BPK 306 Lecture Notes - Partial Pressure, Blood Gas Tension, Hemoglobin

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Published on 1 Dec 2011
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Respiratory System # 8
VIII. GAS DIFFUSION IN THE PULMONARY SYSTEM
A. Introduction
a. Review Fick’s Law of Diffusion (can be for passive diffusion or transfer
of gas through a tissue)
i. Vgas proportional to AD(P1-P2)/T
ii. Body capable of changing surface area and/or driving pressure
differential
iii. Due to difficulty measuring A and T, we simplify it to:
1. Vgasrate = DL*(P1-P2)
a. DL is the diffusing capacity of lung in mL of gas
transferred per min per mm Hg alveolar-arterial
partial pressure gradient (mL/min/mmHg)
B. The Pressure Gradient
a. A gas diffuses down a pressure gradient. Not a conc’n gradient. It goes
against a conc’n gradient
C. Gas in Solution
a. Total volume of gas in blood depends on its solubility in accordance with
Henry’s Law : Gas Volume = Pressure*Solubility Coefficient
D. O2 and CO2 Diffusion
a. Concerned mainly with O2 and CO2 diffusion (examine CO, N2O and He
as well)
b. CO2 molecular weight is 44, O2 is 32
c. Solubility of CO2 is 24x that of O2 at 37 C in saline
d. CO2 diffuses at 20x the rate of O2
E. Thickness or Barrier to Diffusion
a. Already discussed in 1st lecture
F. Interaction of O2 and Hemoglobin
a. w/o Hb, blood can only carry 0.3 mL of O2 per 100 mL blood (at PO2 =
100 mm Hg)
b. 1 gram of Hb carries betw 1.34 – 1.39 mL O2
c. W/ [Hb] = 15 gm/100 mL blood, blood increases O2 content to 20 mL O2
per 100 mL blood (67x increase). Due to:
i. One molec Hb binds 4 O2 molecs
ii. Bound O2 doesn’t exert a partial pressure (reduces PO2 in blood
so there’s a bigger O2 gradient for blood to get more O2 from
alveoli)
G. O2 Dissociation Curve
a. At normal PaO2 = 95 mm Hg, large variation of PO2 (due to
VArate/Qrate differences for example) causes very little change in blood
O2 content
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