Allied Science Physiology 09-10. Respiratory System. Lecture 3.
Allied Science Physiology. Respiratory System. Lecture 3.
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•  Arterial blood O2 and CO2 levels remain relatively
constant
–  Oxygen moves from alveoli to blood at same rate it is
consumed by cells
–  Carbon dioxide moves from blood to alveoli at same
rate it is produced by cells
VCO2 = 200ml/min
Respiratory Quotient :0.8 (rest)
VO2=250ml/min
•  At maximum exercise, RQ > or < than 0.8??
Allied Science Physiology. Respiratory System. Lecture 3.
Figure 17.1
Allied Science Physiology. Respiratory System. Lecture 3.
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•  Many gases are mixtures of different molecules
•  Total pressure is the sum of all Partial Pressures
–  Ptotal = P1 + P2 + P3 + … Pn
•  Partial pressure of a gas depends on:
–  Fractional concentration of the gas
–  Total pressure of gas mixture
Pgas = %gas x Ptotal
Allied Science Physiology. Respiratory System. Lecture 3.
•  Composition of Air
–  79% Nitrogen
–  21% Oxygen
–  Trace amounts carbon dioxide, helium, argon, etc.
–  Water can be a factor depending on humidity
Allied Science Physiology. Respiratory System. Lecture 3.
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•  At sea level (0% humidity):
Pair = 760 mm Hg = PN2 + PO2
–  PN2 = 0.79 x 760 mm Hg = 600 mm Hg
–  PO2 = 0.21 x 760 mm Hg = 160 mm Hg
–  Air is only 0.03% carbon dioxide
•  PCO2 = 0.0003 x 760 mm Hg = 0.23 mm Hg
• 
At 100% humidity:
Pair = 760 mm Hg = PN2 + PO2+ PH2O
–  PN2 = 0.741 x 760 mm Hg = 563 mm Hg
–  PO2 = 0.196 x 760 mm Hg = 149 mm Hg
–  PH2O = 0.062 x 760 mm Hg = 47 mm Hg
–  PCO2 = 0.00027 x 760 mm Hg = 0.21 mm Hg
Allied Science Physiology. Respiratory System. Lecture 3.
Allied Science Physiology. Respiratory System. Lecture 3.
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•  Gas molecules can exist in gas form
or dissolved in liquid
•  When a gas and a liquid in contact: gas molecules dissolve in
the liquid until system reaches equilibrium. At equilibrium:
–  Gas and liquid at the same partial pressure
–  The concentration of gas molecules in the liquid
proportional to the partial pressure of the gas and
solubility of the gas in that liquid
•  Henry’s Law: c = kP (note that k or Henry’s Law
constant differs among gases)
Allied Science Physiology. Respiratory System. Lecture 3.
•  At a given partial pressure, relative
concentrations of different dissolved gases
differ
•  At 100 mm Hg partial pressure in water:
–  [O2] in water = 0.15 mmoles/liter
–  [CO2] in water = 3.0 mmoles/liter
Carbon dioxide is more soluble (20 times!) than
oxygen in water (and blood)
Allied Science Physiology. Respiratory System. Lecture 3.
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Figure 17.3
Allied Science Physiology. Respiratory System. Lecture 3.
–  Diffusion rate (CO2) = 3.62 (diffusion constant) x 6 (ΔP) = 21.7
–  Diffusion rate (O2) = 0.18 (diffusion constant) x 60 (ΔP)= 10.6
•  CO2 diffusion rate is 2 times that of O2
Allied Science Physiology. Respiratory System. Lecture 3.
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•  In gas mixtures, gases diffuse down partial
pressure gradients
–  High partial pressure  low partial pressure
•  A particular gas diffuses down its own partial
pressure gradient
–  Presence of other gases irrelevant
Allied Science Physiology. Respiratory System. Lecture 3.
Figure 17.4
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•  Actual amount of oxygen and carbon dioxide that is
exchanged in any given vascular bed depends on
metabolic activity of tissue
–  Greater rate of metabolism  Greater exchange
•  Normally: PO2=40mmHg and PCO2=46mmHg
•  Strenuous exercise: PO2<40mmHg and PCO2>46mmHg
•  PO2 and PCO2 in different systemic veins varies, but all the
venous blood mixes together before reaching the right atria
Allied Science Physiology. Respiratory System. Lecture 3.
• Equilibration occurs in 0.25 seconds (blood has traveled 1/3
of the length of the capillary): margin of safety
• Rapid because of thinness of the resp membrane Figure 17.5
Allied Science Physiology. Respiratory System. Lecture 3.
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• O2 & CO2 equilibrate at similar rates
• At rest: blood spends 0.75 sec in the capillary
• Normal equilibration within 1/3 of capillary transit (0.25 sec)
• During intense exercise: time reduced to 0.25 sec
• Diffusion process affected by:
•  Exercise
•  Thickening of blood-gas barrier
Allied Science Physiology. Respiratory System. Lecture 3.
•  Fluid accumulation in alveoli and/or interstitial space
•  Impairs diffusion (higher distance from alveoli to blood)
•  Increases work of breathing (decreased lung compliance)
•  Arterial blood: lower P02 and higher PCO2
•  Causes:
•  Increased capillary pressure
–  Via left heart failure
•  Reduced atmospheric pressure
–  At altitude
•  Treatment:
•  Administering oxygen and diuretics
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•  Factors affecting alveolar partial pressures
–  PO2 and PCO2 of inspired air
–  Minute alveolar ventilation
–  Rates at which respiring tissue use O2 and
produce CO2
•  Most critical is rate of alveolar ventilation
relative to rate of oxygen use and carbon
dioxide production
Allied Science Physiology. Respiratory System. Lecture 3.
•  Hyperpnea = increased ventilation due to increased demand
–  Minimal changes in arterial PO2 and PCO2
•  Hypoventilation = ventilation does not meet demands of
tissues
–  Arterial PO2 decreases
–  Arterial PCO2 increases
•  Hyperventilation = ventilation exceeds demands of tissues
–  Arterial PO2 increases
–  Arterial PCO2 decreases
Allied Science Physiology. Respiratory System. Lecture 3.
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