Development of neurohumoral control of fetal, neonatal, and adult cardiovascular functions.

Neurohumoral control of fetal, neonatal, and adult cardiovascular functions have been reviewed. Resting fetal heart rate remains fairly constant but neonatal heart rate declines progressively, reaching adult levels within six to eight weeks; systemic arterial pressure rises while pulmonary pressure falls to adult levels within the first week after birth. Sympathetic and parasympathetic control of circulatory functions matures at different rates during fetal and neonatal development; the sympathetic system becomes active earlier in fetal life than does the parasympathetic system. After birth, the parasympathetic tone of the resting heart rate rises to adult levels while adrenergic tone decreases. Despite changing autonomic activities, resting heart rate is set at given levels through alterations in intrinsic control. In the fetus, peripheral circulation is under neurohumoral tone of increasing magnitude; after birth, neurohumoral tone declines progressively, reaching levels comparable to those of adult nonpregnant sheep. Fetal cardiovascular response to neurotransmitters increases with age because of maturation of the effector system. The pulmonary bed responds primarily to acetylcholine whereas the systemic circulation responds to norepinephrine. After birth, the neonatal cardiovascular system becomes four to five times more sensitive to the action of neurotransmitters mainly because of closure of vascular shunts and elimination of umbilicoplacental circulation. In the neonate and adult, the pulmonary vascular bed loses its reactivity to neurotransmitters.