IB Biology SL

Revision Notes

6.3.2 Blood Clotting


  • When the skin is cut, microorganisms have an entry point to get into the body
    • The first line of defence is compromised
  • In order to minimise the risk of substantial blood loss and entry of unwanted microorganisms, the blood starts to clot to seal the wound
  • In response to blood vessel damage, platelets form a temporary plug to stem bleeding
    • Platelets are cellular fragments that make up one component of the blood
  • They release chemicals called clotting factors that trigger a chemical cascade which results in blood clotting

The components of the blood 1, downloadable IGCSE & GCSE Biology revision notes

The blood is made up of 4 key components including plasma, red blood cells, white blood cells and platelets

Blood Clotting Proteins

  • The chemical cascade, triggered by the clotting factors, involves a large number of steps and several plasma proteins
    • First of all, the clotting factors stimulate the release of the enzyme thrombin
    • Thrombin catalyses the conversion of the soluble protein fibrinogen into fibrin, which is insoluble
    • Fibrin forms a mesh that traps more platelets and blood cells to prevent entry through the wound
      • A small initial stimulus is amplified to produce a large amount of fibrin so that the wound is quickly sealed
    • Exposure to air results in the hardening of the mesh to create a scab

How the blood clots, IGCSE & GCSE Biology revision notesBlood clot formation

Clotting in Coronary Arteries

Causes of blood clots in the coronary arteries

  • A blood clot in the coronary arteries is called coronary thrombosis
  • Several factors may increase the risk of coronary thrombosis developing:
    • Atherosclerosis in the coronary arteries results in a build-up of layers of fatty material (plaque) causing damage to the endothelium wall
      • Bulging of the lumen of the artery causes a blockage which reduces the space for blood flow
      • Deposition of calcium ions can worsen the situation by hardening the endothelium
      • Lesions can also sometimes form due to ruptures in the atheroma

Consequences of blood clot formation in the coronary arteries

  • Occlusion of the coronary arteries is a common problem that can lead to significant health issues such as coronary heart disease
  • The coronary arteries deliver oxygen and nutrients to the cardiac muscle tissue
  • If a blood clot forms in the coronary arteries, it can cause blockages
  • A blockage means that the tissue beyond that point is deprived of oxygen and nutrients, so it is unable to respire aerobically
  • As a result, cells are unable to produce a sufficient amount of ATP which inhibits normal cardiac muscle contraction resulting in irregular and uncoordinated movement called fibrillation
    • If not rectified, either naturally or through medical intervention, fibrillation could lead to death
  • A heart attack (myocardial infarction) may also occur in situations where the blood supply is completely inhibited so that the cardiac muscle tissue starts to die
    • This can be fatal

Risk factors for coronary thrombosis

  • There are several factors which have shown a clear correlation with increased chances of coronary thrombosis or heart attacks
  • The main risk factors for include:
    • Genetic factors
    • Age and sex
    • High blood pressure
    • Smoking
    • High concentrations of low-density lipoproteins (LDLs) 
    • Diabetes
    • Obesity
    • Lack of exercise

Exam Tip

Remember, correlation does not prove causation: There are many contributing factors which will affect the likelihood of developing a coronary thrombosis, as a result, we cannot say that any single factor is causative. We can say that there is a correlation between that factor and the incidence of coronary thrombosis


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