A 43-year-old woman presents to her primary care provider with shortness of breath. She reports a 4-month history of progressively worsening difficulty breathing with associated occasional chest pain. She is a long-distance runner but has had trouble running recently due to her breathing difficulties. Her past medical history is notable for well-controlled hypertension for which she takes hydrochlorothiazide. She had a tibial osteosarcoma lesion with pulmonary metastases as a child and successfully underwent chemotherapy and surgical resection. She has a 10 pack-year smoking history but quit 15 years ago. She drinks a glass of wine 3 times per week. Her temperature is 98.6°F (37°C), blood pressure is 140/85 mmHg, pulse is 82/min, and respirations are 18/min. On exam, she has increased work of breathing with a normal S1 and loud P2. An echocardiogram in this patient would most likely reveal which of the following?

Right ventricular hypertrophy with a dilated pulmonary artery

Biventricular dilatation with a decreased ejection fraction

Left ventricular dilatation with an incompetent aortic valve

Left atrial dilatation with mitral valve stenosis

Left ventricular hypertrophy with a bicuspid aortic valve

Which of the following physiologic changes decreases pulmonary vascular resistance (PVR)?

Inhaling the entire vital capacity (VC)

Inhaling the inspiratory reserve volume (IRV)

Exhaling the entire vital capacity (VC)

Exhaling the expiratory reserve volume (ERV)

Breath holding maneuver at functional residual capacity (FRC)

Which mechanism is primarily responsible for the increase in pulmonary diffusing capacity during exercise?

Pulmonary capillary recruitment

A 37-year-old male presents to your clinic with shortness of breath and lower extremity edema. He was born in Southeast Asia and emigrated to America ten years prior. Examination demonstrates 2+ pitting edema to the level of his knees, ascites, and bibasilar crackles, as well as an opening snap followed by a mid-to-late diastolic murmur. The patient undergoes a right heart catheterization that demonstrates a pulmonary capillary wedge pressure (PCWP) of 24 mmHg. The patient is most likely to have which of the following?

Normal or decreased left ventricular end diastolic pressure (LVEDP)

Decreased pulmonary artery systolic pressure (PASP)

Increased left ventricular end diastolic pressure (LVEDP)

Increased pulmonary vascular compliance

A 35-year-old woman volunteers for a study on respiratory physiology. Pressure probes A and B are placed as follows: Probe A: between the parietal and visceral pleura Probe B: within the cavity of an alveolus The probes provide a pressure reading relative to atmospheric pressure. To obtain a baseline reading, she is asked to sit comfortably and breathe normally. Which of the following sets of values will most likely be seen at the end of inspiration?

Probe A: -6 mm Hg; Probe B: 0 mm Hg

Probe A: 0 mm Hg; Probe B: -1 mm Hg

Probe A: -4 mm Hg; Probe B: 0 mm Hg

Probe A: -4 mm Hg; Probe B: -1 mm Hg

Probe A: -6 mm Hg; Probe B: -1 mm Hg

A 33-year-old woman comes to the physician because of a 6-month history of worsening shortness of breath and fatigue. Her paternal uncle had similar symptoms and died of respiratory failure at 45 years of age. The lungs are clear to auscultation. Pulmonary function testing shows an FVC of 84%, an FEV1/FVC ratio of 92%, and a normal diffusion capacity. An ECG shows a QRS axis greater than +90 degrees. Genetic analysis shows an inactivating mutation in the bone morphogenetic protein receptor type II (BMPR2) gene. Which of the following is the most likely cause of this patient's symptoms?

Elevated pulmonary arterial pressure

Thickening of the interventricular septum

Fibrosis of the pulmonary parenchyma

Chronic intravascular hemolysis

Pulmonary circulation hemodynamics for USMLE Step 3: High-Yield Physiology Lessons

Pulmonary Circulation - The Low-Pressure Player

Hallmark: High-flow, low-pressure, low-resistance system.
- Pulmonary Artery Pressure: ~25/10 mmHg (Mean ~15 mmHg).
- Contrast with Systemic (Aortic) Pressure: ~120/80 mmHg.
Vessels: Shorter, more numerous, and more compliant than systemic vessels, leading to ↓ resistance.
Pulmonary Vascular Resistance (PVR): Calculated as $PVR = (P_{pulm. artery} - P_{left atrium}) / Cardiac Output$.
Gravity Effects: Blood flow is unevenly distributed.
- Apex: Lower flow (Zone 1 & 2 physiology).
- Base: Higher flow (Zone 3 physiology).

⭐ Hypoxic Pulmonary Vasoconstriction (HPV): Unlike systemic circulation, hypoxia in the lungs causes vasoconstriction. This unique reflex diverts blood away from poorly ventilated alveoli, optimizing ventilation/perfusion (V/Q) matching.

West’s Zones of Pulmonary Blood Flow and Pressures

Pulmonary Vascular Resistance - The Lung Squeeze

Pulmonary Vascular Resistance vs. Lung Volume

Pulmonary Vascular Resistance (PVR) is remarkably low (~1/10th of SVR). It varies with lung inflation in a distinct U-shaped relationship. The nadir, or lowest point of resistance, occurs at Functional Residual Capacity (FRC). Both high and low lung volumes increase PVR.

Low Lung Volumes (approaching RV): Extra-alveolar vessels are compressed and become tortuous, increasing resistance.
High Lung Volumes (approaching TLC): Alveolar capillaries are stretched thin and flattened by expanding alveoli, increasing resistance.

⭐ Alveolar hypoxia, unlike in systemic vessels, causes localized vasoconstriction. This "Hypoxic Pulmonary Vasoconstriction" is vital for V/Q matching, diverting blood from poorly ventilated lung regions to better-oxygenated ones.

Flow Regulation - The Oxygen Sensor

Hypoxic Pulmonary Vasoconstriction (HPV): A unique, local response where low alveolar oxygen ($P_{A}O_2$) triggers vasoconstriction. This is the opposite of systemic circulation.
Primary Goal: To optimize ventilation/perfusion (V/Q) matching by diverting blood from poorly ventilated lung segments to well-ventilated segments, thus maximizing gas exchange.

⭐ In conditions of global hypoxia, such as at high altitudes or in advanced COPD, widespread HPV can pathologically elevate pulmonary vascular resistance, leading to pulmonary hypertension.

Modulators:
- Vasoconstrictors: Endothelin-1, Thromboxane A₂.
- Vasodilators: Nitric Oxide (NO), Prostacyclin.

Clinical Tie-ins - Pressure Problems

Pulmonary Hypertension (PHTN): Defined by mean pulmonary arterial pressure (mPAP) > 20 mmHg at rest. Leads to right ventricular hypertrophy (RVH) and eventual cor pulmonale.
- Mechanisms:
  - ↑ Pulmonary vascular resistance (PVR): Idiopathic, heritable (BMPR2), or from chronic hypoxia (lung disease).
  - ↑ Left atrial pressure: Back-pressure from left heart failure (most common cause).
  - Chronic thromboembolism (CTEPH).

Cor Pulmonale: Healthy vs. Right-Sided Heart Failure

Pulmonary Edema:
- Cardiogenic: ↑ PCWP > 18 mmHg from ↑ LA pressure.
- Non-cardiogenic (ARDS): Normal PCWP; due to ↑ capillary permeability.

⭐ PCWP approximates left atrial pressure. It's normal in PHTN from lung disease/hypoxia but elevated in PHTN from left heart failure.

High‑Yield Points - ⚡ Biggest Takeaways

Pulmonary circulation is a low-pressure, low-resistance system, contrasting sharply with systemic circulation.

Pulmonary Vascular Resistance (PVR) is uniquely lowest at functional residual capacity (FRC).

Hypoxic pulmonary vasoconstriction is a critical mechanism to divert blood from poorly ventilated areas, optimizing V/Q matching.

Normal pulmonary artery pressure is ~25/10 mmHg; mean PAP >20 mmHg at rest defines pulmonary hypertension.

Perfusion is gravity-dependent, being highest at the lung bases.

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