Vascular Resistance

The decrease in total peripheral resistance is far greater than can be accounted for by the low resistance of the uteroplacental circulation. There is a widespread decrease in tone of arterioles and larger arteries of the mesenteric, limb, cutaneous, and especially the renal (see below) circulations. Overall vasoconstrictor responses to administered angiotensin II and norepinephrine are attenuated, but to a considerably lesser extent than in uterine arterioles. It is important to note, however, that despite the overall decrease in vascular tone, local and systemic vascular regulatory responses remain operative. Reflex adjustments to changes in posture and to the requirements of exercise are only mildly compromised. Despite changing environmental demands, baroreceptor reflexes maintain blood pressure constant, but at a lower set-point.

The basis for the decrease in vascular resistance is a topic of active research. Clearly, the developing embryo must send out some signals to bring about these changes in vascular tone. Infusion of estrogens in nonpregnant human and animal subjects acutely decreases vascular resistance and produces changes that are similar, though less pronounced, to those seen in normal pregnancy. Human vascular endothelial cells express estrogen

10 20 30

Weeks after last menses

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