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I'm not sure what the mechanism is, but the nurse is only partially right. An increase in pCO2 causes vasoconstriction in the pulmonary vasculature and vasodilation EVERYWHERE else. CO2 causes constriction in the pulmonary vasculature because an increase in pC02 sends the message there is damage/shunting here, sending blood here will not effectively oxygenate it so blood is preferentially sent to a different part of the lung where it will receive better oxygenation.
Vasoconstriction becomes a problem when it's abnormally strong or prolonged. Vaso-occlusion refers to a blockage of one or more blood vessel(s) in the hand or wrist. To understand how vasoconstriction and vaso-occlusion cause cold hands, think of a garden hose watering a lawn. The hose is like your blood vessels; the water is like your blood; and the lawn is like your hand. If you step on the hose, you decrease or block the flow of the water to the lawn.