Final answer:
Vessel narrowing causes reduced blood flow and decreased supply of oxygen to tissues, leading to hypoxia. The cardiovascular system responds to changes in carbon dioxide and hydrogen ion levels by regulating cardiac output and peripheral vasoconstriction.
Step-by-step explanation:
Even without total blockage, vessel narrowing leads to ischemia-reduced blood flow-to the tissue region "downstream" of the narrowed vessel. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function. The chemoreceptors respond to increasing carbon dioxide and hydrogen ion levels (falling pH) by stimulating the cardioaccelerator and vasomotor centers, increasing cardiac output and constricting peripheral vessels. The cardioinhibitor centers are suppressed. With falling carbon dioxide and hydrogen ion levels (increasing pH), the cardioinhibitor centers are stimulated, and the cardioaccelerator and vasomotor centers are suppressed, decreasing cardiac output and causing peripheral vasodilation.