How Does Blood Clot Normally

What is blood?

Blood is a fluid that carries essential substances around the body through 2 types of blood vessels: arteries and veins. Arteries carry oxygenated blood away from the heart to the rest of the body. Veins then bring blood back to the heart and lungs, where it picks up more oxygen.3

Blood is made up of water, proteins, blood cells, and other molecules. There are 3 types of blood cells4:

  • Red blood cells are the most common. They contain hemoglobin, a protein that picks up oxygen from the lungs and releases it throughout the body
  • White blood cells fight infections
  • Platelets seal leaks from broken blood vessels

Blood also contains plasma, which is 90% water. Plasma contains proteins and other molecules necessary for a variety of important functions, including blood-clotting.4

How does blood clot?

Blood-clotting (also called coagulation) is a complex process that involves more than 30 different plasma proteins, also known as blood-clotting factors. The proteins are found in plasma (the liquid part of blood), and work together with platelets (microscopic cells in the blood) to form a clot when there is a wound.5

The clotting process5,6:

  • Within seconds of an injury that causes a wound, platelets assemble around the wound
  • These platelets react together with blood proteins, calcium, and other tissue factors to form fibrin
  • Fibrin forms a blood clot, acting like a net over the wound
  • During the next several days, the clot strengthens
  • Once the wound has healed, the clot dissolves
Clotting Process

What happens with a bleeding disorder?

In people with bleeding disorders, one of the blood clotting factors is missing or doesn’t work properly. This causes wounds to bleed longer than wounds in people whose clotting factor levels are normal. Bleeding problems vary from person to person, and can range from mild to severe.7


  1. Monroe DM, Hoffman M. What does it take to make the perfect clot? Arterioscler Thromb Vasc Biol. 2006;26(1):41-48.
  2. Hoffman M, Monroe DM. Coagulation 2006: a modern view of haemostasis. Hematol Oncol Clin North Am. 2007;21(1):1-11.
  3. Blood clots. American Society of Hematology website. Accessed May 8, 2015.
  4. Blood basics. American Society of Hematology website. Accessed May 8, 2015.
  5. Marieb EN, Hoehn K. Human Anatomy & Physiology. 9th ed. San Francisco, CA: Pearson; 2013.
  6. World Federation of Hemophilia website. The clotting process. Updated January 2014. Accessed May 8, 2015.
  7. What are bleeding disorders? World Federation of Hemophilia website. Updated May 2012. Accessed May 8, 2015.