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Relationship between PAO2 and PACO2

? PACO2 --> ? PAO2 (hyperventilation); ? PACO2 --> ? PAO2 (hypoventilation)


Which factors affect PAO2?

Atmospheric pressure, oxygen concentration of inspired air and PACO2


What determines oxygen content?

Hemoglobin concentration. 1.34ml O2 combines with each gram of hemoglobin.


Amount of dissolved oxygen in the blood

0.3 volumes %; 0.3ml per 100ml of blood. Determines PO2 which acts to keep oxygen bound to Hb


What determines oxygen attachment to hemoglobin?

PO2 and the affinity of the individual attachment sites. The higher the affinity, the less PO2 is needed to keep it attached


What determines PO2?

Amount of oxygen dissolved in plasma. Normally 0.3 volumes %.


Site 4 of hemoglobin

Oxygen is attached at 100mmHg. Least affinity, last site to be saturated.


Site 3 of hemoglobin

Oxygen is attached at 40mmHg. More affinity than site 4, less affinity than site 2.


Site 2 of hemoglobin

Oxygen is attached at 26mmHg which is p50. More affinity, second site to be saturated.


Site 1 of hemoglobin

Oxygen remains attached under physiologic conditions. Highest affinity, first site to be saturated.


Factors that shift oxygen dissociation curve to the right

? CO2, ? 2,3BPG, fever, acidosis


Factors that shift oxygen dissociation curve to the left

? CO2, ? 2,3BPG, hypothermia, alkalosis, HbF, methemoglobin, carbon monoxide, stored blood


How is CO2 carried in the blood?

5% dissolved; 5% attached to Hb (carbamino compounds); 90% as bicarbonate.


Main drive for ventilation

H+ ions from dissociated H2CO3 which stimulate central chemoreceptors. H2CO3 is proportional to PCO2 of CSF


Central chemoreceptors

Sense [H+] which is proportional to PCO2 and H2CO3 of the CSF (not systemic)


Peripheral chemoreceptors

Carotid bodies (afferents via IX), aortic bodies (afferents via X). Monitor PO2 and [H+/CO2]


Main drive for ventilation in severe hypoxemia

Peripheral chemoreceptors sense PaO2 (dissolved oxygen) once PaO2 falls to 50-60mmHg.


Ventilatory response to chronic hypoventilation

Peripheral chemoreceptors are the main drive for ventilation eventhough PaCO2 is increased.


Ventilatory response to anemia

PaO2 and PACO2 are normal, therefore neither peripheral nor central chemoreceptors respond.


Central control of ventilation

Apneustic center in the caudal pons promotes prolonged inspiration. Pneumotaxic center in the rostral pons inhibits apneustic center. Efferents are from the medulla to the phrenic nerve (C1-C3) to the diaphragm


Differences in ventilation between the base and the apex of the lung

Base intrapleural pressure is -2.5, alveoli are compliant and small with a small volume of air but receive a large amount of ventilation; Apex pressure is -10, alveoli are large and stiff and contain a large volume of air but receive small amount of ventilation.


Differences in blood flow between the base and the apex of the lung

Blood vessels of the apex are less distended, have more resistance and receive less blood flow. Blood vessels of the base are more distended, have less resistance and receive more blood flow


Ventilation/perfussion relationship at the base of the lungs

Blood flow is higher than ventilation, the relationship is less than 0.8; the bases are underventilated, ? shunts


Ventilation/perfusion relationship at the apex of the lungs

Blood flow is lower than ventilation, the relationship is more than 0.8; the apex are overventilated, ? dead space


What does a ventilation/perfussion relationship under and over 0.8 mean?

Under 0.8 (at the bases) lungs are underventilated and less gas exchange takes place, therefore PACO2 and end-capillary PCO2 will be higher and PAO2 and end-capillary PO2 will be lower.


What is hypoxic vasoconstriction?

A decrease in PAO2 causes vasoconstriction and shunting of blood through that segment.


What is the effect of a thrombus in a pulmonary artery?

Blood flow decreases, therefore ? Va/Q --> ? PACO2, ? PAO2


What is the effect of a foreign object occluding a terminal bronchi?

Ventilation decreases, therefore ? Va/Q --> ? PACO2, ? PAO2


What constitutes a pulmonary shunt?

Regions of the lung where blood is not ventilated. Low Va/Q relationship.


What constitutes alveolar dead space?

Regions of the lung where there's no blood flow in spite of ventilation. High Va/Q relantionship