Chapter 15 Solutions — Body Fluids and Circulation

The formed elements constitute about 45% of blood and include three types of cells:

1. Erythrocytes (Red Blood Cells, RBCs): Biconcave, anucleate cells containing haemoglobin. Major function: transport of respiratory gases (O2 and CO2) between lungs and body tissues.

2. Leucocytes (White Blood Cells, WBCs): Nucleated, colourless cells. There are two categories:

- Granulocytes (neutrophils, eosinophils, basophils): Neutrophils and monocytes are phagocytic and destroy foreign organisms; basophils secrete histamine and heparin involved in inflammatory reactions; eosinophils resist infections and are associated with allergic reactions.

- Agranulocytes (lymphocytes, monocytes): B and T lymphocytes are responsible for immune responses of the body.

Overall major function: defense against pathogens and immune response.

3. Platelets (Thrombocytes): Cell fragments produced from megakaryocytes in bone marrow. Major function: release substances involved in coagulation (clotting) of blood, thereby preventing excessive blood loss.

Plasma contains three major groups of proteins, each with a distinct physiological role:

1. Fibrinogen: Essential for clotting (coagulation) of blood. It is an inactive precursor that is converted to fibrin threads by the enzyme thrombin during the clotting cascade, forming the clot at the site of injury.

2. Globulins: Primarily involved in the defense mechanisms of the body. They include immunoglobulins (antibodies) that neutralise antigens and pathogens, contributing to humoral immunity.

3. Albumins: Help in maintaining osmotic balance (oncotic pressure) of blood. They prevent excessive loss of water from blood into the interstitial spaces, thus maintaining blood volume and pressure.

In addition, plasma also contains clotting factors (in inactive form) that are essential for the coagulation cascade. Plasma without clotting factors is called serum.

Blood is classified as a connective tissue because it shares the fundamental features that define connective tissues:

1. Mesodermal origin: Like all connective tissues, blood develops from mesoderm during embryonic development.

2. Fluid matrix (extracellular matrix): Blood consists of a fluid matrix called plasma (about 55% of blood volume), which corresponds to the extracellular matrix characteristic of connective tissues. The cells (formed elements) are suspended in this matrix.

3. Specialised cells in a matrix: Blood contains formed elements — erythrocytes, leucocytes, and platelets — suspended in plasma, analogous to cells embedded in the matrix of other connective tissues.

4. Connects and integrates body parts: Blood connects different organs and systems of the body by transporting nutrients, gases, hormones, and waste products between them.

Thus, blood fulfils all criteria of a connective tissue — mesoderm-derived, cells dispersed in an abundant extracellular matrix — and is classified as a special or fluid connective tissue.