Gas Exchange Lecture 1: O2 and CO2 Transport/ Ventilation and perfusion Flashcards
Define Gas Content
- The measure of the number of molecules of a gas contained in a given volume of liquid
- Expressed at litres / litre (vol of gas molecules in a container at STp per litre of liquid)
State Henry’s Law
A constant T, the amount of given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid
P = kC
How are gas content and partial pressures related?
Gases dissolve in liquids by amounts that are in direct proportion to their partial pressures
Partial pressure/gas content relationship for oxygen dissolving in blood
For each mmHg PO2, 0.003 mL O2/100 mL blood is dissolved
3 key characteristics of hemoglobin
- High affinity for O2
- 12 - 15 gm of Hb per 100 mL of blood
- Max carrying capacity for O2 (saturation)
Equations for total blood oxygen content
Total blood O2 content = HbO2 + dissolved O2
Equation for HbO2
[Hgb] x O2 saturation x binding capacity
Equation for dissolved oxygen
PO2 x solubiltiy
What is chloride shift?
The exchange of bicarbonate and chloride ions across the RBC membrane to allow bicarbonate to enter the plasma
3 methods of CO2 transport in the blood
- 70 - 80% as bicarbonate
- 5 - 10% dissolved in plasma
- 5 - 10% bound to Hb as carbamino compounds
CO2 content in blood relationship to partial pressure
Roughly linear disscoaition curve (relative to O2 curve)
How can changes in CO2 shift the HbO2 dissociation curve to the right?
Bohr effect: increased CO2, increased temp, increased 2,3 DPG, decrease pH
How can changes in CO2 lead to a left shift of the HbO2 dissociation curve?
- Decreased CO2
- Decreased temp
- Decreased 2,3 DPG
- Increased pH
What is the influence of O2 on the CO2 curve?
Haldane effect = increased loading of CO2 on deoxygenated Hb (increased CO2 carryign capacity of deoxygenated blood)
Ideal alveolar gas equation fo CO2
Ideal alveolar gas equation for O2
What does the rate of O2 delivery to the alveoli depend on?
Ventilation and inspired PO2
What does the rate of O2 uptake into blood depend on
The O2 needs of the tissues (VO2)
Why is PAO2 lower than PiO2?
Because PAO2 reflects a balance between the rate of O2 delivery to the alveoli and the rate of O2 uptake by alveolar capillary blood
What determines the PO2 and PCO2 levels in the alveolus?
Ventilation-perfusion ratio
Graphic effect of alveolar ventilation on PAO2 and PACO2
Inverse relationship
What exactly does the ideal alveolar gas equation for O2 represent?
The greatest arterial PO2 that such a lung could produce. So if PaO2 = PAO2, then the lung is operating at its highest efficiency
How do you calculate (A-a) DO2?
Ideal alveolar PO2 - measured arterial PO2
What exactly does A-a DO2 measure and why is it useful?
A measure of gas exchange efficiency useful in:
- Determining the efficiency of gas exchange in a given lung
- Helping to detemine the cause of hypoxemia
Define hypoxemia
An abnormally low PO2 in arterial blood for the subject’s age
5 causes of hypoxemia
- Decreased PiO2
- Hypoventilation
- V/Q mismatch
- Shunt
- Diffusion limitation
Pressure values when perfusion is 0
PAO2 = PiO2
PACO2 = PiCO2 = 0
Pressure values when ventilation is 0
PaO2 = mixed venous PvO2
PaCO2 = PvCO2
What is wasted perfusion?
Blood flowing to alveoli with no ventilation = shunt (or shunt-like effect)
What is wasted ventilation?
Air entering alveoli with no perfusion = dead space
2 extremes of V/Q mismatch
Dead space
Shunt
4 examples of pulmonary shunt
- Pulmonary edema
- Consolidtion
- Atelectasis
- Airway obstruction (i.e. tumor, mucus, foreign body)
Define pulmonary shunt
A region of the lung receives blood flow but is not ventilated
Define extrapulmonary shunting
Blood bypasses the pulmonary circulation altogether
3 examples of extrapulmonary shunting
- Ductus arteriosus / heart septal defects
- Thebesian veins (drain a small portion of coronary venous blood directly into the left atrium)
- Bronchial circulation (a portion of bronchial veins drains into the pulmonary veins)
When does hypoxemia occur from V/Q mismatch
When V/Q is LOW
Why is dead space not a cause of hypoxemia?
V is infinitely higher than Q so V/Q = infinity and hypoxemia only occurs if V/Q is LOW
How to calculate dead space
Where and why can diffusion limitation be observed?
Elite athletes due to huge cardiac output and short transit time
Effects of O2 therapy for V/Q inequality and shunt
Effective in V/Q mismatch, but highly ineffective in shunt
Upon discovering a patient with hypoxemia, how do you determine the cause?
