FIGURE 4-1 2 Changes in lung volumes with pregnancy. The most significant changes are reduction in functional residual capacity (FRC) and its subcomponents, expiratory reserve volume (ERV) and residual volume (RV), as well as increases in inspiratory capacity (IC) and tidal volume (). (Redrawn from Hegewald MJ, Crapo RO: Respiratory physiology in pregnancy. Clin Chest Med 32(1):1,2011.) of nasal congestion or total nasal resistance did not signiicantly difer among trimesters or compared with nonpregnant controls.
he amount ofoxygen delivered into the lungs by the increased tidal volume clearly exceeds oxygen requirements imposed by pregnancy. Moreover, the total hemoglobin mass and, in turn, total oxygen-carrying capacity rise appreciably during normal pregnancy, as does cardiac output. Consequently, the maternal arteriovenous oxygen diference is diminished. Oxygen consumption grows approximately 20 percent during pregnancy, and it is approximately 10 percent higher in multifetal gestations (Ajjimaporn, 2014). During labor, oxygen consumption increases 40 to 60 percent (Bobrowski, 2010).
A greater awareness of a desire to breathe is common even early in pregnancy (Milne, 1978). his may be interpreted as dyspnea, which may suggest pulmonary or cardiac abnormalities when none exist. This physiological dyspnea, which should not interfere with normal physical activity, is thought to result from greater tidal volume that lowers the blood Peo2 slightly and paradoxically causes dyspnea. he increased respiratory efort during pregnancy, and in turn the reduction in the partial pressure ofcarbon dioxide in blood (Peo2), is likely induced in large part by progesterone and to a lesser degree by estrogen. Progesterone acts centrally, where it lowers the threshold and raises the sensitivity ofthe chemoreflex response to carbon dioxide (C02) 0ensen, 2005).
To compensate for the resulting respiratory alkalosis, plasma bicarbonate levels normally drop from 26 to 22 mmollL. Although blood pH is increased only minimally, it does shift the oxygen dissociation curve to the left. his shift increases the ainity of maternal hemoglobin for oxygen-the Bohr fectthereby lowering the oxygen-releasing capacity of maternal blood. This is ofset because the slight pH rise also stimulates an increase in 2,3-diphosphoglycerate in maternal erythrocytes.
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This shifts the curve back to the right (Tsai, 1982). hus, reduced Peo2 from maternal hyperventilation aids CO2 (waste) transfer from the fetus to the mother while also aiding oxygen release to the fetus.
he urinary system undergoes several remarkable changes in pregnancy (Table 4-5). Kidney size grows approximately 1.0 cm (Cietak, 1985). Both theglomerulariltration rate (CPR) and renal plasma low increase early in pregnancy. he GFR rises as much as 25 percent by the second week after conception and 50 percent by the beginning of the second trimester. his hyperiltration results from two principal factors. First, hypervolemia-induced hemodilution lowers the protein concentration and oncotic pressure ofplasma entering the glomerular microcirculation. Second, renal plasma flow increases by approximately 80 percent before the end of the irst trimester (Conrad, 2014b; Odutayo, 2012). As shown in Figure 4-13, , 25
FIGURE 4-13 Percentage increment in glomerular filtration rate (GFR) and renal plasma flow (RPF) across gestation and in the puerperium. (Data from Odutayo, 201o2.)
TABLE 4-5. Renal Changes in Normal Pregnancy