Lesson Video: The Mechanism of Blood Clotting Biology • Second Year of Secondary School

Video Transcript

In this video, we will learn how blood clots can form in order to protect our body from the dangerous outcomes of excessive blood loss. We will investigate the step-by-step mechanism of blood clotting and the different factors that are involved in this process. Finally, we will discover how clotting disorders can arise from defective or insufficient clotting factors and how some of these disorders might be treated.

The flow of blood in the human body is essential to survival as it supplies organ systems with the oxygen and nutrients they need to function and carries away their metabolic waste products. Blood is carried throughout the body in blood vessels. We can see a part of a blood vessel magnified in this diagram. When we’re injured, even with a minor cut, the linings of the blood vessels under our skin can be damaged. This is why we bleed. Have you ever noticed though that the bleeding usually stops within a few minutes? The human body cannot afford to lose large volumes of blood, as this would cause the failure of the body’s essential organs, eventually leading to death.

The body therefore has an interesting mechanism to prevent this excess blood loss by causing the blood to clump together in a process called blood clotting or blood coagulation. This prevents excess blood loss, giving the wound time to heal, and usually allows the proper functioning of the body’s vital organs. This is because the blood can continue to supply them with oxygen and nutrients and remove their metabolic waste products. In this video, we’ll be learning about the different factors involved in this process of clotting and the functions that they perform. But first let’s take a look at the different components of healthy blood.

You probably already know that blood is a liquid. And if we were to put it into a test tube and separate it into its different components, it would look something like this. Healthy blood contains a number of different cells and fragments suspended in a fluid medium called plasma. These cells include red blood cells, which are responsible for carrying oxygen and carbon dioxide around the body and also give blood its characteristic red color, white blood cells that are important in our immune defense, and cell fragments called platelets. Let’s take a closer look at platelets, as these are the components that are responsible for forming blood clots.

Platelets, which are sometimes called thrombocytes, are cell fragments that circulate in the blood. The prefix thrombo- is used to describe anything referring to the clotting of blood. In fact, a blood clot is sometimes called a thrombus. And the suffix -cyte means cell, though thrombocytes, or platelets, are actually small fragments of much larger cells called megakaryocytes, which are found in bone marrow. A typical platelet will remain in circulation for about 10 days. And they work together with certain clotting factors in the blood to cause blood coagulation.

Let’s learn a little bit more about platelets and understand the role that they play. When blood flows normally through blood vessels, platelets are in their inactive state. Blood vessels are lined by cells called endothelial cells. When these endothelial cells are damaged due to injury, platelets are attracted to the site of damage. This step is called adhesion, where the platelets adhere to the damaged portion of the blood vessel. Once the platelets adhere to the site of damage, they become activated.

Activated platelets release factors that attract even more platelets, causing these platelets to adhere and be activated too. All of these platelets aggregate together, eventually forming a structure called a platelet plug. This plug prevents the other components of the blood from freely leaking out of the damaged vessel. When blood coagulates like this, it changes from a liquid to a solid, gel-like, thickened mass of blood tissue that plugs up the damaged blood vessel. The formation of this platelet plug triggers a cascade of biochemical reactions.

The damaged blood vessels and the tissue surrounding the blood vessels express the tissue factor thromboplastin. Thromboplastin is a factor responsible for converting the protein prothrombin, which is produced by the liver, into thrombin, which is an active enzyme. This reaction can only take place if calcium ions are present. Blood also contains a protein called fibrinogen. Fibrinogen is also produced by the liver and it’s soluble in blood plasma. When the enzyme thrombin is formed by prothrombin, it acts upon this fibrinogen, converting it into a substance called fibrin, which is insoluble in blood plasma.

Since fibrin is an insoluble protein, it cannot dissolve in the blood plasma once it’s formed. Instead, fibrin precipitates out of the blood in the form of a network of microscopic fibers. The strands of fibrin formed in this way will reinforce the platelet plug by forming a net or a mesh, which traps red blood cells and additional activated platelets. The fibrin, red blood cells, and platelets together form a stronger clot, effectively sealing the wall of the blood vessel and allowing it to heal. Let’s summarize the steps involved in the process of blood clotting with a flowchart, starting with the damage to blood vessels due to injury.

This injury leads to platelets being attracted to the wound. The damaged blood vessels and the tissues that surround these blood vessels express the tissue factor thromboplastin. Thromboplastin in the presence of calcium ions converts prothrombin into the active enzyme thrombin. The enzyme thrombin then converts fibrinogen, which is soluble in blood plasma, into the insoluble fibrin. Fibrin then forms an insoluble mesh of red blood cells and platelets, plugging the wound shut to allow healing.

Aside from preventing excess blood loss from wounds, blood clotting has other advantages. When you have a cut, you might notice that a scab forms over the injury. The scab is actually made of dried blood clots. The scab sits over the site of the wound, allowing the damaged skin to heal underneath it. You might recall that pathogens are biological agents that are capable of causing disease. And scabs can also help to prevent the entrance of these pathogens into the bloodstream through the wound. This also explains why applying a plaster or a band aid to a wound can be helpful as it provides another barrier against pathogen entry.

When a blood vessel sustains some kind of damage to its walls, vasoconstriction occurs. This means that blood vessels constrict or become narrower, which helps to prevent blood loss prior to the formation of a clot. After vasoconstriction, the clot can form, plugging up the damaged site to further prevent blood loss. Eventually, the wound will heal and the normal flow of blood can resume.

Let’s find out what happens when blood is not able to form clots or if they form too readily. As we’ve learned, there are several different proteins, enzymes, and factors involved in the cascade of biochemical reactions that cause blood clotting. If any one of these factors is defective or deficient, this might cause a clotting disorder, in which the body may be unable to form clots at the site of injury. Let’s look at some of the conditions that can cause blood clotting disorders in the body.

Several of the proteins involved in the clotting process are produced in the liver, such as prothrombin, which gets converted into the enzyme thrombin. The production of prothrombin requires vitamin K, which also means that a deficiency of vitamin K can cause a clotting disorder. Any other disease or disorder that impairs the function of the liver, like cirrhosis or hepatitis, can therefore also cause a blood clotting disorder. The production of each of these factors involved in blood clotting is controlled by specific genes. Mutations in these genes can lead to the production of defective or insufficient clotting factors, causing genetic blood clotting disorders.

One example of a genetic clotting disorder is hemophilia. Hemophilia is a blood clotting disorder that’s characterized by excessive and uncontrolled bleeding from wounds. If a patient with hemophilia is wounded, blood flows uncontrollably from the wound without being able to clot. This is highly dangerous because, as we mentioned earlier, the human body cannot tolerate excessive blood loss. Excessive blood loss leads to loss of blood pressure, and this in turn means that less oxygen is delivered to all the cells of the body. The physical effects of this can be dizziness and shortness of breath.

More importantly, the blood flow and oxygen delivery to our essential organs like the heart and brain will decrease with excessive blood loss. This can lead to heart attacks if the blood is not being delivered to the heart, strokes if it’s not being delivered to the brain, or even death. Patients with hemophilia are usually treated with injections of the missing clotting factor, which are called clotting factor concentrates. Hemophilia can also sometimes be treated by supplementing the diet with foods that are rich in vitamin K and iron.

Although blood clotting is important in wound healing, if a clot forms inside a blood vessel, it can restrict the normal blood flow. And when this happens excessively, it can become highly dangerous. Let’s take a look at the risk factors and what they might lead to in excessive clotting next. The risk of blood clots forming are increased through long periods of inactivity, injury to blood vessels, which might occur, for example, through a complex surgery. Obesity also increases the risk of blood clots forming excessively. And some people are more predisposed to blood clots forming due to inherited genetic disorders or autoimmune disorders. The formation of blood clots is extremely dangerous when it happens regularly or unnecessarily, as it can block the supply of blood to essential organs, preventing them from functioning properly, just like excessive blood loss would.

This diagram shows what normal blood flow through a major blood vessel should look like. However, in this diagram, we can see that when a blood clot, or a thrombus, forms in one of these major blood vessels, it blocks the normal flow of blood. This means that it also blocks oxygen from being supplied to these vital tissues and organs. Just like with excessive blood loss, when a thrombus forms in a major blood vessel associated with the heart, it can lead to a heart attack. If the thrombus forms in a major blood vessel delivering the brain with oxygen, it can cause a stroke. Both heart attacks and strokes can cause death. So recognizing their symptoms early and preventing them as much as possible by controlling the risk factors that we’re able to is vital to survival.

You might be wondering, why does blood not usually clot within blood vessels? As we’ve learned, blood clots are initiated by the aggregation of platelets, which then produce clotting factors. The aggregation of these platelets is initially triggered by damage to the endothelial lining of blood vessels. In a healthy blood vessel, the blood should flow without restrictions. This prevents the spontaneous aggregation of platelets within the blood vessels. In addition, the cells of the body, in particular those of the liver and the lungs, can produce a compound called heparin.

Heparin acts as an anticoagulant, which, you may be able to tell from the name, means that it prevents coagulation or clotting. It inhibits the formation of new clots by inhibiting the formation of fibrin. You may recall that fibrin is the insoluble protein that forms a net or mesh around the platelets in red blood cells that reinforces the platelet plug and therefore the blood clots.

While heparin is normally produced naturally by the body, it can also be used as medication to lower the risk of blood clots. For example, if a patient undergoes major surgery that will need them to remain in bed for a long time for their recovery, they might be given heparin to ensure that their blood will not clot from long periods of inactivity. Let’s have a go at applying what we’ve learned so far about blood clotting to a practice question.

Which component of blood is primarily responsible for causing it to clot? (A) Plasma, (B) white blood cells, (C) hemoglobin, (D) red blood cells, or (E) platelets.

The flow of blood throughout our body is essential to the normal functioning of all our organs, as it provides them with oxygen and with nutrients. The body cannot afford to lose large volumes of blood, as this would cause the failure of our essential organs as they’re not supplied with the oxygen or nutrients they require, which will eventually lead to death. Therefore, when blood vessels carrying blood are damaged due to an injury, the blood coagulates at the site of injury into a thickened mass of blood tissue called a blood clot. Blood clots plug the damaged vessel and prevent excessive bleeding.

Blood consists of several different components. So, let’s look at the different components listed in the question and see if we can identify which one of them is responsible for initiating the formation of blood clots. Plasma is the fluid medium of blood. All the components of blood are suspended in the plasma, which is responsible for transporting them throughout the body. Therefore, this option has nothing to do with clotting, so it’s incorrect. White blood cells are the infection-fighting cells. White blood cells are responsible for identifying pathogens in the body and generating immune responses against them. Since these cells play no role in forming blood clots, this option isn’t correct either.

Hemoglobin is found in cells called red blood cells. Hemoglobin is a pigment in these red blood cells that’s responsible for carrying oxygen towards the tissues that need them and carbon dioxide away from them. Hemoglobin is not responsible for forming blood clots however, so this option is also incorrect. Although blood clots do contain some red blood cells as we can see here, these cells are not actually responsible for forming the clots themselves, which is what the question is asking us for. So, this option is also incorrect.

Platelets are fragments of larger cells called megakaryocytes that are found in bone marrow. The platelets work with other clotting factors in the blood to initiate blood coagulation. When the endothelial lining of a blood vessel is damaged, platelets are attracted to the sites of injury and form a structure called a platelet plug. They become activated and trigger cascade of biochemical reactions that leads to the formation of a blood clot. The correct answer to the question is, therefore, platelets.

Let’s summarize everything we’ve learned in this video about blood clotting by reviewing the key points. Blood clots form to prevent excessive blood loss from damaged blood vessels. This process is initiated by cell fragments called platelets. Thromboplastin converts prothrombin into the enzyme thrombin, which in turn can convert soluble fibrinogen into insoluble fibrin. Fibrin forms a net at the site of damage, further plugging up the damaged cells to prevent infection and excess blood loss. Defective or insufficient clotting factors can lead to clotting disorders, like hemophilia. But excessive or unnecessary blood clots can block the blood supply to vital organs. This can be treated with anticoagulants, like heparin.