Stem Cells     Red Blood Cells       White Blood Cells

Platelets        Plasma        Blood Types & Activity

Inactivated (smooth) platelet (stained blue) among spiky, activated platelets as seen through a scanning electron microscope. ©2000 Dennis Kunkel, Ph.D.

Platelets are another important part or component of your blood. Platelets are sticky little pieces that help prevent bleeding and make the blood clot when a cut is made. The picture above shows a great view of platelets using an electron microscope.


When a stem cell decides to make platelets, it turns into a factory cell called a megakaryocyte. This is a very large cell with several nuclei. The megakaryocyte never leaves the bone marrow, but it does produce many, many tiny fragments. These fragments are actually the platelets, small pieces of cell material or cytoplasm.


Platelets do leave the bone marrow and circulate freely in the bloodstream.

Normally, platelets look round and smooth, but when they get busy plugging up cuts and wounds they become spiky and ragged around the edges.

When an injury occurs to a blood vessel wall, the platelets respond by literally throwing themselves over the cut to form a temporary plug within minutes slowing the loss of blood.


The platelets also attract a protein found in plasma called fibrin and use it to form a dense netting that traps red blood cells and quickly becomes a clot.

From the outside of a cut, you can see the scab that forms over the wound. It looks hard and crusty. But on the inside, there's a lot going on! As long as there is still an unhealed hole in the blood vessel wall, the clot is constantly being formed, dissolved and reformed by fresh platelets so that bleeding is prevented. When the wound is finally healed by new cells growing over it, the clot will be cleared away and blood will begin to flow through the vessel again.

I also learned that platelets help keep cuts from bleeding. I really like the movie and I hope that other kids that watch it will enjoy it as I did.

— Terri,
elementary school student



Welcome | Middle & High School | About Us | The Video | Blood Biology | Why Become a Blood Donor
Resources & Glossary | MBYB Teaching Tools | Contact Info | Order Info

Copyright 2002 - My Blood, Your Blood®