Platelets In Human Body


Composition Of Blood


A healthy adult human body comprises approximately 1.325 gallons of blood.

A single drop of blood contains about forty per cent of red blood cells, one per cent of white blood cells, four per cent of platelets and fifty-five per cent of plasma.

Our body starts bleeding during a cut or an injury, but the blood clots and bleeding cease after some time.

This is mainly because of the clotting factors released by the blood platelets.

Let us explore more about blood platelets.

What are blood platelets?

Blood platelets are tiny blood cells that form clots over the injury and stop the bleeding. These blood cells are produced in the bone marrow and appear as a small, disc-shaped, colourless fragment without a nucleus. The total number of platelets present in the blood usually varies between 1,50,000 and 4,50,000 per cubic millimetre in a healthy individual.

Blood platelets are also called thrombocytes.

What is a normal platelet count?

A healthy platelet count varies from 150,000 to 450,000 platelets per microliter of blood.

What is coagulation?

Coagulation of blood is a process of forming clots to prevent excess loss of blood from the body.

Explore More: Blood Coagulation

Mechanisms of blood coagulation

Mechanisms of Blood Coagulation

Blood is a body’s fluid connective tissue, which circulates throughout the body under high pressure and helps maintain homeostasis. Constant blood loss can result in death, and hence we need a mechanism to counteract blood loss.

The mechanism, which helps the body to prevent blood loss is called hemostasis. For hemostasis, the body requires both platelets and a coagulation system. Platelets are specialized blood cells, which help in the healing process.

Under normal conditions, the blood is fluid, but hemostasis leads to the solidification of blood called blood clots.


Hemostasis is a self-protective reaction, which prevents and stops bleeding. It is a physiological defensive reaction, which occurs when blood is present outside of the body or blood vessels and prevents bleeding by sealing the blood vessels. This helps in healing.

Mainly platelets, endothelial cells of blood vessels and blood proteins are responsible for hemostasis.

The mechanism of hemostasis can be divided into the following stages.

  • Constriction of the blood vessel.
  • Formation of a temporary “platelet plug”.
  • Activation of the coagulation cascade.
  • Formation of the “fibrin plug” or the final clot.

Hemostasis takes place in two stages:

Primary hemostasis – The primary hemostasis is caused when bleeding is broken off or reduced by the contraction of the blood vessels. Later, thrombin signals for platelets to assemble and form a loose hemostatic plug or platelet thrombus.

Secondary hemostasis – The secondary hemostasis involves the action of blood proteins and coagulation factors in a sequence to strengthen the platelet plug and mark the healing process’s onset. Blood coagulation is prompted by an extrinsic pathway, i.e., tissue damage, but the intrinsic pathway (internal messengers) increases the clotting.

Explore More: Hemostasis

For more additional information about blood cells, coagulation and haemostasis, check for the links below.

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