Coupon Accepted Successfully!


The Blood

As noted previously, blood is considered a connective tissue, and like all connective tissues is composed of cells separated by an intercellular matrix, referred to as plasma. All blood cells are manufactured in the bone marrow from unspecialized stem cells, and ultimately differentiate into a variety of cells with different functions and characteristics. 
  • Red blood cells (erythrocytes): Erythrocytes, specialized for the transport of oxygen, are the most plentiful type of blood cell, gaining their red color from large quantities of the protein hemoglobin. Hemoglobin contains iron, and binds reversibly to oxygen. Red blood cells are unique among body cells in that they lack a nucleus, which limits their lifetime to a few months. This means that they, like other blood cells, must constantly be produced by the bone marrow. Erythrocytes are also characterized by their unique shape, which resembles a biconcave disk.
  • White blood cells (leukocytes): White blood cells are involved with some aspect of the body’s defense system. There are many types of leukocytes, which can be divided into two major groups based upon their general morphology.
    • granulocytes have granular cytoplasm, and include the neutrophils, eosinophils, and basophils.
    • agranulocytes (which lack cytoplasmic granules) are of two major types, monocytes and lymphocytes.
Neutrophils, monocytes, and macrophages (large cells that develop from monocytes) are involved in the process of phagocytosis (the ingestion and subsequent digestion of foreign agents that enter the body). Eosinophils help to control inflammation (discussed later), while basophils release heparin (which inhibits blood clotting) and histamine (which is involved in allergic reactions). Lymphocytes are involved in specific immunity, which will be discussed later.
  • Platelets (thrombocytes): Platelets are not truly cells, but fragments of giant cells which break apart and enter the circulation. They play an essential role in blood clotting and repairing breaks in blood vessels. Blood clotting is a complex process requiring a cascade of reactions involving at least fifteen different plasma proteins. When a blood vessel break is detected, the end result is the activation of one of these proteins into fibrin, an insoluble protein which clumps and binds together platelets, forming a clot that covers the wound.
  • Plasma: Plasma is the liquid matrix of the blood, in which all the other components are either suspended or dissolved. Plasma is largely made up of water, with many solutes dissolved in it. The dissolved substances include nutrients (amino acids, monosaccharides, and small lipids); gases (oxygen, carbon dioxide, and nitrogen); wastes (urea, uric acid, ketones, etc.); and a wide variety of simple ions, or electrolytes, which influence the pH and osmotic pressure of the blood and tissue fluids. Also found dissolved in the plasma are a variety of proteins, called plasma proteins, which are involved in several processes including clotting, immune reactions, and the maintenance of osmotic balance.

Test Your Skills Now!
Take a Quiz now
Reviewer Name