Ventilation And Perfusion Relationships

Question 1

Lung tissue differences due to gravity

Answer 1

. Base of lungs is compressed
. Apex of lung is expanded
. Weight of lung compresses intrapleural space at bottom of lung creating vertical distribution of intrapleural pressure

Question 2

Alveoli at base of lung at FRC

Answer 2

. Alveoli at base have low resting volume CN of weight from tissue above it w/ reduced expanding pressure bc intrapleural pressure is less neg.
. Alveoli on steep part of compliance curve so any change in intrapleural pressure wil produce large inc. in alveoli volume
. Smaller resting volume, greater expansion during inspiration
. Better ventilated tan apex

Question 3

Alveoli at lung apex at FRC

Answer 3

. Higher resting volume than base
. Large expanding pressure bc intrapleural pressure is more neg.
. At less step part of compliance curve
. Smaller inc. in volume for any change in intrapleural pressure
. Reduced expansion during inspiration

Question 4

Effect of change in ventilation and perfusion in pulmonary system

Answer 4

. Perfusion inc. but ventilation unchanged: more CO2 in blood delivered, more O2 moved from alveolus to blood so PACO2 inc. and PAO2 dec.
. Perfusion dec. but ventilation unchanged: less CO2 brought to alveolus in blood, less O2 removed, PACO2 dec., PAO2 inc.

Question 5

Ventilation to perfusion rations (V/Q)

Answer 5

. Rate of change in perfusion is greater Han corresponding rate of change in ventilation
. Ratio high at apex (over 1) more ventilation than perfusion
. Low at base (under 1) more perfusion than ventilation
. Overall normal ratio is 0.8

Question 6

Gas exchange in well perfused well ventilated alveolus

Answer 6

. PO2 and PCO2 entering pulmonary capillary will be 40 and 46 mmHg
. After gas exchange and equilibration, the blood leaving capillaries will have equilibrate to alveolar PO2 and PCO2
. End capillary blood PO2 100 mmHg and PCO2 40 mmHg

Question 7

Gas exchange in alveolus w/ reduced ventilation

Answer 7

. Blood leaving capillary will be the same as blood entering capillary (40 and 46 mmHg)
. No diffusion gradient for O2 and CO2 an no gas exchange if there is full obstruction

Question 8

Gas exchange in alveolus w/ reduced perfusion

Answer 8

. No blood flow through pulmonary capillary and no gas exchange
. Alveolar PO2 and PCO2 will be the same as inspired air (150 mmHg and 0 mmHg)

Question 9

O2/CO2 diagram findings w/ V/Q ratios

Answer 9

. Low V/Q ratio: high PCO2 and low PO2, depress alveolar PO2 more than they raise alveolar PCO2
. High V/Q: high PO2 and low PCO2, lower alveolar PCO2 more than they inc. PO2

Question 10

Regions lags exchange in lung

Answer 10

. Apex: high V/Q ratio but low perfusion so gas exchange is reduced, PAO2 high, PACO2 low, blood that does pass through capillaries will be well oxygenated
. Base: low V/Q: more ventilation than apex but even more perfusion, low PAO2 and high PACO2, more O2 uptake and CO2 output occurs

Question 11

Causes of non-uniform ventilation

Answer 11

. Uneven airway resistance: collapse, bronchoconstriction, dec. lumen diameter from inflammation, obstruction by mucus, compression of airways
. Uneven lung compliance: fibrosis, regional variations of surfactant production, pulmonary congestion or edema, emphysema, diffuse or regional atelectasis, pneumothorax, compression by tumors/cysts

Question 12

Causes of non-uniform perfusion

Answer 12

. Pulmonary emboli or thrombosis
. Compression of pulmonary vessels 
/ destruction or occlusion of pulmonary vessels
. Pulmonary vascular hypotension
. Collapse or over expansion of alveoli

Question 13

Alveolar-arterial O2 difference

Answer 13

. PO2 of expired gas is higher then the PO2 of arterial blood
. Normally difference is around 5-10 mmHg

Question 14

V/Q inequality as a cause of CO2 retention

Answer 14

. Effect of V/Q inequality is to reduce both O2 uptakes and CO2 output
. V/Q mismatch eventually causes both hypoxemia and CO2 retention
. Inc. in arterial PCO2 will cause an inc. in ventilation through stimulation of the central chemoreceptors
. Inc. in ventilation causes PCO2 to dec. back down to normal levels
. When V/Q mismatch is chronic ventilation may not be enough

Question 15

Right-to-left shunt

Answer 15

. Blood that enters systemic system w/o first passing through ventilated areas of lung
. 5% of blood does this
. Sources: bronchial circulation that supplies intrapulmonary structures and empties into pulmonary vv., thesbian circulation that supplies LV and empties directly into LV
. Effect: further depresses the PaO2 below that of the mixed alveolar PO2

Question 16

How is V/Q inequality distinguished from a true shunt?

Answer 16

. Pure O2 will not correct the depression of the arterial O2 in a true shunt but will in V/Q mismatch
. Blood never passes over alveolar surfaces so the shunted blood is never exposed to supplemental O2