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16.1D: Blood Plasma

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  • Plasma comprises about 55% of total blood volume. It contains proteins and clotting factors, transports nutrients, and removes waste.


    Describe the features of blood plasma


    Key Points

    • The majority of blood volume consists of plasma. This aqueous solution is 92% water. It also contains blood plasma proteins, including serum albumin, blood-clotting factors, and immunoglobulins.
    • Plasma circulates respiratory gases, dissolved nutrients, and other materials. It also removes waste products.
    • Globulins are a diverse group of proteins that primarily transport other substances and inhibit certain enzymes.
    • Albumins maintain osmotic balance between the blood and tissue fluids through exertion of oncotic pressure.
    • Fibrinogen is the main clotting protein found in plasma. It is responsible for stopping blood flow during wound healing.

    Key Terms

    • platelet: A small, colorless, disc-shaped particle found in the blood of mammals. It plays an important role in blood clot formation.
    • immunoglobulin: Any of the glycoproteins in blood serum that respond to invasion by foreign antigens and that protect the host by removing pathogens; an antibody.
    • albumins: A plasma protein that exerts a high degree of oncotic pressure to pull water and other substances into tissues.

    About 55% of blood is blood plasma, a straw-colored liquid matrix in which blood cells are suspended. It is an aqueous solution containing about 90% water, 8% soluble blood plasma proteins, 1% electrolytes, and 1% elements in transit. One percent of the plasma is salt, which helps with pH. Human blood plasma volume averages about 2.7–3.0 liters.

    Molecular Contents of Plasma


    Composition of Blood: Two tubes of EDTA-anticoagulated blood. Left tube: after standing, the RBCs have settled at the bottom of the tube.

    Plasma contains molecules that are transported around the body. Respiratory gases, such as oxygen and carbon dioxide, may be dissolved directly in the plasma. However, most oxygen is hemoglobin bound, and most carbon dioxide is converted to bicarbonate ions in the plasma. Hormones and nutrients such as glucose, amino acids and proteins, lipids and fatty acids, and vitamins are also dissolved in the plasma. Waste products are carried through the plasma during their removal, including urea and ammonia.

    Plasma Proteins

    The largest group of solutes in plasma contains three important proteins: albumins, globulins, and clotting proteins.


    Albumins, produced in the liver, make up about two-thirds of the proteins in plasma. Albumins maintain the osmotic balance between the blood and tissue fluids. These proteins exert a force that pulls water towards them, which is called oncotic or osmotic pressure. During inflammation, albumins leave the vascular endothelium and enter the tissues, which transports water and some of the plasma into the interstitial fluid. This is the principal cause of exudate edema, which is the swelling that indicates inflammation.

    Albumins also assist in transport of different materials, such as vitamins and certain molecules and drugs (e.g. bilirubin, fatty acids, and penicillin) due to the force exerted by their oncotic pressure. Plasma that is pulled into the tissues by albumin-exerted oncotic pressure becomes interstitial fluid. This gradually drains into the lymphatic system which it turn recirculates it back into the plasma of the circulatory system.


    Globulins are a diverse group of proteins designated into three groups, gamma, alpha, and beta, based on how far they move during electrophoresis tests. Their main function is to transport various substances in the blood. For example, the beta globulin transferrin can transport iron. Most gamma globulins are antibodies (immunoglobulin), which assist the body’s immune system in defense against infections and illness. Alpha globulins are notable for inhibiting certain proteases, while beta globulins often function as enzymes in the body.

    Clotting Factors

    Clotting proteins are mainly produced in the liver. Twelve proteins known as “clotting factors” participate in the cascade clotting process during endothelial injury. One important clotting factor is fibrinogen. Fibrinogen generates fibrin when activated by the coagulant thrombin, which forms a mesh that clots blood with the assistance of a platelet plug. Normally, anticoagulants and fibrinolytics in the plasma, such as plasmin and heparin, break up fibrin clots and inactivate thrombin. However, during endothelial injury, damaged cells will release tissue factor, another type of clotting factor that causes a cascade of thrombin production that will overpower the anticoagulants and cause a clotting response.

    Serum is a term used to describe plasma that has been removed of its clotting factors. Serum still contains albumin and globulins, which are often called serum proteins as a result.



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