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?
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?
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
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:
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