CDS (I) 2024 General Knowledge · Question 63 of 120

Correct — C, Platelets. Platelets, also called thrombocytes, are not whole cells at all but fragments shed from megakaryocytes in the bone marrow, and NCERT puts their function in one line — they can release a variety of substances, most of which are involved in the coagulation or clotting of blood. When a vessel wall is cut, platelets stick to the exposed collagen, change shape, clump together and degranulate, and the factors they pour out drive the cascade that converts prothrombin to thrombin and then fibrinogen to fibrin. The fibrin threads trap blood cells and the clot forms. A fall in the platelet count is followed by clotting disorders and excessive bleeding, which is exactly why a dengue patient's platelet count is watched. Of the four options only platelets have any role in coagulation.

• (a)RBCs — Red cells in mammals lose their nucleus and are packed with haemoglobin for carrying oxygen and carbon dioxide. They are passively trapped in the fibrin mesh of a clot but release no clotting factor.
• (b)Eosinophils — Eosinophils are granulocytes that act against parasitic worms and take part in allergic reactions. They make up only two to three per cent of white cells and have no coagulation role.
• (d)Monocytes — Monocytes are the largest white cells and become macrophages that engulf pathogens and debris. That is phagocytosis and immunity, not clotting.

Blood is plasma plus three sets of formed elements — erythrocytes, leucocytes and platelets — which together make up about 45 per cent of its volume. Clotting draws on two of those compartments. The plasma already carries the coagulation factors, but in an inactive form; plasma with those factors removed is called serum. Platelets supply the trigger and the extra factors that switch the inactive plasma proteins on.

The honest complication worth knowing is that platelets are not the only place clotting factors come from. Most of the plasma factors are synthesised in the liver and circulate inactive until needed, which is why liver disease and vitamin K deficiency both cause bleeding. The question, though, offers only four blood components, and among RBCs, eosinophils, monocytes and platelets there is exactly one that releases clotting substances. Read the option list first and the item becomes easy. A healthy person carries roughly 1.5 to 3.5 lakh platelets per cubic millimetre of blood, and it is the drop in that count, not any change in the white cells, that makes dengue dangerous.

• Platelets, or thrombocytes, are cell fragments produced from megakaryocytes in the bone marrow.
• Blood normally contains about 1,50,000 to 3,50,000 platelets per cubic millimetre.
• Platelets release substances most of which are involved in the coagulation or clotting of blood.
• Clotting factors also circulate in plasma in an inactive form; plasma without them is called serum.
• The clot itself is a mesh of fibrin, formed from fibrinogen by the enzyme thrombin.

• Reading 'clotting factors' as something the plasma alone supplies and rejecting platelets on that basis.
• Confusing thrombocytes with lymphocytes because both names are unfamiliar.
• Assuming red cells must be involved because a clot looks red — they are trapped in the mesh, not part of making it.

Asked as a one-line recall item where three of the four options are cell types with completely unrelated jobs.

• practice — not a real PYQ

  Plasma from which the clotting factors have been removed is called

  • (a)lymph
  • (b)serum
  • (c)tissue fluid
  • (d)haemolymph

  Answer(b) serum — plasma minus the coagulation factors.

• practice — not a real PYQ

  Platelets are produced in the bone marrow from which one of the following?

  • (a)Megakaryocytes
  • (b)Lymphoblasts
  • (c)Reticulocytes
  • (d)Osteoblasts

  Answer(a) Megakaryocytes — platelets are fragments shed from these large marrow cells, which is why they lack a nucleus.