During the last trimester, maternal plasma volume expands by approximately 40–50% while red cell mass increases only 20–30%, resulting in the physiological anaemia of pregnancy. What is the primary stimulus for this disproportionate plasma expansion?

• A Progesterone and oestrogen cause systemic vasodilation, reducing mean arterial pressure and activating the renin-angiotensin-aldosterone system, leading to Na⁺ and water retention that preferentially expands plasma volume ✓
• B Human placental lactogen directly stimulates renal tubular Na⁺ reabsorption without activating the renin-angiotensin system
• C Fetal erythropoietin crosses the placenta to suppress maternal erythropoiesis, creating a relative red cell deficit
• D Increased maternal GFR dilutes plasma proteins, reducing oncotic pressure and driving fluid from interstitium into plasma

Explanation

Oestrogen and progesterone (primarily oestrogen-mediated nitric oxide and prostacyclin production) cause systemic arterial vasodilation from early pregnancy. This reduces effective arterial filling, activating baroreceptors that stimulate the renin-angiotensin-aldosterone axis and ADH, resulting in substantial Na⁺ and water retention. The retained fluid preferentially expands plasma volume (because oncotic forces initially resist oedema formation). Red cell mass also increases but lags behind because erythropoiesis is slower to respond than fluid retention, creating the haemodilution of pregnancy. Fetal erythropoietin does not cross the placenta to suppress maternal erythropoiesis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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