In comparison to the systemic circuit, the pulmonary circuit _____

Question

BlackTom AI · Accepted Answer

Because cardiac output (volume flow rate) from the right and left ventricles must be essentially the same at steady state, the relationship ΔP = Q × R (where ΔP is pressure difference, Q is flow, R is resistance) shows that a much smaller pulmonary pressure gradient implies a much lower pulmonary resistance. Relevant concepts: blood flow (cardiac output) is equal through systemic and pulmonary circuits; systemic mean arterial-to-right-atrial pressure difference is large (~95 mmHg), while the pulmonary arterial-to-left-atrial pressure difference is small (~15 mmHg); vascular resistance depends on vessel length, radius, and number of parallel pathways and obeys R ∝ 1/r^4 (Poiseuille’s principle) qualitatively.

Step-by-step reasoning with typical values:
1. Take a representative cardiac output Q = 5 L/min (same through both circuits).
2. Systemic pressure gradient ΔP_sys ≈ mean arterial pressure (≈ 95 mmHg) minus right atrial pressure (≈ 0 mmHg) → ≈ 95 mmHg.
3. Pulmonary pressure gradient ΔP_pulm ≈ mean pulmonary artery pressure (≈ 15 mmHg) minus left atrial pressure (≈ 0 mmHg) → ≈ 15 mmHg.
4. Use R = ΔP / Q to compute resistances:
   - R_sys = 95 mmHg / 5 L·min^-1 = 19 mmHg·min·L^-1
   - R_pulm = 15 mmHg / 5 L·min^-1 = 3 mmHg·min·L^-1
5. Ratio R_pulm / R_sys = 3 / 19 ≈ 0.158, showing pulmonary resistance is much lower than systemic.

Why this is so physiologically: pulmonary vessels are shorter, have greater total cross-sectional area and are more compliant, and the pulmonary circuit is arranged to favor low pressure and low resistance to allow gas exchange without high capillary pressures that would cause pulmonary edema.

Why other choices are incorrect:
- "Has lower blood flow": incorrect because flow (cardiac output) is essentially equal in both circuits.
- "Is under greater pressure": incorrect; pulmonary pressures are much lower than systemic.
- "Has an equal pressure gradient to drive blood flow": incorrect; the pulmonary pressure gradient is much smaller.
- "None of these": incorrect because a correct statement is present.

Therefore, the correct answer is: has lower resistance to blood flow. This matches the provided answer.

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