During normal pregnancy, cardiac output increases by approximately 40–50%. Which of the following contributes MOST to this increase?

• A Increased peripheral vascular resistance
• B Increased haemoglobin concentration increasing oxygen delivery
• C Increased heart rate and stroke volume due to expanded plasma volume and progesterone-mediated vasodilation ✓
• D Increased afterload from uterine compression of the aorta

Explanation

Pregnancy-related increases in cardiac output (to 6–7 L/min by 32 weeks) result from two main mechanisms: (1) plasma volume expansion of 40–50% (driven by RAAS activation from progesterone/oestrogen, increasing preload and stroke volume) and (2) heart rate increase of 15–20 bpm from progesterone-mediated reduction in peripheral vascular resistance. SVR actually falls in pregnancy due to progesterone, prostacyclin, and relaxin-mediated vasodilation — reducing afterload. Haemoglobin concentration typically falls (dilutional anaemia of pregnancy).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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