Platelets and Hemostasis
Objective 5
Describe the production and function of platelets. Define hemostasis and describe the three mechanisms that contribute to hemostasis. Describe the process of coagulation: stimuli, pathways, and products. Explain the purpose and basic functioning of the fibrinolytic system.
Platelets, also called thrombocytes, are cytoplasmic fragments of large cells in the bone marrow called megakaryocytes. These cells are 10-15 times larger than a red cell, and due to their size, they don’t escape the bone marrow and show up in the blood stream. An individual megakaryocyte can release 2000-5000 platelets. The normal range for platelets in the blood is 150–400 × 103/ µL.
Platelets have a very short lifespan, approximately 5–9 days. Platelets help limit blood loss by forming a platelet plug and releasing chemicals to encourage vasoconstriction and activate the clotting process.
The term hemostasis, not to be confused with homeostasis, has a Greek origin, meaning blood stagnation, or blood stopping. It is a very complex process. When activated, the process causes bleeding to cease. There are three mechanisms involved: vascular spasm, platelet plug formation, and coagulation.
Vascular spasm is the constriction of damaged blood vessels. This limits the amount of blood lost. The vascular constriction is due to chemicals released from platelets, damage to the smooth muscle of the vessels, and pain receptor reflexes.
Platelets are very active in the hemostatic process. Think of them as little bags of procoagulant chemicals. Platelets form a plug through a three-step process. First, the platelets adhere to the wall of the blood vessel. Second, the platelets release their chemical contents. This encourages further vasoconstriction and recruitment of other platelets. Lastly, due to chemical release, the activated and newly-recruited platelets become sticky. The clumping of platelets is called platelet aggregation. With the activation of enough platelets, a loose platelet plug is formed.
Everyone witnesses this event sometime in their life, some more than others. When someone cuts themselves shaving, it is common practice to place a piece of toilet paper over the cut. This keeps blood off their clothing and it makes an attempt at stopping the bleeding. Because the individual is usually in a hurry, (that is why they cut themselves in the first place) they remove the toilet paper prior to going out into public. They are disappointed to find out that they begin bleeding again. What happened to the platelets? They are on the toilet paper, so there isn’t an intact platelet plug. Now, that is embarrassing.
The last mechanism in hemostasis is coagulation. An easy way to think of coagulation is taking that which is liquid (plasma) and make it a solid (clot) or at least a semi-solid.
Coagulation is a complex series of enzymatic reactions that occur in a stepwise or cascading fashion. Think of the steps as dominos standing next to each other, read