IGCSE Biology Paper-2: Specimen Questions with Answers 156 - 156 of 203

Question 156




Which change occurs during blood clotting?


Choice (4)
a.Haemoglobin to oxyhaemoglobin
b.Glucose to cellulose
c.Glucose to glycogen
d.Fibrinogen to fibrin




Blood Coagulation (Blood Clotting)

  • When an injury is caused, the wound does not continue to bleed for a long time. Usually the blood stops flowing outside after sometimes due to blood coagulation or blood clotting.
  • This is a mechanism to prevent excessive loss of blood from the body. Inside an intact blood vessel, blood does not coagulate due to the presence of active anticoagulants, heparin, and antithrombin. Procoagulants also occur in the blood but are in an inactive state.

The process of coagulation can be described in three major steps.

  • At the site of an injury, the blood platelets disintegrate and release a phospholipid, called platelet factor- 3 (platelet Thromboplastin). Injured tissue also release a lipoprotein factor called Thromboplastin. These two factors combine with calcium ions and certain proteins of blood plasma to form an enzyme called Prothrombinase.
  • In presence of calcium, the Prothrombinase inactivates heparin (or antipro thrombin-anticoagulant). Prothrombinase also catalyses the conversion prothrombin (an inactive plasma protein) into an active protein called thrombin and some small peptide fragments.
  • Thrombin acts as enzyme and first causes depolymerization of fibrinogen (a soluble plasma protein) into its monomers. Later thrombin stimulates repolymerization of these monomers into long insoluble fibre like polymers called fibrin. The thin long and solid fibres of fibrin form a dense network upon the wound and trap blood corpuscles and platelets to form a clot. The clot seals the wound and checks the bleeding. A clot is formed at the wound in about 2 - 8 minutes after injury. Soon after, the clot starts contracting (clot retraction) and a pale yellow fluid called serum, starts oozing out from it. This serum is blood minus the corpuscles and fibrinogens.
Blood Coagulation (Blood Clotting)

Blood Coagulation (Blood Clotting)

Loading image