Answers to Quiz 106 · Pulmonary Circulation and Ventilation-Perfusion Ratio
The mean pressure in the pulmonary artery is about:
Answer: 15 mmHg
Mean pulmonary arterial pressure is about 15 mmHg, which is what makes this a low-pressure circuit. 8–10 mmHg is the pulmonary capillary hydrostatic pressure and 25 mmHg the plasma oncotic pressure. Revise pulmonary circulation
Which of these is NOT true of the pulmonary circulation?
Answer: Its vascular resistance is about ten times the systemic value
Pulmonary vascular resistance is about one-tenth of systemic, not ten times it. The other three are all genuine special features. Revise pulmonary circulation
Pulmonary vascular resistance is lowest at:
Answer: Functional residual capacity
Resistance is minimal at FRC and rises on both sides of it: at high volumes wide alveoli compress the alveolar vessels, and at low volumes a positive pleural pressure compresses the extra-alveolar vessels. Revise pulmonary circulation
When cardiac output rises, pulmonary vascular resistance falls. The main mechanism is:
Answer: Opening of capillaries that were partly closed
Capillary recruitment, mostly in the poorly perfused apex, is the primary mechanism. Capillary distension also lowers resistance but is the secondary one, and there is almost no resting sympathetic tone to withdraw. Revise pulmonary circulation
In the middle zone (Zone 2) of an upright lung, blood flow is determined by:
Answer: The difference between arterial and alveolar pressure
In Zone 2 venous pressure is below alveolar pressure, so the veins are pinched from outside and blood dams in the capillaries before escaping — the waterfall effect. Flow driven by the arterio-venous difference is Zone 3. Revise pulmonary circulation
Alveolar hypoxia causes:
Answer: Vasoconstriction of the small pulmonary arteries
Hypoxia constricts pulmonary vessels, the opposite of its dilating action in the systemic circulation: it inhibits K⁺ channels, the muscle depolarizes, voltage-gated Ca²⁺ channels open and Ca²⁺ enters. A high CO₂ and a low pH accentuate it. Revise pulmonary circulation
Which of these is true of the bronchial circulation?
Answer: It can grow new vessels, providing collaterals when pulmonary flow is blocked
Angiogenesis is a property of the bronchial, not the pulmonary, circulation, and it lets parenchyma survive an embolus. It supplies only the conducting airways, its pressure is near aortic and its resistance is much higher; it normally takes 1% of cardiac output. Revise pulmonary circulation
Fluid is normally absorbed rather than filtered at the pulmonary capillary because:
Answer: Capillary hydrostatic pressure of 8 to 10 mmHg is well under the oncotic pressure of 25 mmHg
The low pulmonary capillary pressure against a 25 mmHg oncotic pressure favours absorption. Surface tension lowers interstitial pressure and so favours filtration, and the lymphatics drain the interstitium rather than the capillary. Revise pulmonary edema and drowning
A patient with long-standing mitral stenosis develops pulmonary edema. The mechanism is:
Answer: A raised pulmonary capillary hydrostatic pressure
Mitral stenosis raises pulmonary venous and therefore capillary pressure, the commonest cause of pulmonary edema; treatment aims to lower it with diuretics, digitalis and vasodilators. Low oncotic pressure belongs to hypoproteinemia, raised permeability to endotoxin or head injury, and surfactant loss to ARDS. Revise pulmonary edema and drowning
In fresh-water drowning, death is usually due to:
Answer: Ventricular fibrillation
Aspirated fresh water is drawn into the blood by the low capillary hydrostatic and high oncotic pressure, dilutes the plasma and haemolyses red cells, giving hyperkalemia and hyponatremia; hyperkalemia with hypoxemia triggers fibrillation. Pulmonary edema and asphyxia belong to salt water. Revise pulmonary edema and drowning
In salt-water drowning:
Answer: Hypertonic fluid in the alveoli produces pulmonary edema, and death is from asphyxia
Sea water is rich in Na⁺ and Cl⁻, so it is hypertonic and pulls fluid into the alveoli, and the patient asphyxiates. Haemolysis, plasma dilution and fibrillation are the fresh-water sequence. Revise pulmonary edema and drowning
The normal ventilation-perfusion ratio for the whole lung at rest is:
Answer: 0.8
Alveolar ventilation 4 L/min divided by pulmonary blood flow 5 L/min gives 0.8. 0.6 is the value at the base and about 3 the value at the apex. Revise ventilation–perfusion ratio
Going from apex to base of an upright lung:
Answer: Both ventilation and perfusion rise, but perfusion rises more, so the ratio falls
Perfusion varies about five-fold from apex to base while ventilation varies only about two-fold, so the ratio falls from roughly 3 at the apex to 0.6 at the base. Both increase downwards; neither falls. Revise ventilation–perfusion ratio
Pulmonary tuberculosis is commonest at the apex of the lung because there:
Answer: The high ventilation-perfusion ratio gives a high alveolar oxygen tension
A ratio of about 3 at the apex means ventilation outruns perfusion, so alveolar PO₂ is high, and Mycobacterium tuberculosis grows better in plenty of oxygen. Apical blood flow is the lowest in the lung, not the highest. Revise ventilation–perfusion ratio
The normal physiological shunt of the lung is due to:
Answer: Bronchial venous blood draining into the pulmonary veins
Deoxygenated bronchial venous blood joins oxygenated pulmonary venous blood, and normally accounts for about 2% of cardiac output. A septal defect and a direct artery-to-vein connection are anatomical shunts, and ventilated but unperfused alveoli are dead space. Revise ventilation–perfusion ratio
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