Blood Clotting (Edexcel International A Level Biology): Revision Note

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 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