How do ANP and NO interact to regulate blood pressure?

• A. ANP has no effect on blood pressure; NO raises BP
• B. ANP increases vasodilation and reduces blood volume; NO also promotes vasodilation; together they can decrease BP
• C. ANP causes vasoconstriction; NO causes vasoconstriction
• D. ANP increases blood volume; NO increases viscosity

Answer: (B)

The main idea is that both ANP and NO promote vasodilation and together they lower blood pressure by reducing vascular resistance and circulating blood volume. ANP is released when the atria are stretched and acts to make the kidneys excrete sodium and water, which lowers blood volume (preload) and, through suppressing the RAAS, can further help reduce pressure. In blood vessels, ANP also raises cGMP in smooth muscle, causing relaxation and vasodilation. NO, produced by endothelial cells, freely diffuses into smooth muscle and activates guanylate cyclase to raise cGMP, leading to smooth muscle relaxation and reduced vascular resistance. With both systems driving vasodilation and ANP reducing volume, the net effect is a decrease in blood pressure.