Final answer:
Decreased blood pressure causes tissue ischemia mainly because of reduced oxygen delivery to tissues, leading to hypoxia and potential cell death in critical organs like the heart and brain.
Step-by-step explanation:
Decreased blood pressure might cause tissue ischemia primarily due to reduced oxygen delivery to tissues. When blood pressure falls, the flow of blood through the vascular system, including arteries, arterioles, and capillaries, is reduced. This diminution in blood flow means that less oxygen and fewer nutrients reach the tissues, which can lead to hypoxia—a state where tissues are deprived of adequate oxygen. Hypoxia can have serious consequences, such as cell death, particularly in sensitive organs like the heart and brain where it can severely impair function. Hence, the correct answer to why decreased blood pressure might cause tissue ischemia is 'a. Reduced oxygen delivery to tissues.'