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5.3.4: Fluid Tissues

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    Fluid Tissues

    Blood and lymph are the two types of connective tissue in this sub-category. Both are fluid, rather than solid, and both lack the network of extracellular protein fibers found in the other types of connective tissue.


    The blood has a fluid matrix (plasma) and specialized cells or formed elements of the blood: red blood cells (erythrocytes), white blood cells (leukocytes), and cell fragments called platelets. The primary function of blood is to deliver oxygen (O2) and nutrients to and remove wastes from body cells, but that is only the beginning of the story. The specific functions of blood also include defense, distribution of heat, and maintenance of homeostasis. Mature erythrocytes lack organelles, including a nucleus, so they can accommodate a high concentration of hemoglobin, a protein that carries O2 and CO2. Hemoglobin carries O2 from the lungs to body cells which use O2 to make ATP (energy-rich molecule) in the process of cellular respiration. Hemoglobin also carries CO2 from body cells to the lungs to dispose of this waste product of the cellular respiration process. Leukocytes are involved in immune responses and platelets are important for blood clotting.

    Microscopic image of blood.

    Above: Blood. Tissue is magnified by 400x.


    Lymph is the fluid located in the lymphatic vessels and contain lymphocytes. Lymphocytes are the primary cells of adaptive immune responses. The two basic types of lymphocytes, B cells and T cells, are identical morphologically with a large central nucleus surrounded by a thin layer of cytoplasm. They are distinguished from each other by their surface protein markers as well as by the molecules they secrete.