Central nervous system regulation of reflex responses to hypotension during fetal life

The ability of the fetus to survive, grow, and successfully complete the tr ansition from fetal to neonatal life is critically dependent on the appropr iate regulation of fetal blood pressure, blood volume, and fluid dynamics. This is a short review of the physiological mechanisms controlling the feta l cardiovascular system, focusing mainly on the neural and endocrine elemen ts in the schema of cardiovascular function and control. The fetal cardiova scular system is arranged anatomically to provide for perfusion of the umbi lical-placental circulation, the organ of gas exchange of the fetus, and to largely bypass the lungs. Fetal blood volume and pressure, maintained at l evels that are appropriate for this function, are influenced by neural and endocrine control mechanisms, which are similar to, but quantitatively diff erent from, the adult animal. Baroreceptors and chemoreceptors located in t he carotid sinuses and aortic arch sense changes in blood pressure and bloo d gases and comprise the afferent limb of the major reflexes that maintain normal fetal blood pressure and volume. Fetal hypotension stimulates reflex decreases in fetal heart rate, which are apparently mediated by chemorecep tor input. Arginine vasopressin responses to hypotension are most likely me diated by baroreceptor input. Recent evidence suggests that the reflex resp onses to hypotension in the fetus are modulated by paracrine or endocrine f actors. For example, baroreceptor or chemoreceptor reflex pathways are modu lated by the endogenous production of prostanoids and by the preparturient changes in fetal plasma estrogen concentration.