In recent months we have discussed changes in the physiology of organ systems that come about during pregnancy. These have included the circulatory system, the respiratory system, the endocrine system, the urinary system, the digestive system, the musculoskeletal system, the blood clotting system, and systems to buffer the pH of the blood. But today we’re going to discuss an organ that is present in your body, only when you are pregnant. This organ is the placenta and it has five main functions. It has a function in respiration, it has a function in excretion, a function in immunity, a function in nutrition, and an endocrine function. Let’s go through each of these functions, beginning with respiration.
The developing fetus has developing lungs, but the fetal lungs never function in the exchange of the gases —oxygen and carbon dioxide— while the fetus is inside the mother. Rather, the fetus depends entirely on the placenta for its respiratory function. Of course, the mother has lungs, which she uses to receive oxygen from the surrounding environment and to exhale carbon dioxide. Through the maternal lungs, these gases exchange between the atmosphere and the mother’s blood, where nearly all of circulating oxygen is carried by a special protein called hemoglobin, which is packed into red blood cells. Fetal hemoglobin —the hemoglobin inside fetal red blood cells— has a higher affinity for oxygen than adult hemoglobin has. This means that oxygen prefers to stick to fetal hemoglobin than to the mother’s hemoglobin. Inside the placenta, the mother’s blood does not come into direct contact with the fetal blood (unless there is some tissue damage), but there are maternal capillaries (microscopic blood vessels) very close to fetal capillaries. Red blood cells do not exchange between these maternal and fetal capillaries, but the oxygen does. It moves from the maternal hemoglobin to the fetal hemoglobin. At the same time, carbon dioxide from the fetal blood moves into the maternal blood. In contrast with oxygen, which is carried almost entirely by hemoglobin, carbon dioxide is transported in the blood in a few different ways. A fraction of the carbon dioxide attaches to hemoglobin, but much of it dissolves in the blood and converts into acid (hydrogen ions), plus a base called bicarbonate (HCO3–) and this is how it travels in the blood. Another substance that exchanges through the placenta related to all of this is lactic acid.
As with respiration, the nutrition of the fetus moves entirely through the placenta. This nutrition consists mostly of blood sugar, glucose. The fetus uses glucose for energy and growth. Minerals also transfer from the mother through the placenta, but sometimes potentially harmful substances can pass through as well. Such substances may include medications, alcohol, and a plethora of harmful substances present in tobacco smoke.
The placenta also functions in excretion, like the kidneys of the mother. Waste products from the fetal blood that the placenta filters out include urea and creatinine. These waste products, along with those from the mother herself, are then filtered out of the mother’s body through her kidneys. As for the immune function of the placenta, the organ allows antibodies from the mother to transfer from the maternal blood into the fetal blood. Although maternal antibodies act as a kind of passive immunity for the fetus (immunity resulting from antibodies received from someone else), they provide the benefit of the mother’s well-developed, long-term immunity. This is true both during pregnancy, and for a time after birth, when maternal antibodies are still circulating within the newborn. On account of this phenomenon, newborns tend to be protected better against infectious agents to which the mother has been exposed for a long time than against infectious against to which she has been exposed very recently. This is because the antibodies resulting from long-term immune responses are more protective compared with antibodies resulting from a short-term immune response.
Finally, the placenta has an endocrine function. It acts like an actual endocrine gland. The placenta produces a hormone called human chorionic gonadotropin (hCG). This is the hormone for which you are tested to see if you are pregnant. Concentrations of hCG increase throughout early pregnancy and plateau around 10 weeks of gestation, then begin to fall. The job of hCG is to maintain a structure called the corpus luteum in the ovary, until the placenta is ready to take over the production of the hormones estrogen and progesterone. hCG is one of the things that causes nausea when you are pregnant. Another placental hormone is estrogen, which has various functions. It helps to soften tissues, so that they will be more flexible. This allows the muscles and ligaments of the uterus and pelvis to expand and for the cervix to expand and be ready for birth. Estrogen also allows the breasts and nipples to expand in preparation for breastfeeding. Another hormone from the placenta is progesterone. The placenta takes over the production of this hormone from the corpus luteum, beginning around 5 weeks after gestation. Progesteron has an important job maintaining pregnancy. It relaxes uterine muscles and also maintains the endometrium, the layer inside the uterus where the pregnancy implants and develops. Because of progesteron’s relaxing effects on tissues, it can also cause various problems for the mother, such as reflux of acid into the esophagus, constipation, and low blood pressure, skin flushing, and headaches.