Erythrocyte ion and water balance and membrane potential in the puerperium of normal pregnancy

Objective To examine the balance of erythrocyte ions and water during the rapid changes in plasma osmolality in the early puerperium, and during the subsequent period of sustained readjustment. Design A serial study from the third trimester of pregnancy to 20 weeks after delivery. Participants Thirty‐five primiparous women who had experienced no antenatal complications. Main Outcome Measures Plasma osmolality, erythrocyte hydration, potassium, chloride and sodium were measured and nondiffusible ion content and erythrocyte membrane potential calculated. Plasma sodium, potassium and chloride were also measured. Results During the first week after delivery plasma osmolality increased (280 (SEM 0.52)–289 (SEM 064) mosmol/kg; P < 0.001) but erythrocyte hydration did not decrease (2.060 (SEM 0.018)–2.067 (SEM 0.021) l/kg dry cells) because of an increase in total cell osmole content (577 (SEM 5.31)–597 (SEM 6.15) mosmol/kg dry cells; P= 0.001). This increase included nondiffusible anions, chloride and potassium. These changes in ionic balance did not affect membrane potential. After the first week of the puerperium and up to the 20th week, plasma osmolality was stable but erythrocyte osmole content and hydration both decreased. This was due to a decrease in nondiffusible anions and potassium with a smaller increase in chloride leading to a decrease in membrane potential (–14.31 (SEM 0.34) mV to −12.66 (SEM 0.28) mV; P < 0.001). Conclusions A rapid increase in intracellular osmoles can occur in the mature erythrocyte and probably precedes the decrease in plasma osmolality in the puerperium. Changes in erythrocyte homeostasis in the first week of the puerperium can be accounted for by alterations in nondiffusible anions. After the first week of the puerperium it appears that the functional organisation of the membrane is changing.