5.2 Prevention of Blood Coagulation

• How long does blood normally take to clot in a glass test tube?

  About 6 minutes
• Why does blood in siliconized containers take longer to clot?

  Silicone prevents contact activation of platelets
• What are the two principal factors that initiate the intrinsic clotting mechanism?

  Platelets and Factor XII
• What happens in untreated glass containers regarding blood coagulation?

  They allow rapid contact activation of platelets
• What is heparin used for?

  Preventing coagulation of blood
• In what surgical procedures is heparin especially used?

  Heart-lung machine or artificial kidney machine
• What type of substances can prevent blood coagulation outside the body?

  Substances that decrease calcium ion concentration
• How does a soluble oxalate compound prevent coagulation?

  It precipitates calcium oxalate from plasma
• What ion is especially valuable for preventing coagulation?

  Citrate ion
• How does citrate prevent coagulation?

  It combines with calcium to form un-ionized compound
• What is an advantage of citrate anticoagulants over oxalate anticoagulants?

  Citrate is non-toxic in moderate quantities
• What happens to citrate after injection into the blood?

  It is removed by the liver and metabolized
• How much citrated blood can be transfused safely?

  500 milliliters
• What can happen if large quantities of citrated blood are given too quickly?

  Citrate may depress calcium ion levels
• What are the potential consequences of depressed calcium ion levels in the blood?

  It can result in tetany and convulsive death