Blood, tissue fluid and lymph

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  • Blood, tissue fluid and lymph
    • Blood
      • Is the liquid held in our blood vessels. It consists of blood cells in a watery fluid called plasma.
        • The plasma contains many dissolved substances, including oxygen, carbon dioxide, salts, glucose, fatty acids, amino acids, hormones and plasma proteins
        • The cells include the red blood cells (erthrocytes), various white blood cells (leucocytes) and fragments called platelets
    • Tissue fluid
      • Similar to blood, doesn't contain most of the cells found in blood or plasma proteins.
      • The role of tissue fluid is to transport oxygen and nutrients from the blood to the cells and to carry carbon dioxide and other wastes back to the blood
      • Bathes the cells of individual tissures
    • How tissue fluid is formed
      • When an artery reaches the tissues it branches into smaller arterioles, then into a network of capillaries.
        • These eventually link up with venules to carry blood into the veins.
        • So blood flowing into an organ or tissue is contained in the capillaries.
        • At the arterial end of a capillary, the blood is under high pressure due to the contraction of the heart, known as hydrostatic pressure.
          • It will tend to push the blood fluid out of the capillaries. The fluid can leave through the tiny gaps in the capillary wall.
            • The fluid that leaves the blood consists of plasma with dissolved nutrients and oxygen.
            • All RBC, platelets and most of the WBC remain in the blood as do the plasma proteins (too large to be pushed out)
            • The fluid that leaves the capillary is known as the tissue fluid.
    • How does the fluid return to the blood
      • The tissue fluid has hydrostatic pressure, which will push the fluid back into the capillaries
      • Both the blood and the tissue fluid also contains solutes, giving them a negative water potential. The water potential of the tissue fluid is less negative than that of the blood.
        • This means that water tends to move back into the blood from the tissue fluid by osmosis, down the water potential gradient.
      • At the venous end of the capillary, the blood has lost its hydrostatic pressure. The combined effect of the hydrostatic pressure in the tissue fluid and the osmotic force of the plasma proteins is sufficient to move fluid back into the capillary.
        • It carries with it any dissolved waste substances, such as carbon dioxide, that have left the cells.
    • Formation of lymph
      • Excess tissue fluid drains into the the lymphatic system.
      • The lymphatic system consists of a number of vessels that are similar to capillaries. They start in the tissues and drain the excess in fluid into larger vessels which  eventually rejoin the blood system in the chest cavity
      • Lymph fluid is similar to tissue fluid and contains the same solutes. There will be less oxygen and nutrients. There will also be more carbon dioxide and wastes that have been released from the body cells.
        • Lymph also has more fatty material that has been absorbed from the intestines.
      • Lymph contains many lymphoctyes. These are produced in the lymph nodes.
        • The lymph nodes are swellings found at intervals along the lymphatic system. They filter any bacteria and foreign material from the lymph fluid. They phagocytes can then engulf and destroy these bacteria and foreign particles. The part of the immune system that protects the body against infection.
    • Lymph
      • Held within the lymphatic system.

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