Respiratory physiology underpins every aspect of oxygen delivery to both the mother and the developing fetus. During pregnancy, maternal metabolic rate increases significantly to support the growing placental unit and fetal tissues. Understanding how gases move from the atmosphere into the maternal lungs, across the placental barrier, and into fetal circulation is a core competency for Nigerian midwives. This physiological exchange relies on basic physical laws of pressure and diffusion rather than active cellular pumping of gas molecules.
Gas molecules in a mixture exert their own pressure, known as partial pressure. In external and internal respiration, gases always travel down a partial pressure gradient—moving from a region of high partial pressure to a region of low partial pressure.
If fluid builds up in the lungs, as seen in severe pulmonary edema or eclamptic complications, the diffusion distance increases, slowing down this vital transfer.
External respiration takes place across the respiratory membrane inside the lungs. Deoxygenated blood arriving from the pulmonary artery unloads carbon dioxide into the alveoli while simultaneously picking up fresh oxygen. The thinness of the alveolar wall ensures this exchange happens rapidly with every heartbeat. Midwives monitor this process indirectly by assessing maternal respiratory rate, depth, and capillary refill or oxygen saturation levels.
Only a tiny fraction of oxygen dissolves directly into blood plasma. The vast majority binds chemically to hemoglobin inside red blood cells. Each hemoglobin molecule can carry four oxygen molecules. As blood passes through active tissues, factors like warmth, acidity, and high carbon dioxide levels encourage hemoglobin to release its cargo—a phenomenon known as the Bohr effect. In pregnancy, a higher baseline cardiac output helps compensate for the high oxygen demands of maternal and fetal organs.
The body carries carbon dioxide away from metabolizing cells through three main pathways: dissolved in plasma, bound to amino acids on hemoglobin as carbaminohemoglobin, and mostly converted into bicarbonate ions ($HCO_3^-$). This bicarbonate conversion acts as a crucial chemical buffer system, keeping blood pH within a safe, narrow range. When venous blood reaches the lungs, these chemical reactions reverse, allowing carbon dioxide gas to reform and escape in the exhaled breath.
Internal respiration, or tissue respiration, involves the exchange of gases between systemic capillaries and surrounding body cells. Oxygen diffuses out of the oxygen-rich capillary blood into the tissue fluid and cells, while carbon dioxide diffuses in the opposite direction from the cells into the blood. In pregnant women, efficient internal respiration ensures that uterine tissues, the placenta, and all maternal organs receive the continuous oxygen supply necessary to maintain normal cellular function.
Fetal gas exchange occurs across the placental membrane, where maternal and fetal blood vessels lie in close proximity without mixing. Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin, allowing it to successfully pull oxygen away from maternal blood in the intervillous space. Midwives must remember that any drop in maternal blood oxygenation or severe maternal hypotension directly reduces oxygen transfer to the fetus, potentially leading to fetal distress.
Impaired oxygenation in obstetric patients can stem from respiratory infections, asthma exacerbations, pulmonary embolism, or severe hemorrhage. When gas exchange fails, maternal hypoxemia quickly translates into fetal hypoxia. Midwives play a crucial role in recognizing early signs of respiratory distress—such as tachypnea, cyanosis, or abnormal fetal heart rate tracings—and implementing prompt interventions like positioning the mother on her left side and administering supplemental oxygen.