Maternal acid-base changes in pregnancy

Sex differences in breathing

Breathing is a vital behavior that ensures both the adequate supply of oxygen and the elimination of CO₂, and it is influenced by many factors. Despite that most of the studies in respiratory physiology rely heavily on male subjects, there is much evidence to suggest that sex is an important factor in the respiratory control system, including the susceptibility for some diseases. These different respiratory responses in males and females may be related to the actions of sex hormones, especially in adulthood. These hormones affect neuromodulatory systems that influence the central medullary rhythm/pontine pattern generator and integrator, sensory inputs to the integrator and motor output to the respiratory muscles. In this article, we will first review the sex dependence on the prevalence of some respiratory-related diseases. Then, we will discuss the role of sex and gonadal hormones in respiratory control under resting conditions and during respiratory challenges, such as hypoxia and hypercapnia, and whether hormonal fluctuations during the estrous/menstrual cycle affect breathing control. We will then discuss the role of the locus coeruleus, a sexually dimorphic CO₂/pH-chemosensitive nucleus, on breathing regulation in males and females. Next, we will highlight the studies that exist regarding sex differences in respiratory control during development. Finally, the few existing studies regarding the influence of sex on breathing control in non-mammalian vertebrates will be discussed.

The Role of Placental Hormones in Mediating Maternal Adaptations to Support Pregnancy and Lactation

During pregnancy, the mother must adapt her body systems to support nutrient and oxygen supply for growth of the baby in utero and during the subsequent lactation. These include changes in the cardiovascular, pulmonary, immune and metabolic systems of the mother. Failure to appropriately adjust maternal physiology to the pregnant state may result in pregnancy complications, including gestational diabetes and abnormal birth weight, which can further lead to a range of medically significant complications for the mother and baby. The placenta, which forms the functional interface separating the maternal and fetal circulations, is important for mediating adaptations in maternal physiology. It secretes a plethora of hormones into the maternal circulation which modulate her physiology and transfers the oxygen and nutrients available to the fetus for growth. Among these placental hormones, the prolactin-growth hormone family, steroids and neuropeptides play critical roles in driving maternal physiological adaptations during pregnancy. This review examines the changes that occur in maternal physiology in response to pregnancy and the significance of placental hormone production in mediating such changes.

Plasma osmolality and urinary concentration and dilution during and after pregnancy: evidence that lateral recumbency inhibits maximal urinary concentrating ability

In nine women studied serially before conception and through the first trimester, plasma osmolality (Posm) started to decline with the first missed menstrual period, was significantly decreased by the fifth week of pregnancy and was 10 mosmol/kg lower than preconception values by the tenth week, changing little thereafter. Changes in plasma sodium (and its attendant anion) accounted for the majority of the decrement. In separate studies, urinary concentration and dilution, assessed by water deprivation and loading, studied in nine women during their last trimester and again 10-12 weeks post partum. Basal Posm was 9 mosmol/kg lower in the last trimester than post partum (p less than 0.001) but the results of concentration and dilution tests were similar during and after pregnancy. Basal urinary arginine vasopressin (AVP) excretion was similar during and after pregnancy and water loading suppressed AVP excretion in both pregnant and postpartum women. An unanticipated observation was that lateral recumbency interfered with urinary concentration tests in both pregnant and postpartum women. The results demonstrate that the decrement in Posm during pregnancy is an early event. The data (urinary AVP excretion, its suppression by water loading and normal concentration and dilution despite a lower plasma tonicity) also suggest that human pregnancy is accompanied by a resetting of the threshold for vasopressin secretion to a lower Posm.

Plasma solute concentrations during gonadotrophin-induced ovulation

The effects of ovulation induction on some plasma solute concentrations were investigated serially in eleven women, nine of whom conceived in that cycle. Increasing oestrogen output during the follicular phase was associated with decreasing levels of osmolality, urea, total protein and the albumin free protein fraction. Smaller decreases in the concentrations of sodium, potassium, chloride, carbon dioxide combining power (CO2CP) and albumin were not statistically significant. The luteal phase was marked by a rise in osmolality and a fall in CO2CP but no other significant changes. Levels of osmolality, sodium and potassium fell in early pregnancy when oestrogen output was further increased. These results appear consistent with an oestrogen induced expansion of the extracellular fluid compartment by retention of water in excess of solute, which might also account for the similar changes in pregnancy.