Floreani: Alveolar Gas Exchange Flashcards
3 things that gas exchange depends upon
- Ventilation
- Perfusion
- Barriers: tissue, blood cell and air-liquid interface across which gases diffuse
In the lung, at what level does gas exchange begin?
At the level of the respiratory bronchiole
What is the thickness of the blood-gas barrier in microns?
0.5 microns
What is the area of the blood-gas barrier?
Very large! About 50-100 square meters
Is ventilation per unit volume greater at the apex or the base?
Greatest at the base!
Is intrapleural pressure more negative at top or bottom of the lung?
More negative at the top or apex
Is there a larger pressure at the base or the apex?
At the base, to balance downward acting weight
Is the resting volume of alveoli less at lung apex or base?
Less at lung base
Which part of lung gets greater blood flow?
The base
What is the result of a mismatch in V/Q?
Decrease in partial pressure of arterial oxygen (PaO2)
What do these variables stand for:
PAO2 = PIO2 or FiO2 (PB - PH2O) – PCO2/R
Where PB = barometric pressure
PH2O = 47 mm at 37 o
FiO2 is fraction of inspired Oxygen ( .21 to 1.0)
Where R = the Respiratory quotient
( VCO2 / VO2 ) with a usual resting value of 0.8 to 1.0
What is the normal range for PaO2?
70 to 100mm
Decreased PaO2
hypoxemia
Basic causes of hypoxemia
Lack of O2 in alveoli or decreased PAO2
Abnormalities in lung structure and function
Hypoxemia can be caused by hyperventilation. What are some things that can cause this?
Sedation Neuromuscular diseases Muscle disorders Pharmacologic blockade High spinal cord lesions Brainstem infarction
What is the most common cause of hypoxemia in clinical disease states?
Mismatch in V/Q ratio or ventilation-perfusion mismatch
What does a high V/Q ratio indicate?
Increased dead space, or ventilation to non-perfused units
Another cause for hypoxemia could be diffusion defects. Discuss this.
Alveolar-capillary membrane compromised. Diffusion of gas across those membranes impaired, resulting in hypoxemia.
Hypoxemia can be caused by a shunt. Intra-cardiac shunts and extra-cardiac shunts. What are some intra-cardiac shunts?
Patent foramen
Atrial septal defects
Ventricular septal defects
Congenital heart disease
What are some extra-cardiac shunts?
Pulmonary AVMs
A-V fistulas
V/Q that approaches 0
No ventilation as blood bypasses
Does right to left shunt effect PaCO2 or PaO2?
Dramatic effect of PaO2, but minimal effect on PaCO2
