Blood Clotting (Edexcel International A Level Biology)

• 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 thromboplastin, which acts as an enzyme
  • 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

Stroke

• 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 heat 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