Platelets Structure And Functions Of The Blood Notes

Platelets Introduction

Platelets or thrombocytes are the formed elements of the blood. Platelets are small colorless, non-nucleated, and moderately refractive bodies. These formed elements of blood are considered to be fragments of cytoplasm.

Size of Platelets

Diameter : 2.5 p (2-4 μ)

Volume : 7.5 cuμ (7-8 cu.μ).

The shape of Platelets: Normally, platelets are of several shapes, viz. spherical or rod-shaped, and become oval or disc-shaped when inactivated. Sometimes, the platelets have a dumbbell shape, comma shape, cigar shape, or any other unusual shape.

Read And Learn More: Medical Physiology Notes

Structure And Composition Of Platelets

Platelets are constituted by:

1. Cell membrane or surface membrane
2. Microtubules
3. Cytoplasm.

Cell Membrane: It is 6 nm thick and contains lipids in the form of phospholipids, cholesterol and glycolipids, carbohydrates as glycocalyx, and glycoproteins and proteins. Of these substances, glycoproteins and phospholipids are functionally important.

• Glycoproteins: Glycoproteins prevent the adherence of platelets to normal endothelium but, accelerate the adherence of platelets to collagen and damaged endothelium in ruptured blood vessels. Glycoproteins also form the receptors for ADP and thrombin.
• Phospholipids: Phospholipids accelerate the clotting reactions. The phospholipids form the precursors of thromboxane A₂ and other prostaglandin-related substances.

Microtubules: Microtubules form a ring around the cytoplasm below the cell membrane. Microtubules are made up of polymerized proteins called tubulin. These tubules provide structural support for the inactivated platelets to maintain the disklike shape.

Cytoplasm: The cytoplasm of the platelets contains the cellular organelles, Golgi apparatus, endoplasmic reticulum, mitochondria, microtubule, microvessels, filaments, and different types of granules. Cytoplasm also contains some chemical substances like proteins, enzymes, hormonal substances, etc.

• Proteins
  • Contractile proteins
    • Actin and myosin – contractile proteins which are responsible for contraction of platelets
    • Thrombosthenin – third contractile protein which is responsible for clot retraction
  • von Willebrand factor: Responsible for adherence of platelets and regulation of plasma level of factor 8
  • Fibrin stabilizing factor: A clotting factor
  • Platelet-derived growth factor (PDGF): Responsible for the repair of damaged blood vessels and wound healing. It is a potent mytogen (a chemical agent that promotes mitosis) for smooth muscle fibers of blood vessels
  • Platelet-activating factor (PAF): Causes aggregation of platelets during the injury of blood vessels resulting in the prevention excess loss of blood
  • Vitronectin (serum spreading factor): Promotes adhesion of platelets and spreading of tissue cells in culture
  • Thrombospondin: Inhibits angiogenesis (formation of new blood vessels from pre-existing vessels).
• Enzymes
  1. ATPase
  2. Enzymes necessary for synthesis of prostaglandins.
• Hormonal Substances
  1. Adrenaline
  2. 5HT (serotonin)
  3. Histamine.
• Other Chemical Substances
  1. Glycogen
  2. Substances like blood group antigens
  3. Inorganic substances – calcium, copper, magnesium, and iron.
• Platelet Granules: Granules present in the cytoplasm of platelets are of two types, alpha granules, and dense granules. Alpha granules contain clotting factors 5 and 13, fibrinogen, and platelet-derived growth factor. Dense granules contain nucleotides, serotonin, phospholipid, calcium, and lysosomes.

Normal Count And Variations Of Platelet

Normal platelet count is 2,50,000. It ranges between 2,00,000 and 4,00,000/cu mm of blood.

• Physiological Variations
  1. Age: Platelets are less in infants (1,50,000-2,00,000/ cu mm) and reaches normal level at 3rd month after birth.
  2. Sex: There is no difference in the platelet count between males and females. In females, it is reduced during menstruation.
  3. High altitude: Platelet count increases.
  4. After meals: After taking food, the platelet count increases.
• Properties Of Platelets: Platelets are three important properties (three ‘A’s):
  1. Adhesiveness
  2. Aggregation
  3. Agglutination

1. Adhesiveness: Adhesiveness is the property of sticking to a rough surface. While coming in contact with any rough surface the platelets are activated and stick to the surface. The factors, which cause adhesiveness are collagen, thrombin, ADP, Thromboxane A₂, calcium ions, von Willebrand factor, P-selectin, and vitronectin.

2. Aggregation (Grouping Of Platelets): Aggregation is the grouping of platelets. Activated platelets group together and become sticky. The stickiness is due to ADP and thromboxane A₂. Platelet aggregation is also accelerated by platelet activating factor which is a cytokine secreted by neutrophils and monocytes.

3. Agglutination: Agglutination is the clumping together of platelets. The agglutination of platelets occurs due to the actions of some platelet agglutinins and platelet-activating factor.

Functions Of Platelets

Normally, the platelets are inactive, and execute their actions only when activated. The activated platelets immediately release many substances. This is process is known as the platelet release reaction. The functions of platelets are carried out by these substances.

1. Role In Blood Clotting: The platelets are responsible for the formation of intrinsic prothrombin activator. This substance is responsible for the onset of blood clotting.

2. Role In Clot Retraction: In the blood clot, the blood cells including platelets are entrapped in between the fibrin threads. The cytoplasm of platelets contains the contractile proteins namely actin, myosin, and thrombosthenin which are responsible for clot retraction.

3. Role In Prevention Of Blood Loss (Hemostasis): Platelets accelerate the hemostasis by three ways

1. Platelets secrete 5 HT, which causes the constriction of blood vessels
2. Due to the adhesive property, the platelets seal the damage in blood vessels like capillaries
3. By formation of temporary plug also platelets seal the damage in blood vessels.

4. Role In Repair Of Ruptured Blood Vessel: The platelet-derived growth factor (PDGF) formed in cytoplasm of platelets is useful for the repair of the endothelium and other structures of the ruptured blood vessels.

5. Role In Defense Mechanism: By the property of agglutination, platelets encircle the foreign bodies and destroy them by phagocytosis.

Activators And Inhibitors Of Platelets

Activators Of Platelets

1. Collagen which is exposed during damage of blood vessels
2. von Willebrand factor
3. Thromboxane A₂
4. Platelet-activating factor
5. Thrombin
6. ADP
7. Calcium ions
8. P selectin – cell adhesion molecule secreted from endothelial cells
9. Convulxin – a purified protein from snake venom.

Inhibitors Of Platelets

1. Nitric oxide
2. Clotting factors – 2, 9, 10, 11 and 12
3. Prostacyclin
4. Nucleotidases which break down the ADP.

Development Of Platelets

• Platelets are formed from bone marrow. The pluripotent stem cell gives rise to the CFU-M. This develops into a megakaryocyte. The cytoplasm of megakaryocyte form pseudopodium. A portion of pseudopodium is detached to from the platelet, which enters the circulation.
• Production of platelets is influenced by colony-stimulating factors and thrombopoietin. Colony-stimulating factors are secreted by monocytes and T lymphocytes. Thrombopoietin is a glycoprotein like erythropoietin. It is secreted by liver and kidneys.

Lifespan And Fate Of Platelets

Average lifespan of platelets is 10 days. It varies between 8 and 11 days. Platelets are destroyed by tissue macrophage system in spleen. So, splenomegaly (enlargement of spleen) decreases platelet count and splenectomy (removal of spleen) increases platelet count.

Applied Physiology Platelet Disorders

Platelet disorders occur because of pathological variation in platelet count and dysfunction of platelets. The platelet disorders are:

1. Thrombocytopenia
2. Thrombocytosis
3. Thrombocythemia
4. Glanzmann thrombasthenia

1. Thrombocytopenia: Decrease in platelet count is called thrombocytopenia. It leads to thrombocytopenic purpura. Thrombocytopenia occurs in the following conditions:

1. Acute infections
2. Acute leukemia
3. Aplastic and pernicious anemia
4. Chickenpox
5. Smallpox
6. Splenomegaly
7. Scarlet fever
8. Typhoid
9. Tuberculosis
10. Purpura
11. Gaucher’s disease.

2. Thrombocytosis: The increase in platelet count is called thrombocytosis. It occurs in the following conditions:

1. Allergic conditions
2. Asphyxia
3. Hemorrhage
4. Bone fractures
5. Surgical operations
6. Splenectomy
7. Rheumatic fever
8. Trauma (wound or injury or damage produced by external force).

3. Thrombocythemia: It is a condition with persistent and abnormal increase in platelet count. It occurs in:

1. Carcinoma
2. Chronic leukemia
3. Hodgkin’s disease.

4. Glanzmann Thrombasthenia: It is an inherited hemorrhagic disorder caused by a structural or functional abnormality of platelets. It leads to thrombasthenic purpura. However, the platelet count is normal. It is characterized by normal clotting time, and normal or prolonged bleeding time but defective clot retraction.