Edexcel A (SNAB) A Level Biology:复习笔记1.1.8 Blood Clotting

Blood Clotting

  • Blood clotting is an important process; it prevents excess blood loss, the entry of pathogens, and provides a barrier, or scab, under which wound healing can occur
  • A break in the mucous membranes or skin membranes causes the release of molecules that trigger a chemical cascade which results in blood clotting
    • The process of blood clotting is known as thrombosis
  • The chemical cascade involves a large number of steps and several plasma proteins
  • The process of thrombosis involves
    •  The damaged blood vessel releases a protein called thromboplastin
    • Calcium ions from the plasma, along with thromboplastin, trigger the conversion of soluble prothrombin protein into the enzyme thrombin
    • Thrombin catalyses the conversion of the soluble protein fibrinogen to the insoluble protein fibrin
    • Fibrin fibres mesh and tangle together, trapping platelets and red blood cells
    • A blood clot is formed

Blood clotting, or thrombosis, is brought about by a cascade of chemical reactions

Blood clots and atheromas

  • Atheromas can increase the risk of blood clotting
    • The plaque deposit of an atheroma can rupture through the endothelium of the artery, damaging the endothelium and forming a rough surface
    • The damage to the endothelium triggers the process of thrombosis
  • The combination of atheromas and blood clotting can be dangerous to the health of an individual
    • The blood clot that forms can completely block the artery
      • Blood clots reduce blood flow which restricts the movement of oxygen in the blood, therefore reducing respiration of the surrounding cells, tissues and organs
    • The blood clots can dislodge and travel to different blood vessels in the body; if they reach the brain this can cause a stroke to occur
  • Blood clotting can also lead to
    • Heart attack
    • Deep vein thrombosis


  • A stroke is a sudden loss of brain function in a localised area due to disruption of blood flow to the brain
    • A blood clot leads to a blockage of the arteries supplying the brain
    • This leads to reduced blood flow and delivery of oxygen to the cells of the brain, reducing respiration
    • Cells in the affected part of the brain cannot produce ATP and their function is reduced
  • A stroke caused by a blood clot is called an ischemic stroke

Heart attack

  • Coronary heart disease (CHD) is caused by the formation of atheromas and blood clots in the coronary arteries
    • The coronary arteries flow over the surface of the heart, supplying the heart muscle itself with blood
  • Blood flow to certain areas of the heart is restricted and delivery of oxygen to the affected cells decreases, thereby reducing respiration in these cells
    • The cells can no longer produce ATP
    • The cells can no longer contract, reducing the force generated by the heart when it beats
    • The cells can die, causing permanent damage to heart tissue
  • This leads to a myocardial infarction, also known as a heart attack
  • Complete heart failure may occur if large areas of the heart are affected by blood clots; this can be fatal
  • Symptoms of a heart attack include
    • Chest pain
    • Shortness of breath
    • Sweating

Atheroma and blood clots in the coronary arteries can restrict blood flow to the heart muscle, leading to a heart attack

Deep vein thrombosis

  • If a blood clot forms in a vein deep inside the body, it is known as deep vein thrombosis (DVT)
  • This is most common in the veins of the legs
  • Causes include
    • Prolonged inactivity
    • Old age
    • Some medications