THE MAJOR BODY FLUIDS BLOOD
By definition, blood is a tissue, composed of several types of cells in a matrix called plasma. Plasma consists of about 90% water and 10% of a long list of other sub stances (7% protein, 3% urea, amino acids, carbohydrates, organic acids, fats, steroid hormones, and other inorganic ions). Within the plasma are three types of cells: erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Red blood cells, erythrocytes, are legion in the blood—roughly 5 million per milliliter of blood! The purpose of these 6–8 µm diameter cells is to transport oxygen and carbon dioxide throughout the body via the circulatory system; this is accomplished by hemoglobin. Hemoglobin, the respiratory pigment of many animals, is a conjugated protein consisting of four polypeptides, each of which contains a heme group. The heme groups contain iron and have the ability to bind with oxygen; this association is reversible, allowing for respiration. Erythrocytes are produced in the bone marrow and have about a 4-month life span. They discard their nuclei as they mature and, therefore, contain no DNA. White blood cells, or leukocytes, are active in the immune system but are not as numerous as erythrocytes, about 10–15,000 per milliliter of blood. The two types of leukocytes differ in their specific functions but work in a coordinated fashion to provide the body’s defense against disease. The first type, neutrophils, is part of the first line of defense and offers up a complicated response to invaders: the immune response. Neutrophils, which are the most abundant type of white blood cell, work in conjunction with lymphocytes, which are produced in the bone marrow and the thymus gland, to engender the immune response. Lymphocytes produce antibodies, which are protein molecules that can bind to foreign molecules. Any foreign molecule that induces antibody formation is called an antigen. Macrophages support the immune response. Once bacteria or damaged platelets are identified, macrophages swarm the area and consume the offending materials until they literally eat themselves to death. The dead cells are exuded from the body as pus. Finally, platelets are only fragments of cells and contain no nuclei; they number around 15–300,000 per milliliter of blood. Platelets are involved in the clotting process.
Table 10.1 The Blood Groups in the ABO System and Their Antigens, Antibodies, and Frequency in the Population
