4 QUESTIONS GENERATION.docx

OVERVIEW OF BLOOD AND CLOTTING Blood circulates throughout the cardiovascular system in order to supply oxygen and nutrients to the body's tissues and organs. Without adequate blood supply, tissues become ischemic and can die. The essential functions of the blood are carried out by the many and var...

OVERVIEW OF BLOOD AND CLOTTING Blood circulates throughout the cardiovascular system in order to supply oxygen and nutrients to the body's tissues and organs. Without adequate blood supply, tissues become ischemic and can die. The essential functions of the blood are carried out by the many and various components of the blood. Red blood cells (RBCs) deliver oxygen to the tissues and remove byproducts of cellular metabolism, such as carbon dioxide. Blood also transports white blood cells, platelets, and proteins of the coagulation, complement, and kinin systems to sites of injury and pathogen invasion. Additional functions of blood include regulating body temperature, maintaining acid--base balance, transporting nutrients to tissues and discarding wastes, and carrying hormones to target tissues. Blood is composed of plasma and formed elements. Plasma makes up approximately 55% of the whole blood volume. Ninety percent of plasma is water; the remaining volume is composed of electrolytes (sodium \[Na+\] and chloride \[Cl−\] being the most abundant), plasma proteins (such as albumin, globulins, fibrinogen, and prothrombin), nutrients (glucose, amino acids, and lipids), wastes (creatinine, bilirubin, and urea), and dissolved gases (oxygen and carbon dioxide). The remaining 45% of the whole blood volume is composed of formed elements. More than 99% of the cells in the formed elements are RBCs, with white blood cells and platelets making up the remaining cells. White blood cells and their disorders are discussed in Chapters 6, The Immune System and Leukocyte Function, and 7, Neoplasia. RED BLOOD CELLS RBCs (also known as red cells or erythrocytes) are the most abundant blood cells. Each milliliter of blood contains approximately 5 billion RBCs. The primary purpose of the RBC is to travel between the lungs and the tissues carrying oxygen to the tissues (site of oxygen utilization), where it picks up carbon dioxide for removal by the lungs. RBCs lack nuclei, organelles, and ribosomes in order to accommodate a large amount of hemoglobin, which maximizes oxygen- carrying capacity. Without nuclei and other organelles, RBCs are unable to undergo mitotic division or conduct cell repair, resulting in their limited life span (100 to 120 days). Continual production of new RBCs through erythropoiesis is required to maintain an adequate red cell count. RBCs constitute approximately 34% to 47% of the blood volume in adult women and 39% to 54% in adult men; this value is termed the hematocrit. Total hemoglobin is measured by lysing the cells in a blood sample to measure the total protein. Hemoglobin in adult women averages 11 to 16 g/dL; in adult men it averages 14 to 18 g/dL. Lower values for hematocrit or hemoglobin are diagnosed as anemia. In order to carry oxygen to the tissues of the body, RBCs must have a size and shape that allow them to travel through very small capillaries. The biconcave disk of the RBC is 7 to 8 μm in diameter and 2 μm thick, which allows for optimal diffusion of gases across the membrane. RBCs also have the capacity to be reversibly deformed, which allows the cells to become more compact to squeeze through smaller capillaries, which can be only 5 μm in diameter, and then return to their original shape. Specific characteristics of the cells are quantified using several key indices: RBC size is estimated as the mean cell volume (MCV), calculated by the laboratory as the hematocrit divided by the red cell count. Differential diagnosis of many RBC disorders includes identification of macrocytosis (increased MCV, \>100 fL), microcytosis (decreased MCV, \