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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