Platelets are fragments of larger cells and normally circulate in the blood without adhering to blood vessel walls. When the wall of a blood vessel is damaged, collagen fibers in the wall are exposed to the interior of the blood vessel. The exposed fibers and chemicals released from the endothelial cells that line the blood vessel attract platelets, which start to form a plug and release other chemicals (Figure 1).
Which of the following best explains the feedback mechanism illustrated in Figure 1?
a. This is an example of positive feedback, because the few platelets that initially bind attract more platelets to the damaged area.
b. This is an example of positive feedback, because it results from the interactions among collagen, endothelial cells, and platelets.
c. This is an example of negative feedback, because a large clump of platelets can block the blood vessel and prevent blood flow through it.
d. This is an example of negative feedback, because the accumulation of platelets returns the open blood vessel wall to a closed state.a. This is an example of positive feedback, because the few platelets that initially bind attract more platelets to the damaged area.