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Pulmonary Gas Exchange and
Gas Transport
Dr. Meg-angela Christi Amores
Physiologic Anatomy
One of the most important problems in all the
respiratory passages is to keep them open to
allow easy passage of air to and from the alveoli
• Trachea – with cartilage rings 5/6 of the way around
• Bronchi – walls have less extensive cartilage plates
• Bronchioles – no plates. Diameter <1.5mm, all smooth
muscles
– Kept expanded by same transpulmonary pressures that expand
the alveoli
Physiologic Anatomy
All areas of the trachea and bronchi not occupied by
cartilage plates, walls are composed of smooth
muscles
Resistance to flow is greatest NOT in the minute air
passages of terminal bronchioles but in some of
the larger bronchi near to the trachea.
Smaller airways are easily occluded ;
smooth muscles = contract easily
Pulmonary Circulatory System
• Pulmonary Vessels
– Pulmonary artery (5 cm, thin, 2x VC, 1/3 aorta)
• Right and Left main pulmonary branches – lungs
• Large compliance (7 mL/mmHg)
• Allows pulmonary arteries to accommodate 2/3 of
stroke volume output of Right Ventricle
– Bronchial Vessels – arterial supply to the lungs
• 1/3 of cardiac output
• Supplies supporting tissues (CT, septa, bronchi)
• Drains to pulmonary veins
Pulmonary vs. Alveolar Ventilation
• Pulmonary Ventilation
– Inflow and outflow of air
between the atmosphere
and the lung alveoli
• Alveolar Ventilation
– Rate at which new air reaches the areas in the
lung where it is in proximity to the pulmonary
blood or gas exchange areas (alveolar sacs, ducts,
respiratory bronchioles)
Diffusion of Gases
• Diffusion
– Random molecular motion of
molecules with energy provided by
kinetic motion of the molecules
– All molecules are continually
undergoing motion except in
absolute zero temperature
– Net diffusion
• Product of diffusion from high to low
concentration
Gas Pressures
• Partial Pressure
– Pressure is directly proportional to the
concentration of gas molecules; caused by impact
of moving molecules against a surface
– In respiration, there’s mixture of gases: O2, N2, CO2
– Rate of diffusion of each gas is directly
proportional to the pressure caused by each gas
alone
• AIR = total Pressure 760 mmHg
• 79% N, 21% O2
= PP
N = 600mmHg , PP O2 =160mmHg
Gas Pressure in Fluid
• Determined by its concentration and by
solubility coefficient
• If gas is repelled, pressure increases
• HENRY’s LAW : Pressure = concentration
solubility coefficient
Solubility of Gases in body temp.
•
•
•
•
•
O2 = 0.024
CO2 = 0.57 - 20x more soluble than O2
CO = 0.018
N2 = 0.012
He = 0.008
Factors that affect Rate of Gas
Diffusion thru Respiratory Membrane
• Respiratory Unit:
– Respiratory bronchiole
– Alveolar ducts
– Atria
– Alveoli (300 Million in
both lungs) (0.2mm)
*their membranes make up the
respiratory membrane
Respiratory Membrane
• Layers:
1. Layer of fluid lining
alveolus (surfactant)
2. Alveolar epithelium
3. Epithelial basement
membrane
4. Interstitial Space
5. Capillary basement
membrane
6. Capillary endothelial
membrane
Overall thickness: 0.2um (ave: 0.6 um)
Total surface area: 70 m2
Factors that affect Rate of Gas
Diffusion thru Respiratory Membrane
1. Thickness of membrane
•
Inc. in edema and fibrosis
2. Surface area of membrane
•
Dec. in removal of lung and emphysema
3. Diffusion coefficient of Gas in substance of
membrane
•
Gas’ solubility
4. Pressure difference
•
Difference between partial pressure of gas in alveoli
and pressure of gas in pulmonary capillary blood
Ventilation-Perfusion Ratio
• A concept developed to help us understand
respiratory exchange where there is
imbalance between alveolar ventilation and
alveolar blood flow
• Areas in lung with well ventilation but no
bloodflow or excellent blood flow but no
ventilation
• Va – alveolar ventilation
• Q – blood flow
Ventilation-Perfusion Ratio
Va/Q = normal
• If Va is 0 (zero), but with perfusion: Va/Q = 0
• If Va is present, but no perfusion Va/Q = infinity
• In both: there is no gas exchange
Ventilation-Perfusion Ratio
• Normal person :
– Upright: Va and Q are less in Upper part but Q is more
– At top of lung: Va/Q 2.5x > as ideal = physiologic dead
space (ventilation but less blood flow)
– At bottom: Va is less than Q
• Va/Q is 0.6 < as ideal = physiologic shunt
• COPD patient:
– Smoker, emphysema, alveolar walls destroyed
– Wasted blood flow = severe shunting
Transport of O2 and CO2
• Pressure differences causes gas to diffuse
Alveolus
Capillaries
Tissues (fluid)
Tissues (cells)
pO2
104 mmHg
95 mmHg
40 mmHg
5-40 (ave 23)
mmHg
pCO2
40 mmHg
45 mmHg
45 mmHg
46 mmHg
Transport of O2 and CO2
• CO2 can diffuse about 20 times as rapidly as O2
Transport of O2 in blood:
• 97% of O2 from lungs to tissues are carried in
combination with hemoglobin
• O2 combines loosely and reversibly with heme
pO2 – O2 combines with heme (pulm capi)
pO2 – O2 is released (tissue capillaries)
• For the next meeting, read on Regulation of
Respiration
• Guyton Textbook of Medical Physiology, 10th
edition Chapter 41