Question

Damaged tissue releases chemicals that activate platelets and stimulate the formation of blood clots. Which of the following predictions about the activity of platelets best describes a positive feedback mechanism?
A. Activated platelets release chemicals that inhibit blood clot formation.
B. Activated platelets release signaling molecules that inhibit cell division in damaged tissue.
C. Activated platelets constrict the blood vessels, stopping blood flow.
D. Activated platelets release chemicals that activate more platelets.

Answer 1

Answer:

D. Activated platelets release chemicals that activate more platelets.

Explanation:

When a stimulus or a change in some condition triggers a response that intensifies the changing condition, the mechanism is called a positive feedback mechanism.

Any damage to tissues makes the platelets to contact and stick to parts such as collagen fibers of the connective tissue present under the damaged endothelial cells. This process of platelet adhesion activates the platelets and brings about changes in them. The activated platelets grow many projections to contact and interact with one another. They also release ADP and thromboxane A2 which in turn activates the nearby platelets. The ADP makes other platelets in the area sticky. The stickiness of the newly recruited and activated platelets causes them to join the originally activated platelets. Eventually, the accumulation and attachment of large numbers of platelets form a platelet plug.

Here, release of chemicals from the activated platlets to activate more platlets represents the positive feedback mechanism.