L34 – Pulmonary Circulation Flashcards Preview

MBBS I CPRS > L34 – Pulmonary Circulation > Flashcards

Flashcards in L34 – Pulmonary Circulation Deck (35):

Describe the Distribution of blood flow in Upright lung. Explain.

 Apical region: less perfusion
 Basal region: better perfusion

effect of gravity > hydrostatic pressure differences within blood vessels > uneven distribution of blood flow


What is Pulmonary artery and pulmonary vein pressures?

Pa= 15mmHg
Pv= 5mmHg


The upright lung can be divided into 3 zones. Explain the little perfusion to Zone 1 (apical).

Alveolar pressure > Pul. Arterial pressure > Pul. Venous pressure

Pulmonary artery enters lung at zone 2 level. Pa decreases higher up the lung (flow against gravity)

Ventilated but unperfused (capillaries squashed flat by higher alveolar pressure outside capillaries)


What is total height of lung and pressure difference?



Does alveolar pressure change over entire height of upright lung?

No, remains constant


Explain the perfusion in zone 2 of upright lung.

p.arterial pressure > Alveolar pressure > p. venous pressure

Less effect of gravity on arterial blood and pulmonary artery enters lung at Zone 2 level (doesnt have to travel up against gravity)

Alveolar P. remains constant and arterial pressure increases > increase hydrostatic pressure > dilate capillaries > increase perfusion

Arterial pressure not too high so some compression near venous side


Explain perfusion in zone 3 of upright lungs.

p. arterial pressure > p. venous pressure > Alveolar pressure

Hydrostatic pressure inside capillary grows further down the lung, flow increases as alveolar pressure remains constant

Increased intravascular hydrostatic pressure on both Pa, Pv , distend vessels = less resistance

Opening of closed capillaries = more blood pass through


How does alveolar oxygen tension regulate blood flow in pulmonary circulation?

 Increase PAO2 = vasodilation (dilates small arteries, arterioles)

 Decrease PAO2 = vasoconstriction (constricts small arteries, arterioles)


Why does hypoxia cause vasoconstriction?

Hypoxic vasoconstriction to divert blood flow away from hypoxic regions

> reduce deleterious effects on gas exchange (divert blood to where ventilation is better, better use of blood)


What are the two types of reduction in PAO2?

-Local reduction > local hypoxic vasoconstriction

-Global reduction > generalized pulmonary vasoconstriction (affects entire lung)


How can Right heart failure arise from global PAO2 regulation to blood flow?

Cor pulmonale (= right heart disorder caused by primary lung disorder)

Global Hypoxic vasoconstriction >generalized pulmonary vasoconstriction

> Increase pulmonary arterial pressure > increase resistance of flow for right heart > develop right heart hypertrophy > RH failure


Efficiency** (not rate) of gas exchange in lungs is affected by which 4 factors?

1. Alveolar ventilation
2. Perfusion (blood flow)
3. Gas diffusion (across alveolocapillary membrane)
4. Matching of alveolar ventilation and perfusion (V/Q matching)


At equilibrium, the concentration of CO2 in lungs depend on what?

Blood flow (Q) & Alveolar Ventilation (V)

e.g. hypercapnia requires both increase in V and Q


What is critical for theoretical normal gas exchange?

Ventilation matches blood flow within various regions of the lung

If mismatched in various regions of the lung > impairment of gas transfer


What is Normal alveolar ventilation/perfusion ratio for the lung unit as a whole?


Normal V = 4L/min, Q= 5L/min


What does O2 addition to lungs depend on?



What does perfusion (Q) depend on?

Cardiac output


What causes V/Q to become zero? How does zero V/Q change the pO2 and pCO2 in alveolar?

Airway obstruction > Ventilation becomes zero, perfusion unchanged> V/Q becomes zero

pO2 drops, pO2 in alveolar becomes close to pO2 in venous blood

pCO2 rises, pCO2 in alveolar becomes close to pCO2 in venous blood


What causes V/Q to become infinite? How?

Blood flow constriction > Ventilation unchanged, perfusion decreases to zero> V/Q becomes infinite

pO2 rises, pO2 in alveolar becomes close to atmospheric pO2

pCO2 drops, pCO2 in alveolar becomes close to atmospheric pCO2 (negligible amount)


How does regional V/Q change across the lung height?

Regional differences in V/Q increases from bottom to apex,

apex has least perfusion so V/Q is highest

Both ventilation and perfusion increases further down the upright lung

*Change in blood flow is more significant down the lung than change in ventilation


What is PO2 and PCO2 of the mixed effluent blood from the lungs? Explain.

Effluent blood is an average/ mixture of all lung regions of different V/Q ratios

assuming each of the three alveolar types from the 3 zones (high, normal and low V/Q) contribute the same amount to effluent blood vol. %.

(in reality, basal region contributes most)


How does O2 dissociation correlate with difference in V/Q across lung height, leading to an averaged effluent blood vol. %?

O2 dissocation: 60mmHg = intersection between flat and steep section of curve

Alveoli with high V/Q (150mmHg) has O2 vol. % increase of 0.5 compared to normal V/Q

Alveoli with low V/Q (40mmHg) has O2 vol.% decrease of 4.9 compared to normal V/Q

Gain in O2 cannot compensate loss so PaO2 is average of high, normal and low V/Q alveoli


In reality, the contribution to effluent blood is not equal between high, normal and low V/Q alveoli. Effect on effluent blood?

Basal region with low V/Q contributes most to effluent blood

Lower V/Q alveoli means larger decrease in O2 vol. % than gain in O2 from high V/Q alveoli

Some hypoxemia remains, meaning effluent blood is not as high in O2 conc as inspired air


In healthy lungs, V/Q mismatching causes slight "hypoxemia" due to which two effect (limitations on gas composition)? How is this compensated?

1) Sigmoid shape of O2 dissociation curve
2) Blood leaving the lung mainly comes from basal region

Ventilatory compensation


What is shape of CO2 dissociation curve?



How does V/Q correlate with pCO2?

Low V/Q = high CO2
High V/Q = low CO2

CO2 retention is unrelated to CO2 dissociation curve


What causes CO2 retention?

Gain in CO2 by lower V/Q alveoli = loss of CO2 by high V/Q alveoli

Since Most efferent blood comes from basal region with low V/Q >> overall high pCO2 blood


Compare the ventilatory compensation between CO2 retention and Hypoxemia. Why?

CO2 retention: compensation is COMPLETE

Hypoxemia: compensation is INCOMPLETE

Due to the shape of their dissociation curves


What is the overall consequence of V/Q mismatching across the lung in normal subject?

overall V/Q tend towards lower

Lower O2, higher CO2 in reality


Explain the partial ventilatory compensation caused by hypoxemia.

Low V/Q alveoli have low pO2, High V/Q alveoli have high pO2

Increase in ventilation leads to negligible increase in pO2 in high V/Q alveoli and some increase in pO2 in low V/Q alveoli (due to O2 association curve)

Hypoxemia never fully compensated but pO2 still within normal range


Explain the complete ventilatory compensation caused by CO2 retention.

CO2 retention is not related to CO2 dissociation curve, but ventilatory compensation is related.

Chemoreceptors sense pCO2 rise > Increase in ventilation causes decrease in pCO2 in ALL ALVEOLI > restore normal pCO2


How can pathological conditions cause respiratory failure, severe hypoxemia and hypercapnia?

Lung disease causing abnormal ventilation and inability of ventilatory compensation
Cardiovascular disease causing uneven perfusion
severe V/Q MISMATCH = Respiratory failure


Why should V/Q inequality not be measured by decrease in PaO2?

Decrease in PaO2 is unreliable for measurement of V/Q mismatch

Although it can be cause by V/Q mismatch, other factors include:

-Diffusion impairment


What are the 2 types of shunt?

 Physiological: artery directly to vein, no gas exchange, V/Q to zero

 Anatomical: arterial-venous anastomosis, alveolar has no ventilation but only blood flow


What are the ways to measure V/Q inequality? (check L09)

1) Index for gas exchange function: difference in ideal PaO2 and actual PaO2

2) Physiological dead space > More dead space (with ventilation but no blood supply) > V/Q tend to infinite

3) Physiological shunt: Oxygen test

Decks in MBBS I CPRS Class (78):