Platelets

What are Platelets?

The formed elements of the blood include three principal components: red blood cells (RBCs), white blood cells (WBCs), and platelets. RBCs or erythrocytes transport oxygen from the lungs to body cells and deliver carbon dioxide from body cells to the lungs. WBCs or leukocytes protect the body from invading pathogens and other foreign substances. Platelets release chemicals that promote blood clotting when blood vessels are damaged.

Platelets, or thrombocytes, are small cell fragments that are irregularly discshaped and have no nucleus. Platelets break off from the megakaryocytes in red bone marrow and then enter the blood circulation. Between 150,000 and 400,000 platelets are present in each microliter of blood. Their granules contain chemicals that promote blood clotting. They help stop blood loss from damaged blood vessels by forming a platelet plug.

Structure of Platelets

The structure of platelets includes:

Size and Shape

Platelets are small in size, typically 2-4 micrometres in diameter. They are shaped like a disc which enables platelets to flow freely through blood vessels.

Granules

Platelets contain numerous granules that store clotting factors, growth factors, enzymes, and other molecules operative for their functions.

Cell membrane

The platelet cell membrane is endowed with receptors that enable it to interact with other cells and elements of the hemostatic system.

Functions of Platelets

The functions of platelets in human body include:

Hemostasis

Platelets, being the usual physical vessel holds, make sure integrity is maintained by preventing bleeding with a platelet plug at the site of vascular injury.

Clot Formation

The release of specific chemicals from platelets exaggerates the coagulation cascade to form a stable blood clot.

Wound Healing

Platelets release growth factors involved in tissue repair and regeneration after an injury.

Platelet Activation and Aggregation

Platelet activation and aggregation are essential steps in hemostasis that prevent blood loss after vascular injury.

Mechanism of Activation

The platelets themselves get activated on contact with denuded blood vessels or exposed collagen. Activation is also accompanied by a shape change that allows the platelets to hang around at the site of injury.

Platelet Plug Formation

Initially, platelets contact and stick to parts of a damaged blood vessel. Due to adhesion, the platelets become activated. They interact with one another, and they begin to aggregate. Eventually, the accumulation and attachment of large numbers of platelets form a mass called a platelet plug.

Production of Platelets (Thrombopoiesis)

Platelets are formed by the process called thrombopoiesis. The formation of platelets takes place in the bone marrow from large cells called megakaryocytes.

Role of Megakaryocytes

Myeloid stem cells develop into megakaryoblasts. Megakaryoblasts transform into megakaryocytes, huge cells that splinter into 2000 to 3000 fragments. Each fragment, enclosed by a piece of the plasma membrane, is a platelet. Platelets break off from the megakaryocytes in red bone marrow and then enter the blood circulation.

Regulation by Thrombopoietin

Under the influence of the hormone thrombopoietin, myeloid stem cells develop into megakaryocyte colony forming cells that in turn develop into precursor cells called megakaryoblasts and eventually form platelets.

Platelet Lifespan and Removal

Platelets are relatively short-lived with about 7-10 days of an average lifetime in circulation. At the end of this period, they are phagocytosed from the bloodstream by the spleen and liver.

Platelet Disorders

Any change in the number of platelets can cause disorders in the human body.

Thrombocytopenia

A low count of platelets can result in excessive bleeding and bruising.

Thrombocytosis

A high count of platelets can predispose to thrombosis.

Platelet Dysfunction

Disorders that affect platelet function can lead to bleeding disorders despite an adequate count.

Clinical Significance of Platelets

Clinical significance of platelets include:

Diagnostic Testing: Platelet counts are part of blood tests that are routinely conducted to ascertain if a patient is at risk for bleeding or clotting disorders.

Platelet Transfusions: Very severe thrombocytopenia may require platelet transfusion to reconstitute normal clotting.

Importance Of Platelets To Health

Platelets are currently performing their function of maintaining homeostasis and preventing a heavy outflow of blood. Their involvement in the process of tissue healing and scarring can be yet discussed about general health and healing after injuries.

Platelets NEET MCQs (With Answers & Explanations)

Important questions asked in NEET exam are:

• Formation of Platelets (thrombopoiesis)

• Characteristics of Platelets (shape, count, lifespan)

• Disorders of Platelets

Practice Questions for NEET

Q1. The life span of thrombocytes is

1. 3 to 4 weeks

2. 4 to 5 weeks.

3. 3 to 7 days

4. None of the above

Correct answer: 3) 3 to 7 Days

Explanation:

They are non-nucleated round, biconvex cells. They originate from megakaryocytes by cytoplasmic extension. They release clotting factor (thromboplastin) for blood coagulation at the site of injury. Platelets play a crucial role in wound healing by forming a platelet plug, which helps seal blood vessels and stop bleeding. They also release other substances that help in the constriction of blood vessels and the promotion of tissue repair. Platelet count and activity are essential for maintaining proper clotting and preventing excessive bleeding.

Hence, the correct answer is option 3) 3 to 7 days.

Q2. A decrease in platelets count is termed as

1. Leucocytopenia

2. Thrombocytopenia

3. Leukemia

4. Thrombocytic

Correct answer: 2) Thromobocytopenia

Explanation:

Thrombocytopenia is a condition where there's a decrease in the platelet count. This can lead to what are known as bleeding disorders like purpura. The normal life of a platelet is typically 5 to 9 days. When platelet count goes below 150,000 per microliter of blood, individuals might experience increased bruising and prolonged bleeding due to the impaired ability of blood to clot, which requires medical evaluation and possible treatment.

Hence, the correct answer is Option 2) Thrombocytopenia.

Q3. One of the common symptoms observed in people infected with Dengue fever is

1. Significant decrease in RBC count.

2. Significant decrease in WBC count.

3. Significant decrease in platelet count.

4. Significant increase in platelet count.

Correct answer: 3) Significant decrease in platelet count.

Explanation:

One frequent side effect of dengue fever, a virus spread by mosquitoes, is a low platelet count
Normal platelet count
Between 150,000 and 450,000 platelets per microliter (µL) of blood is considered normal.
Dengue platelet count
Platelet counts in dengue can fall sharply, occasionally to 10,000/µL or even less. A count below 40,000 mcL indicates a significant risk of bleeding, whereas a count below 100,000 mcL is deemed concerning.

Hence, the correct answer is option 3) Significant decrease in platelet count.

Also Read:

• MCQs on Platelets

• Connective Tissue

• Epithelium Tissue

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