CBSE Class 10 Answered

Blood platelets are tiny cell fragments that play a critical role in blood clotting. Platelets normally start the clotting process when they are exposed to air, such as in a cut or a wound. Inside the body, the blood flows in a closed system where the platelets do not come in contact with air. Hence, the blood does not clot normally in our body.

Typically, our body naturally dissolves the blood clot after the injury has healed. Sometimes, however, clots form on the inside of the vessels without an obvious injury or do not dissolve naturally. 

In some diseases, the number of platelets rises so high that blood clots too easily, even without a wound. This is also dangerous because the blood clot can become detached and travel through blood vessels until it becomes stuck. This can cut off the flow of blood to the tissue downstream from the clot. If it happens in the heart, the clot causes a heart attack. In the brain, a blood clot causes a stroke.

Atherosclerosis, or hardening of the arteries, can also cause internal clotting. Deposits of cholesterol and calcium build up inside arteries, narrowing them and providing a rough, sticky surface that attracts platelets.