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In contrast to the adult, who is surrounded by air with a changing environmental temperature, the developing fetus is surrounded by amniotic fluid at 37°C. The fetus relies upon the maternal circulation for provision of nutrients, removal of metabolites and respiration, including oxygen supply and carbon dioxide removal. The fetus is a rapidly developing organism, but it exists in a state of relative hypoxia. The developing brain is the most sensitive organ to hypoxia, and the fetal cardiovascular system ensures delivery of the highest oxygenated blood to the brain, whilst blood is distributed to the remaining organs depending on local requirements. Comparison of the Adult and Fetal Circulations In health, the adult circulation contains approximately 5 litres (66 mL/kg) of blood, equivalent to 6 to 8 per cent of body weight. Eighty per cent of the circulating volume is in the systemic veins, right side of the heart and pulmonary circulation. The cardiac output is approximately 5 litres/min (~66 mL/kg/min) secondary to a left ventricle contracting 70 times per minute and ejecting a stroke volume of 70 mL with each contraction. Deoxygenated blood returns to the right ventricle, and the entire cardiac output passes via the pulmonary arteries to the lungs for the purposes of gas exchange. Oxygenated blood returns via the pulmonary veins to the left ventricle and systemic circulation.
The right and left sides of the heart are separate, and the pulmonary and systemic circulations are in series. The pulmonary circulation is low pressure (25/10 mmHg) compared to the systemic circulation (120/80 mmHg). In the fetus, gas exchange occurs at the placenta. For oxygenated blood to return to the systemic circulation and deoxygenated blood to the placenta there are several communications or shunts present, and the two circulations are in parallel with both ventricles contributing to the total cardiac output. The fetal circulation is illustrated in Figure 1.1.
Chapisha Maoni