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Platelets: Hemostasis and Clotting

Platelets: Hemostasis and Clotting

Author: Amanda Soderlind

Identify how blood clots work and disorders associated with blood clotting.

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Platelets: Homeostasis and Clotting

Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on platelets.

Today we will be investigating the role of platelets in helping blood to clot, and therefore maintaining homeostasis.

First of all platelets, are a component of blood that are involved in clotting. So within our blood we have our red blood cells, our white blood cells, and our platelets. And the platelets are the part of our blood that are involved in clotting.

Hemostatis is what we call the process that slows or stops bleeding. So if we had a tear in a vessel, for example, something would have to happen in order to repair that tear, to keep blood from constantly flowing out of that vessel. Hemostasis is that process that stops or slows the bleeding.

We're going to take a look at this diagram right here, and we're going to talk about what happens when a small vessel does tear. The process that happens in order to stop that bleeding.

If we take a look at step number one right here. So let's say there was some sort of injury or tear in a vessel. What's going to happen then is where that tear is located, blood is going to start to flow out. Now if this isn't fixed, and too much blood flows out of the vessels, we're going to have an imbalance in homeostasis will not be able to be maintained. So our blood needs to stay within our vessels to deliver those important nutrients to our different tissues. So something has to be done very quickly in order to repair this tear so that homeostasis can be maintained.

So what's going to happen first, after the tear has happened, is that the vessel will actually contract or constrict. And by doing this, there's going to be less leakage out of the vessel. So it's going to constrict, restricting the flow of blood through that vessel, therefore less blood is going to flow out of the tear.

The next thing that's going to happen is platelets. So our red dots here are red blood cells, and the little orange dashes are going to represent our platelets. So platelets will stick to the wall of the vessel. So you'll notice in this area right here, we have platelets sticking to the wall of that vessel, helping to clog up that tear. Therefore allowing less blood to be able to escape.

Then our last thing that's going to happen, in step four, is that the blood will coagulate. And what this means is that it becomes thicker, almost a gel-like substance. Therefore, less of it is going to escape. And also, a more permanent blood clot will form. So how this happens is that within your plasma, you have a protein called factor X This protein will be activated. And then, an enzyme called thrombin will be produced. This enzyme will cause fibrous threads to form. As those fibrous threads form-- they kind of form a net-- and they'll trap red blood cells and platelets, and they'll help to clot the tear that's in that vessel. So they'll form this net of red blood cells and platelets, and it will form over that tear in the vessel, forming an actual clot. That's a more permanent solution. And then that will start to pull that vessel back together until it can permanently heal.

There are a couple different disorders associated with the blood not clotting properly, or there being some sort of disorder with clotting. One of those disorders is thrombosis. Thrombosis is a disorder when a clot forms in an undamaged vessel. It's normal for a clot to form in a damaged vessel, but sometimes clots will form in an undamaged vessel, and then that clot will stay there. So it will stay inside that vessel. And what that does is, it affects blood flow through the vessel. So blood is not allowed to flow through that vessel as freely, because you have this clot just kind of sticking in there. So that's thrombosis.

An embolism is kind of related to thrombosis. It's when a clot will form in an damaged vessel, but then instead of staying in that vessel, it will actually start roaming throughout the body. This can be very dangerous because you don't want a clot roaming throughout the body, because if they get stuck in a vessel somewhere, it can affect blood flow to tissues and organs, which can be very dangerous.

A stroke is a clot that blocks blood flow to the brain. If we have a specific clot that's blocking that flow of blood to the brain, it's called a stroke. And then the brain is not going to receive enough oxygen to be able to function properly. So these can be very, very dangerous, as well.

Hemophilia is a clotting disorder that's a genetic disorder. It's inherited. In this type of disorder, blood doesn't clot properly. So if a person were to get a cut or tear in their vessel, the clots don't form as quickly as they would normally, which can lead to more serious problems, especially if the person is in a very serious accident where they're losing a lot of blood. That can be very, very dangerous. So their blood doesn't clot properly, and it doesn't clot as quickly as it should.

This lesson has been an overview on the role of platelets in clotting blood.

Terms to Know

A circulating mass in the blood. Examples: fatty embolus or an air embolus.


A group of hereditary disorders that prevent clots from being formed.


The process of controlling/stopping bleeding from a broken vessel; platelets are the main component of hemostasis by working with the plasma protein fibrinogen to form a blood clot.


Platelets (also know as thrombocytes) are actually cell fragments of bone marrow cells called megakaryocytes. Platelets are used to form blood clots to control hemostasis.


When blood flow is obstructed to an area of the brain, this is called a stroke. It causes brain tissue to become ischemic; if blood flow is not immediately restored brain damage will ensue.


An enzyme in the blood that plays a critical role in hemostasis by converting the plasma protein fibrinogen into fibrin; fibrin secures platelets as they plug a damaged vessel (platelet plug) and the end result is a clot.


The formation of a blood clot within a vessel; the clot obstructs blood flow through the vessel.