Gas Exchange Flashcards
Partial Pressure of Gases
Each gas in the mixture of air exerts a portion of the total pressure of gas
PO2
Air is 20.93% oxygen
PO2 = 0.2093 * 760 = 159mmHg
Inspired air is
100% humidified and warmed
Lung partial pressure is calculated as a ___ value
dry values, so you have to subtract the effect of water vapor
PH2O = 47mmHg
Trachea = 760 - 47 * 0.21 = 150mmHg
When oxygen moves to a fluid medium (blood) PO2 is defined by…
O2 dissolved in solution
Gas Exchange depends on…
Diffusion
Gas diffuses from high to low partial pressures
Fick’s Law of Diffusion
A/T * D * (P1-P2)
Primary driving force is the pressure gradient
D - coef of gas is proportional to its solubility in the membrane
CO2 is more soluble in membrane
Rate of diffusion of gas is proportional to ____ and inversely related to ____
tissue area (A) and inversely related to tissue thickness (T)
Diffusion path for lung
- Layer of fluid lining the alveolus containing surfactant
- Alveolar epithelium
- Epithelial basement membrane
- Interstitial space
- A capillary basement membrane
- Capillary endothelial membrane
Lung Interstitum
Very small under normal conditions
Can become enlarged with inflammatory cells and fluid (edema) and this would interfere with gas exchange
Diffusion depends on…
O2
CO2
Pressure gradient
O2 = 100 to 40
CO2 = 40 to 46
CO2 = more soluble in membrane, better diffusion coef.
Hemoglobin
Pigment in RBCs gives blood red color
Oxygen ____ when binding heme
Ox reversibly binds heme
Consists of porphyrin ring and central iron atom
Hemoglobin # of Ox molecules can bind
Hemoglobin cooperatively binds 4 oxygen molecules -
Oxygen of 1st heme inc the affinity for 2nd molecule of O2 - when 1st binds, there is more attraction for second and so on until 4 are bound
O2 uptake depends on
- diffusion characteristics (Ficks Law)
2. Pulmonary capillary blood flow
Oxygen diffusion along pulmonary capillary - how long to saturate hemoglobin under norm conditions
Only 1/4 of a sec to saturate hemoglobin under normal conditions
Oxygen diffusion along pulmonary capillary - how long to saturate hemoglobin with exercise
Changing time blood cells take to fly across the capillary, generally saturation stays the same
Pathology –> can have problem with saturation and may never get fulls aturation
Oxygen carrying capacity of blood
Hemoglobin = 15Hb/dL
1 g Hb binds 1.34 mL of O2
20.1 mL O2/dL blood = oxygen content of arteriole blood
O2 saturation is ratio of…
)2 content (amount actually bound) over O2 capacity (usually 98%)
Ratio of how much is bound to what the capacity is
Bohr Effect
Inc H+ combines with Hb changing its shape so oxygen cant bind
Increase in PCO2 causes…what with pH
Decrease in pH
Carbonic anahydrase causes CO2 and water to combine reversibly and form H2CO3 that then ionizes H+ and HCO3-
Increasing temperature…
Decreases tendency for oxygen to remain bound to hemoglobin
BPG released by…
RBCs as they break down glucose for energy
Lowers affinity for O2
Binds to hemoglobin and increases release of O2
Increased with anemia, chronic hypoxia, and acclimatization to altitude
Oxygen content is critical factor affecting…
metabolism
Anemia
Dec. in RBCs
Can have normal PO2 and SO2 but a dec oxygen content
O2 content and O2 capacity are both decreased, so the ratio stays the same
Carbon Monoxide
Affinity of Hb for CO is 210X that of O2
No feedback mechanism when O2 is low
CO shifts curve to the left - dec in O2 unloading at the tissues
CO2 transport in blood
Dissolved = 7%
Bound to Hb = 23%
Bicarbonate = 70%
CO2 diffusion along pulmonary capillary
46 mmHg to 40 mmHg as you go across pulmonary capillary
Takes 1/3 of a sec for CO2 to get to 40
With pathology might not get this, would get to 41 or 42
Primary driver for inspiration
CO2
Just a little change will impact inspiration
CO2 Equilibrium - Dissociation Curve
Describes relationship btw PCO2 and CO2 content of the blood
Oxygen helps CO2 diffusion
Haldane Effect
Increased PO2 drives the curve down (facilitates unloading of CO2 at the lungs)
Decreased PO2 drives curve up (facilitates CO2 loading at the tissues)
Integration of Dissociation Curves - blood can carry more….
Blood can carry more CO2 than O2
Integration of Dissociation Curves
Shape of CO2 equilibrium curve allows lungs to unload lots of CO2 with small change in PCO2
Change in CO2, large change in content
O2 change, not much impact on content
