How do pulmonary and systemic circulations differ in pressure and vascular structure?

• A. Pulmonary pressure higher than systemic
• B. Systemic has much lower pressure with more compliant vessels
• C. Capillary networks are only in systemic circulation
• D. Pulmonary has much lower pressure (~25/10 mmHg) and thinner walls

Answer: (D)

Pulmonary circulation is built for gas exchange and operates at a low pressure with vessels that are relatively thin-walled. The blood in the lungs travels through a vast capillary network surrounding the alveoli, so keeping pressures modest (around a few dozen mmHg in the pulmonary arteries) helps prevent capillary rupture and edema while still allowing adequate diffusion of oxygen and carbon dioxide. The walls don’t need the thick, muscular structure that systemic arteries use to push blood through the entire body, so they’re thinner and more compliant.

Systemic circulation, by contrast, must distribute blood to every tissue, which requires higher pressures to overcome the resistance of the whole vascular network. That means thicker, more muscular arterial walls to generate and sustain that pressure and to regulate flow to different organs. Capillaries are present in both circuits, including the alveolar capillaries in the lungs, so it’s not accurate to say capillary networks exist only in systemic circulation. The lower, thinner-walled, low-pressure nature of the pulmonary vessels is the hallmark difference reflected in the statement.