The effects of these two drugs are additive and using them together could precipitate a dangerous drop in blood pressure. Link to page devoted to the many physiological roles of nitric oxide. Under some circumstances, capillary beds may open without others closing in compensation. Although the volume of blood remains unchanged, blood pressure declines abruptly as blood pools in the capillary beds. If untreated shock is usually fatal.
Shock can also result from severe bleeding.
The heart can only pump as much blood as it receives. The tissues fail to receive enough oxygen. This is especially critical for the brain and the heart itself. To cope with the problem, arterioles constrict and shut down the capillary beds - except those in the brain and heart. This reduces the volume of the system and helps maintain normal blood pressure.