I have enough content from the textbook. Let me write the short note.

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1. Low-pressure, high-flow, low-resistance system The pulmonary circulation carries the entire cardiac output (unlike systemic circulation, which is high-pressure). Systolic pulmonary artery pressure averages 25 mm Hg, diastolic 8 mm Hg, and mean pulmonary arterial pressure only 15 mm Hg - about one-sixth of the mean aortic pressure. Mean pulmonary capillary pressure is about 7 mm Hg, and mean left atrial pressure is about 2 mm Hg. This low pressure is possible because pulmonary vascular resistance is very low, allowing the thin-walled right ventricle to pump the same output as the left ventricle with far less work.

2. High distensibility and compliance (short, thin-walled vessels) The pulmonary artery has a wall thickness only one-third that of the aorta, and pulmonary vessels (even small arteries and arterioles) have larger diameters than corresponding systemic vessels. This makes the pulmonary arterial tree highly distensible, with a compliance of about 7 mL/mm Hg (similar to the entire systemic arterial tree), allowing it to accommodate the right ventricular stroke volume. Pulmonary arterial distensibility is about six times that of systemic arteries, since pulmonary vessels normally operate at roughly one-sixth the systemic pressure.

3. Acts as a blood reservoir and shows unique hypoxic vasoconstriction Pulmonary blood volume is about 450 mL (~9% of total blood volume), of which ~70 mL is in the capillaries, the rest divided between arteries and veins; this volume can vary from half to twice normal, letting the lungs act as a blood reservoir (e.g., during forced expiration or hemorrhage compensation). Uniquely, a fall in alveolar O2 causes local pulmonary vasoconstriction (opposite to the vasodilation seen in systemic vessels with hypoxia), diverting blood away from poorly ventilated alveoli to optimize ventilation-perfusion matching.