Which one of the following is the major function of platelets?

Correct — A, Clotting of blood. Platelets, also called thrombocytes, are cell fragments shed from large bone-marrow cells. When a vessel is cut they stick to the damaged wall and to each other, forming a temporary plug, and they release factors that set off the cascade converting fibrinogen into fibrin. The fibrin mesh traps blood cells and hardens the plug into a clot. A shortage of platelets shows up as easy bruising and prolonged bleeding.

• (b)O2 transport — Oxygen transport is the work of red blood cells, which carry it bound to haemoglobin. Platelets have no haemoglobin and, being fragments, no nucleus either.
• (c)Immunity — Immunity is handled by the white blood cells — lymphocytes making antibodies, and others attacking pathogens directly.
• (d)Phagocytosis — Phagocytosis, the engulfing of microbes and debris, is carried out by neutrophils and monocytes among the white cells.

Blood has three formed elements suspended in plasma. Red cells carry oxygen, white cells defend the body, and platelets stop bleeding. Platelets are the smallest of the three and are not complete cells at all — they are fragments budded off from megakaryocytes in the bone marrow, and they survive in circulation for about a week to ten days.

Each wrong option here names the job of a different blood component, so the item is really asking whether the three elements have been kept apart. The clinical hook worth remembering is dengue: the virus drives the platelet count down, and a falling count is what doctors watch for because it signals a risk of bleeding. Vitamin K is required for the liver to make several clotting factors, which is why its deficiency also prolongs bleeding even when the platelet count is normal.

• Platelets are cell fragments from bone-marrow megakaryocytes, without a nucleus, and live about a week to ten days.
• They plug damaged vessels and release factors that convert fibrinogen into a fibrin mesh.
• A normal count is roughly 150,000 to 400,000 per microlitre, far above the white cell count.
• Dengue and some other viral infections cause a sharp fall in platelet count, raising the risk of bleeding.
• Vitamin K is needed for the liver to synthesise several clotting factors, so its deficiency also delays clotting.

• Assuming platelets are complete cells; they are anucleate fragments.
• Attributing immunity or phagocytosis to platelets because all three elements circulate together.
• Confusing a low platelet count with anaemia, which is a red-cell or haemoglobin problem.

A function-to-component item, often set in a dengue context so that the clinical clue and the physiology have to be linked.

• practice — not a real PYQ

  A sharp fall in the platelet count in a patient with dengue is dangerous mainly because it leads to

  • (a)reduced oxygen transport
  • (b)impaired blood clotting and a risk of bleeding
  • (c)loss of immunity to bacteria
  • (d)an inability to digest food

  Answer(b) impaired blood clotting and a risk of bleeding — platelets initiate the plug and the cascade that seals a damaged vessel.

• practice — not a real PYQ

  The soluble plasma protein converted into an insoluble mesh during clotting is

  • (a)albumin
  • (b)globulin
  • (c)fibrinogen
  • (d)haemoglobin

  Answer(c) fibrinogen — it is converted to fibrin, whose threads trap blood cells and form the clot.