5.4.1 Fetal and maternal blood vessels
In order to understand how maternal and fetal blood is brought close together in the placenta, but does not mix, you must first learn some basic facts about blood and blood vessels. The cardiovascular system consists of the heart, the blood vessels (veins and arteries – see Box 5.2), and the blood that circulates around the body. It is the transport system that supplies oxygen and nutritive substances absorbed from the gastrointestinal system (gut) to all the cells, tissues and organs of the mother’s body and that of the growing fetus, enabling them to generate the energy they need to perform their functions. It also returns carbon dioxide, the waste product of respiration, to the lungs where it is breathed out. The chemical processes that go on in the body generate many waste products, which the blood transports to the kidneys and liver, where they are removed. Other functions of the cardiovascular system include the regulation of body temperature, and the circulation and delivery of hormones and other agents that regulate body functions.
Box 5.2 gives some key facts about the interaction of the maternal and fetal cardiovascular systems, and their relationship to the placental circulation.
Box 5.2 Key facts about the fetal, maternal and placental circulation
Oxygenated blood: Blood that carries enough oxygen and nutrients to the body’s cells, tissues and organs to enable them to function normally.
Deoxygenated blood: Blood that contains less oxygen and a higher proportion of dissolved wastes and carbon dioxide than is found in oxygenated blood.
Umbilical arteries: Arteries usually carry oxygenated blood, but the two umbilical arteries (see Figure 5.4) collect deoxygenated blood from the body of the fetus and carry it to the placenta. The blood in the umbilical arteries is pumped to the placenta by the fetal heart.
Maternal veins: The mother’s veins collect deoxygenated blood from the placenta; as the blood passes through her liver and kidneys, dissolved wastes are removed — including those collected from the placenta by her endometrial veins. When the deoxygenated blood reaches the mother’s heart, it is pumped to her lungs to pick up more oxygen.
Maternal arteries: The mother’s arteries carry oxygenated blood around her body, pumped by her heart. Her endometrial arteries bring blood to her uterus and into the placenta, delivering oxygen from the mother’s lungs, and nutrients from her digestive system.
Umbilical vein: Veins usually carry deoxygenated blood, but the single umbilical vein (see Figure 5.4) carries oxygenated and nutrient-rich blood from the placenta and delivers it to the fetal heart, which pumps it around the body of the fetus.
5.4 The placenta
5.4.2 The placental circulation