L11: Respiratory II

Properties of gases

• Gas molecules constantly moving (collide) = exert pressure on the container

• Total pressure: determined by the total number of gas molecule per volume unit

• Partial pressure of gases: in a mixture = total pressure x % of that gas

• Independent of other gases present in the mixture

Air (dry) P =

PN2 + PO2 + PCO2 = 760 mm Hg

Is the air in lungs dry?

NOT dry

• Inspired air is 100% saturated with water when it reaches the respiratory zone

• Some of the gas molecules colliding with water dissolve in water (slightly different than for dry air)

At equilibrium the relative amount of dissolved gas is..

CONSTANT

• Depends of the solubility of the gas

• Partial pressure of the gas

Presence of water ___ gas content

DILUTES

• At 37 C, water vapor pressure is 47 mm Hg

• PO2 = (0.21 x 760)-(0.21x47) = 150 mm Hg

• PO2 = (amount present in dry air) – (amount dissolved in water)

Gas exchange is done by

Diffusion- passive movement of gas molecules from regions of high concentration (partial pressure) to regions of low concentration (partial pressure)

Air velocity in the alveoli

In alveoli the air velocity = 0 (due to large cross-sectional area)

• Movement from alveoli to alveolar-capillary membrane by diffusion only

Factors affecting diffusion of gases in the alveoli

At constant atmospheric partial pressures diffusion of gases will depend on the alveolar and blood partial pressures:

• Depends on ventilation for alveoli (how much air replaced in the alveoli)

• Depends on the tissue consumption for blood

At higher altitude pO2 in atmosphere ↓; significant impact on diffusion

Atmospheric pCO2 affect on alveoli

• pCO2 in atmosphere is way lower than in alveoli and blood

• changes in atmospheric pressure does not impact CO2 diffusion dramatically (except in enclosed spaced)

How does gas exchange occur in the lungs

• Gas exchange occurs between alveoli and blood capillary network

• Blood from right ventricle flows in capillaries; lungs also have some capillaries from the left side of the heart to bring oxygen to lung tissue

• Blood entering alveolar capillaries has low pO2

• O2 diffuses from alveoli to the blood

• At the end of capillary, pO2 is the same in blood and alveoli

• For CO2: pCO2 higher in blood entering capillary then diffuses out toward alveoli

Main determinants affecting gas exchange in the lungs

1. Driving partial pressure gradient

• PalveoO2 – PcapO2

2. Surface area available for diffusion (A)

• Increased area = more exchange

• Capillaries can open during exercises to increase exchange

3. Thickness of the air-blood barrier (X)

• Deep inspiration during exercises reduces the distance

  between alveoli and capillary epitheliums

4. Physical properties of the gas (D)

• VO2 = D x A x (PAO2 – PcapO2)/X

Perfusion and its requirements

• Perfusion correspond to the blood entering the lung

• Need to match ventilation with the blood flow for optimum gas exchange

Perfusion for bipeds (humans) and quadrupeds

For bipeds (human) at rest: not optimum

• Gravity = perfusion (blood flow) lowest at the top of the lung (capillaries can collapse)

• Not so pronounced for ventilation

Not so bad for quadrupeds

V/Q

term used for ventilation/perfusion (0.8 in humans)

• becomes more uniform during exercise (more blood pump throughout the lung)

V/Q inequality

Certain diseases can affect either the ventilation or perfusion of certain alveol

Natural mechanism to minimize the impact of V/Q inequality

Hypoxic vasoconstriction

• closing of the poorly ventilated alveoli

• redirecting blood towards the well ventilated

How is Hypoxic vasoconstriction initiated

Mechanism initiated by a reduction of pO2 and/or increase in pCO2 in the interstitial fluid of affected areas

How can impaired pulmonary gas exchange occur

1. Thickening of alveolar-capillary membrane

• Increases the time for diffusion across the membrane and decreases rate of diffusion

2. High altitude or low air pO2

• Decrease alveolar oxygen pressure, hence decrease driving pressure

3. Hypoventilation

• Inadequate ventilation of lung

4. Ventilation-perfusion inequality

• Ventilated alveoli with no blood supply or vice versa

How does gas exchange in tissues occur

• Follows the same principle as in the alveoli

• Diffusion driven by difference in partial pressure

• Cells consume O2 and produce CO2

• Blood from the left heart loaded with O2

• Diffusion of O2 will occur from blood to interstitial fluid to the cells

• Diffusion of CO2 will occur from the cell to the interstitial fluid to the blood