gas exchange partial pressures of gases composition of lung gases alveolar ventilation diffusion...
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Gas Exchange
• Partial pressures of gases
• Composition of lung gases
• Alveolar ventilation
• Diffusion
• Perfusion = blood flow
• Matching of ventilation to perfusion
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Partial Pressure of Gas
P =nRT
Vgas equation
Pgas 1 + Pgas 2 + Pgas 3……= Total P
(e.g. 760 mmHg)
partial pressures of gases
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Gas Transfer
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. Henry’s law:
conc dissolved gas partial pressure of gas x solubility index
• Also:
Dissolved gas in equilibrium with gas combined with other agents e.g. hemoglobin
• Gases in liquids diffuse from area of higher partial pressure to area of lower partial pressure.
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PA > PB
O2
CO2
mixed venous blood
venous
40 mmHg 100 mmHg
PAO2
100 mmHg
46 mmHg 40 mmHg
PACO2
40 mmHg Pulmonary vein = arterial
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Gas diffusion cont.
Rate of diffusion
pressure difference
surface area
solubility index
membrane thickness molecular weight
Pressure difference: ≈ 60 mmHg for O2
≈ 6 mmHg for CO2
Surface area: large
depends on ventilation:
perfusion matching
Solubility index if: O2 = 1, CO2 = 20
Membrane thickness: small
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Respiratory membrane
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Fig. is a scanning electron micrograph of mouse lung to show an interalveolar septum.
Red blood cells
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A surface view of capillaries in the alveolar wall.
Cross-sectional view of the alveolar wall and their vascular supply.
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Diffusing capacity:
= volume of gas that diffuses through respiratory membrane per minute for each mmHg of gas partial pressure difference
with exercise — capillary diameter
~ better matching of ventilation to perfusion (blood flow)
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Alveolar Ventilation
Air in alveoli + airways
Dead space = air not involved in gas exchange~ anatomical: air in airways
(in ml ≈ body weight in pounds)
~ alveolar: air in alveoli with blood supply
little gas exchange ventilation wasted
very small normally
V T = VA + VD
tidal volume air to alveoli dead space
500 ml = 350 ml + 150 ml
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Minute/total ventilation = VT breaths / min
Alveolar ventilation =
VA = (VT VD ) breaths / min
(500 150 ) 12
≈ 4.2 L / min
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Ventilation : Perfusion
Matching
Pulmonary circulation
flow rate in systemic circulation
=flow rate in
pulmonary circulation
blood pressure in pulmonary circulation
mean ≈ 15 mmHg
<<blood pressure in
systemic circulation
mean ≈ 100 mmHg
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Even with exercise
Mean P < 30 mmHg
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Shunts = blood that returns to L side of heart without being oxygenated
~ anatomical: ≈ 2%
directly to heart from central chest tissues
~ physiologic / pathologic:
blood from areas of lung where
blood flow > airflow
so some blood flow wasted
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Factors which tend to disturb
ventilation / perfusion ratio:
• non-uniform pulmonary blood flow
• non-uniform ventilation
• pathological
gravity effects
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Effects of Gravityat normal breathing volumes:1. change in volume at lung bases is greater than at apicesi.e. alveolar ventilation > alveolar ventilation
(at bases) (at apices)2. blood flow (perfusion) > blood flow (perfusion)
(at bases) (at apices)
but Overall
Base perfusion > ventilation
Apex ventilation > perfusion
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Protective reflexes which match ventilation with perfusion:
1. Hypoxic pulmonary vasoconstriction – NBif blood flow > airflow
2. PCO2 control of airway smooth muscleif airflow > blood flow
PO2 in lung tissuevasoconstriction
blood flow
PCO2 in lung tissue
broncho-constriction air flow
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Gas Exchange (summary)
• Alveolar vs total ventilation• Alveolar gas composition (partial pressure)• Gas transfer
partial pressure of gas in liquidrate of gas diffusiondiffusing capacitypulmonary circulationventilation : perfusion matching
dead spaceshuntsprotective reflexes