Lesson Explainer: The Mechanism of Blood Clotting Biology

In this explainer, we will learn how to describe the mechanism of blood clotting in humans.

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 waste products. Blood is carried throughout the body in blood vessels, but when we are injured, even with a minor cut or a graze, 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 excessive blood loss by causing the blood to coagulate or clump together. When blood coagulates, it changes from a liquid to a solid, gel-like, thickened mass of blood tissue that plugs the damaged blood vessel. This is called blood clotting, or blood coagulation. In this explainer, we will be learning about the different factors involved in this process and the functions they perform.

Key Term: Blood Clotting

Blood clotting is the process by which blood coagulates to form a thickened mass of blood tissue.

Blood is made of several components, suspended in a fluid medium called plasma. These components include red blood cells, white blood cells, and platelets. Of these, the platelets, also called thrombocytes, are the ones responsible for forming blood clots. The prefix thrombo- is used to describe anything relating to the clotting of blood. Platelets are fragments of larger cells called megakaryocytes, which are cells found in the bone marrow. Platelets 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 they play.

Definition: Platelets (Thrombocytes)

Platelets are small cell fragments that circulate in the blood and are responsible for forming blood clots.

When blood flows normally through blood vessels, the platelets are in their inactive state. Blood vessels are lined by cells called endothelial cells. When these cells are damaged due to injury, platelets are attracted to the site of damage.

Key Term: Endothelial Cells

Endothelial cells are cells that form the thin, endothelial lining of blood vessels.

This step is called adhesion, in which platelets adhere to the damaged portion of the blood vessel.

Once the platelets adhere at the site of damage, they become activated. Activated platelets release factors that attract more platelets, causing these platelets to adhere and be activated, too! All of the platelets aggregate together, eventually forming a temporary platelet plug that prevents the other components of the blood from freely leaking out of the damaged vessel, as shown in Figure 2.

Example 1: Identifying the Components of Blood Involved in Clotting

Which component of blood is primarily responsible for causing it to clot?

  1. Plasma
  2. White blood cells
  3. Hemoglobin
  4. Red blood cells
  5. Platelets

Answer

The flow of blood throughout our body is essential to the normal functioning of all our organs as it provides them with oxygen and nutrients. The body cannot afford to lose large volumes of blood, as this would cause the failure of our essential organs, eventually leading to death. Therefore, when the blood vessels carrying blood are damaged due to an injury, the blood coagulates to form thickened masses of blood tissue called clots. Blood clots plug the damaged vessel and prevent excessive bleeding.

Blood consists of several different components. Let’s look at the different components listed in the question and see if we can identify which of them is responsible for forming blood clots.

Plasma is the fluid medium of the blood. All of the components of blood are suspended in the plasma, which is responsible for transporting them throughout the body. This option is, therefore, incorrect.

White blood cells are the body’s 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 the protein found in red blood cells, and it gives blood its characteristic red color. It is also responsible for carrying molecules of oxygen to the organs of the body and carrying carbon dioxide away from them. This protein is not involved in the process of blood clotting, which means that this option is also wrong.

Red blood cells are the most abundant cells in the blood. They carry hemoglobin and transport oxygen and carbon dioxide, as we mentioned earlier. Red blood cells are also called erythrocytes; erythro means “red” and cyte means “cell.” Although blood clots do contain some red blood cells, these cells are not responsible for causing the blood to clot, which is what the question is asking. Therefore, this option is also incorrect.

Platelets are fragments of larger cells called megakaryocytes. These platelets work with certain clotting factors in the blood to cause blood coagulation. When the endothelial lining of a blood vessel is damaged, platelets are attracted to the site of damage to form a platelet plug. They become activated and trigger a cascade of biochemical reactions that lead to the formation of a blood clot.

The correct answer is, therefore, platelets.

The formation of the platelet plug triggers a cascade of biochemical reactions. The damaged blood vessels and the tissues surrounding blood vessels express the tissue factor, thromboplastin. This factor is responsible for converting a protein called prothrombin, which is produced by the liver, into thrombin, which is an active enzyme. This reaction can only take place if calcium ions (Ca)2+ are present.

Blood also contains a protein called fibrinogen, which is produced by the liver and is soluble in blood plasma. When the enzyme thrombin is formed from prothrombin, it acts upon fibrinogen and converts it into fibrin, which is insoluble in blood plasma. The fibrin precipitates out of the blood as 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 more 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, as shown in Figure 3.

The flowchart in Figure 4 summarizes the steps involved in the process of blood clotting.

Example 2: Identifying the Factors Involved in Blood Clotting

The flowchart outlines the basic process of how a blood clot forms at the site of a wound.

What word would correctly replace the gap in statement 2?

  1. Hemoglobin
  2. Collagen
  3. Fibrinogen
  4. Thromboplastin

Answer

When the blood vessels carrying blood are damaged, the blood coagulates to form thickened masses of blood tissue called clots, which plug the damaged vessel and prevent excessive bleeding. There are several biochemical reactions involved in this process, which require different proteins called clotting factors. The flowchart in the question outlines the steps and the factors involved in the process of blood clotting.

According to the flowchart, the missing word in statement 2 is the name of a tissue factor that is produced by damaged blood vessels, after a blood vessel is damaged. Let’s have a look through the options in the question and see which one fits best.

Hemoglobin is a protein found in red blood cells and is responsible for carrying molecules of oxygen to the organs of the body and carrying carbon dioxide away from them. This does not fit with the description in statement 2!

Collagen is a structural protein found in muscles, skin, blood vessels, and several other body parts. Again, this does not match with the description of the word we are looking for, so this option is also wrong.

Fibrinogen is an important soluble protein involved in the process of blood clotting. It’ is produced in the liver and circulates in the blood. An enzyme called thrombin converts soluble fibrinogen into insoluble fibrin, which is described in statement 4 of the flowchart in the question. We can, therefore, rule out fibrinogen, as it is involved in a different stage of the clotting cascade.

Thromboplastin is a tissue factor expressed by the tissues surrounding blood vessels. It works to convert prothrombin into the active enzyme thrombin.

The option that matches the description of the factor in statement 2 is, therefore, thromboplastin.

Example 3: Explaining the Role of Thrombin in Blood Clotting

What is the role of thrombin in the process of blood clotting?

  1. To initiate the release of calcium ions to encourage the aggregation of blood cells
  2. To catalyze the conversion of soluble fibrinogen into insoluble fibrin
  3. To initiate the conversion of the protein prothrombin into the enzyme thrombin
  4. To form a net to trap red blood cells that aggregate into a clot

Answer

Blood clotting, the process of coagulation of blood into a thickened mass of blood tissue, involves a cascade of biochemical reactions requiring several different enzymes, proteins, and factors.

Thrombin is one of these factors; it is an active enzyme produced when the tissue factor thromboplastin acts on the protein prothrombin. Thrombin is responsible for the generation of insoluble protein fibers called fibrin, which are important in the formation of a strong clot to plug damaged blood vessels.

To produce fibrin, the enzyme thrombin catalyzes a protein called fibrinogen, which is soluble in blood plasma, to convert it into insoluble fibrin. Another important function of fibrin is to form a “net” that traps red blood cells and activated platelets that aggregate into a clot.

The role of thrombin in the process of blood clotting is, therefore, to catalyze the conversion of soluble fibrinogen into insoluble fibrin.

Aside from preventing excessive blood loss from wounds, blood clotting also has other advantages. When you have a cut or a graze, you might have noticed that a scab forms over the injury. This scab is actually made of dried blood clots! The scab sits over the site of a wound, allowing the damaged skin to heal underneath it. Blood clots also help by preventing the entry of disease-causing pathogens into the bloodstream through the wound.

When a blood vessel sustains some form of damage to its walls, it automatically constricts to prevent excessive blood loss. When a blood clot forms, it plugs the damaged site. This allows the blood vessel to open up again, restoring the free flow of blood. The blood vessel can therefore open up again, allowing blood to flow more freely through it. This is also helpful in the process of healing the wound, as the blood can deliver the oxygen and nutrients required for healing to the site of the wound.

So far, we’ve clearly understood the mechanism of blood clotting and why this is important. What happens if, in some conditions, blood is not able to form clots?

As we have 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 can cause a clotting disorder, in which the body may be unable to form clots at the site of an injury. Let’s look at some conditions that can cause blood clotting disorders in the body.

Key Term: Blood Clotting Disorder

A blood clotting disorder is a condition in which a patient’s blood is unable to clot normally.

Several of the proteins involved in the clotting process are produced in the liver. We learned about the protein prothrombin, which gets converted into the enzyme thrombin. Prothrombin is produced by the liver in a process that requires vitamin K, which means that a deficiency of vitamin K can also cause a clotting disorder. Any other disease or disorder that impairs the function of the liver, like cirrhosis or hepatitis, can therefore cause a blood clotting disorder.

The production of each of the 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. If a patient with hemophilia is wounded, blood flows uncontrollably from the wound without being able to clot. This is dangerous because, as we mentioned earlier, the body cannot tolerate excessive blood loss. Patients with hemophilia are usually treated with injections of the missing clotting factor, called “clotting factor concentrates.” Hemophilia can also be treated by supplementing the diet with foods rich in vitamin K and iron.

Key Term: Hemophilia

Hemophilia is a genetic blood clotting disorder characterized by excessive and uncontrolled bleeding from wounds.

Example 4: Understanding Genetic Clotting Disorders

The factors that initiate the clotting of blood are coded for by particular genes, but sometimes these genes can mutate. An example of a condition caused by these mutations is hemophilia. What symptom would you expect someone with hemophilia to have?

  1. Inability to bleed from wounds
  2. Frequent formation of clots within blood vessels
  3. Excessive or uncontrolled bleeding from wounds
  4. Formation of an excessive amount of scabs

Answer

Blood clotting, the process of coagulation of blood into a thickened mass of blood tissue, involves a cascade of biochemical reactions requiring several different enzymes, proteins, and factors.

The production of the different blood clotting factors is controlled by specific genes. If any of these genes are mutated, this could disrupt the normal production of these factors. Depending on the type of mutation, the clotting factors produced may be defective, or they may not be produced in sufficient quantities, causing a genetic clotting disorder in which the blood cannot form clots. Hemophilia is an example of a genetic clotting disorder.

If a patient with hemophilia is wounded, the blood cannot clot normally, which means that the wound will bleed uncontrollably. This matches the statement that mentions excessive or uncontrolled bleeding from wounds.

The correct symptom of someone with hemophilia is, therefore, excessive or uncontrolled bleeding from wounds.

Although clots are important in wound healing, if a clot forms inside a blood vessel, it can restrict the normal blood flow. This is extremely dangerous, as it can block the supply of blood to essential organs, preventing them from functioning. For example, if a blood clot blocks the supply of blood to the brain, it can cause a stroke. A blood clot that forms within the blood vessels is called a thrombus. On the right-hand side of Figure 5, you can see how a blood clot can block the normal flow of blood in a blood vessel.

So why does blood usually not clot inside blood vessels? As we have learned, blood clots are initiated by the aggregation of platelets, which then produce clotting factors. The aggregation of platelets is triggered by damage to the endothelial lining of blood vessels. In a healthy person, blood should flow smoothly and without restrictions, which prevents the spontaneous aggregation of platelets inside the blood vessels.

In addition, the cells of the body, in particular, those of the liver and lungs, can produce a compound called heparin. Heparin acts as an anticoagulant, preventing the formation of new clots by inhibiting the formation of fibrin. While heparin is produced naturally by the body, it is also used as a 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!

Key Term: Heparin

Heparin is an anticoagulant produced naturally by the liver, lungs, and other tissues in the body, or administered as a medication.

Let’s summarize everything we have learned in this explainer about blood clotting.

Key Points

  • To prevent excessive blood loss, blood coagulates when a blood vessel is damaged, forming a blood clot that plugs the damaged blood vessel.
  • Platelets are small cell fragments responsible for initiating the formation of blood clots.
  • Thromboplastin is a tissue factor that converts prothrombin into the active enzyme thrombin.
  • Thrombin converts soluble fibrinogen into insoluble fibrin, which forms a “net” at the site of damage.
  • Defective or insufficient clotting factors can cause clotting disorders. Hemophilia is a genetic clotting disorder characterized by uncontrollable bleeding from wounds.
  • Clots within blood vessels can block blood supply to essential organs. The body produces heparin, an anticoagulant, to prevent this.

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