Glossary of 4 - Alveolar ventilation

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What is PO2 in dry air? How does this change when air is inspired and conditioned?
*in dry air, PO2 = 0.2093 x 760mmHg = 159mmHg
*when the air is inspired, water vapor is added which reduces the percentage of O2, so PO2 = 149mmHg
Why is the PO2 of inspired air greater than that of air in the alveoli?
Some O2 is consumed in respiratory bronchioles and alveolar ducts.
Why is the PO2 of expired air higher than that of alveolar air?
As the alveolar air (PO2 = 104) is expired, it mixes with air in physiological dead space (PO2 = 149).
What is the PCO2 of inspired air? Of alveolar air? Of expired air? What does this imply about the lungs?
*PCO2 (inspired) = 0
*PACO2 = 40
*PCO2 (expired) = 27
*they are a source of CO2
What is the relationship between PCO2 in the alveoli and that in arterial blood?
They are equal.
How are alveolar ventilation and PaCO2 related?
They are inversely related - faster ventilation reduces PaCO2 and vice versa.
What does the alveolar ventilation equation allow one to calculate?
Alveolar ventilation from PaCO2.
Why is PAO2 a poor indicator of alveolar ventilation?
There is no linear relationship between PAO2 and alveolar ventilation because O2 is obtained from outside the lung. This means PAO2 will never exceed 149mmHg despite faster ventilation.
What is hyperpnea?
An normal increase in alveolar ventilation in response to increased metabolic CO2 production.
What is hyperventilation? In what condition does this often occur?
An abnormal increase in alveolar ventilation in the absence of increased metabolic CO2 production. Commonly seen in patients with vascular shunts.
What is hypoventilation? In what disease type does this occur?
An abnormal decrease in alveolar ventilation in the absence of decreased metabolic CO2 production. Commonly seen in obstructive lung disease.
What is the alveolar gas equation? What does it allow you to calculate?
*PAO2 = 147 - (PACO2/0.80)
*PO2 in the alveoli from ABG (PaCO2)
What is the A-a difference? What is the normal range for this value?
*the difference between calculated PAO2 and measured PaO2 as an indicator of lung function
*normal = 2-15mmHg; abnormal is >20mmHg
What is the "slinky" effect?
Alveolar interdependence and gravity cause those alveoli in the upper regions to be pulled more open than those in the lower regions. This decreases their ability to ventilate.
What is the bellows effect?
Higher IPP and alveolar interdependence causes higher transmural pressure, preventing the alveoli from expelling air efficiently and thus ventilating poorly.
Where is compliance greater - in the lower or upper regions of the lungs?
CPL is greater in the lower regions
What is the difference between arterial and venous pressures in the lungs?
Name three variables that can affect the rate of perfusion in pulmonary capillaries?
1. Arterial pressure
2. Venous pressure
3. Alveolar pressure
Where is perfusion greatest in the lungs - in the upper or lower regions?
In the lower regions.
What inhibits efficient perfusion in the upper regions of the lungs?
Alveolar pressure is greater than arterial pressure and thus partially occludes the capillary beds in this region.
Why are capillaries in the lower regions of the lung well-perfused?
Arterial and venous pressures are greater than alveolar pressure, thus blood is effectively moved through the capillary beds.
Describe capillary perfusion in the middle regions of the lungs.
Blood enters the capillary bed well because Pa>PA, however, at the venous end of the bed flow is somewhat occluded because PA>Pv.

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