Question

Why doesn’t the blood clot in the blood vessels? Blood contains platelet cells which help in clotting of blood when there is any cut on our body. Why doesn’t it clot when blood is present inside the blood vessel in a normal healthy body?

Answer 1

Before answering the question, let us learn a bit about the platelets. The platelets are colorless, rounded/oval, non-nucleated fragments of cells. These platelets are thus not true cells, but rather fragments of cells. These fragments contain blood coagulating substances that promote coagulation of blood when we suffer an injury. The process by which blood coagulates is popularly known as cascade process and involves many factors.

Complete answer:
Now, whenever we suffer an injury, blood oozes out from the wound. The platelets that are present in the blood get ruptured due to the rupture of blood vessels and friction due to roughness of the skin surface to which the blood gets exposed.
An injury or trauma stimulates the platelets in blood to release coagulation promoting substances called thromboplastins which in turn activate the mechanism of coagulation. Tissues at the site of injury also release tissue thromboplastins.
This is the reason why in a healthy body there is no clotting of blood. When there is no injury, then the blood platelets and the tissues are intact. So, no thromboplastins are released, and there is no coagulation of blood. Thus, blood flows freely in the blood vessels.

Note:
There are certain instances where blood may clot inside the blood vessels. These are rare instances but prove to be fatal if the clot is not treated in time. These clots may occur at any part like the blood vessels of legs, arms, heart or brain. The clinical term associated with these clots is commonly known as thrombosis. The site of occurrence of thrombosis also decides the risk of the clot. A clot in the heart or brain is obviously more fatal than a clot in the limbs. The risk of internal clots is high in patients having high cholesterol, blood pressure, diabetes, and related clinical manifestations.