oxide (Fetal life).
  O2 = 150 mm Hg
  CO2 = 0 mm Hg
  O2 = 100 mm Hg
  CO2 = 40 mm Hg
  O2 = 40 mm Hg
  CO2 = 45 mm Hg
  O2 = 100 mm Hg
  CO2 = 40 mm Hg
  O2 = 150 mm Hg
  CO2 = 0 mm Hg
  Decreased O2
  Increased CO2
  02 = 40 mm Hg
  CO2 = 45 mm Hg
  Decreased O2
  Increased CO,
  A
  The reflex
  constriction of the
  vessel is the
  response to the
  alveolar hypoxia
  O2 = 150 mm Hg
  CO2 - 0 mm Hg
  Decreased O2
  Increased CO2
  O2 - 40 mm Hg
  CO2 = 45 mm Hg
  Decreased O2
  Increased CO2
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  B
  O2 = 100 mm Hg
  CO2 = 40 mm Hg
  Decreased O,
  Increased CO2
  Î
  The PO2 is still low,
  but less amount of
  blood is going
  through this unit
  D

Water Balance in the Lung

WATER BALANCE IN THE
LUNG

• Only 0.3 micrometers of tissue separates
  the capillary blood from the air in the lung
• Fluid exchange across the capillaries
  walls is believed to obey "Starling's Law"
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  (h) Exchange surface of alveoli
  Alveolar
  epithelium
  Nucleus
  endothelia
  Capillary
  Endothelium
  0.1-
  1.5
  um
  Surfactant
  Fused basement
  membranes
  Alveolar air space
  Silverthorn. Human Physiology.2001

Capillary Pressure Values

VALUES
Capillary Colloid osmotic
pressure:
28 mmHg.
Capillary hydrostatic pressure:
Higher at bottom of the lung than at the
top.
Colloid osmotic pressure on
interstitial fluid
20 mmHg in lung lymph.
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Pulmonary Edema Development

EDEMA

• Early stage of edema. Interstitial
  edema. Engorgement of
  perivascular and peribronchial
  spaces.
• Later stages -> Water crosses the
  alveolar epithelium into the
  alveolar space. Unventilated and
  no gas exchange.
  Alveoli
  Alveolar space
  2
  Interstitium
  Capillary
  Alveolar wall
  1
  Bronchus
  O
  Artery
  /
  Perivascular space
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  West. Respiratory Physiology. 2008

Diffusion of Gases

DIFFUSION OF GASES
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Diffusion Objectives

Objectives

• State the Fick's law for diffusion and determine the limitations of gas transfer.
• Define oxygen diffusing capacity and describe the use of carbon monoxide to
  determine oxygen diffusion capacity.
• Name the factors that affect diffusive transfer of gas.
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Characteristics of Gas Transfer

CHARACTERISTICS
PA
C
A
A
A
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• Area of blood-gas barrier in the
  lung: 50 to 100 m2.
• Thickness: only 0.3 m in many
  places.
• The rate of transfer is
  proportional to a diffusion
  constant, which depends on
  the properties of the tissue and
  the particular gas.
  12
  CO2
  Transfer of gases
  through cellular
  membranes or
  capillary walls
  functions through
  "SIMPLE
  DIFUSSION"
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Fick's Law and Gas Transfer Limitations

WHAT STATES FICK'S LAW REGARDING GAS EXCHANGE
AND WHAT FACTORS LIMIT GAS TRANSFER?
PAO2 = 100
PACO2 = 40
CO2 O2
PvO2 40
PvCO2 45
I
I
0
time/sec
0.75
100
I
PaO2
-
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Start of
capillary
End of
capillary
Alveolar
N2O
O2 (Normal)
O2 (Abnormal)
Partial pressure
CO
0
0.25
0.50
0.75
Time in capillary (s)
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Picture from Respiratory Physiology, West

Diffusion vs. Perfusion Limited Gas Transfer

DIFFUSION VS PERFUSION LIMITED
Diffusion limited

• E.G. Carbon monoxide (CO)
  " CO strong bonds with Hb
  " Increases in CO content result in very
  minimal increase in partial pressure
  Partial pressure difference still exists (when
  blood finishes its passage through the alveoli)
  Transfer of CO is limited by the rate of
  diffusion, not the amount of blood available
  02 in certain conditions (emphysema,
  fibrosis, intense exercise)
  Perfusion limited
• E.G. Nitrous oxide (N20)
  . N20 doesn't form bond with Hb
  . Increase in N20 content result in rapid rise
  in partial pressure
  · Equilibrium is reached very early on
  . Transfer of N20 is limited by the amount of
  blood available
  . 02 in normal conditions is perfusion limited
  · CO2 is perfusion limited
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  A
  Mild affection of O2
  and CO2 exchange
  B
  Severe affection Alveoli
  completely filled
  105
  Normal
  B
  PaO2
  exercise
  I
  40
  0
  time
  .75B
  Alveolar
  50
  I
  Normal
  I
  I
  Abnormal
  Po2 mm Hg
  Î
  Grossly abnormal
  Exercise
  0
  0
  0.25
  0.50
  0.75
  Time in capillary (s)
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  Picture from Respiratory Physiology, West
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  Pco2 mmHg
  45
  I
  ABNORMAL
  I
  NORMAL
  I
  1
  40
  +
  ALVEOLAR
  I
  EXERCISE
  1
  0
  .25
  .5
  .75
  Time in Capillary - sec
  Picture from Respiratory Physiology, West

Diffusing Capacity of the Lung

DIFFUSING CAPACITY
Vgas . A.D. P -P2 )
T
Vgas = DL . (P1 - P2)
DL = Diffusing capacity of the lung

• Rate at which the gas is diffused through the alveolar capillary
  membrane (ml/min)
• Includes area, thickness and diffusion properties
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Measurement of Diffusing Capacity

MEASUREMENT OF DIFFUSING CAPACITY
Carbon Monoxide is used because is diffusion limited
Vgas = DL . (P1 - P2)
VCO
DL =
P1 -P2
Example : Carbon monoxide
P1 is partial pressure at alveolar gas
P2 is partial pressure at capillary blood
Partial pressure of CO in capillary blood is virtually zero (at non-lethal PAco)
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DL =
VCO
PACO

Conditions Decreasing Diffusion Capacity

What conditions decrease the diffusion capacity?

• Thickening of barrier
• Edema
• Fibrosis (sarcoidosis, scleroderma)
• Decreased uptake by red cells
• Anemia
• Reduced surface area
• Emphysema
• Tumors
• Low Cardiac Output, embolus
• Uneven VA - Q
• Reduced uptake by red cells
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  A
  CO
  B
  CO
  C
  CO
  Alveolar
  fibrosis
  Alveolus
  Capillary
  Exudate
  D
  CO
  E
  CO
  F
  CO
  - Loss of
  alveoli
  Alveolar
  fibrosis
  Pulmonary
  embolus
  Edema
  No
  blood flow
  Copyright 2009 by Saunders, an imprint of Elsevier Inc.