Blood, Lymphatic System and Immunity

Blood Composition and Clinical Specimen Collection

Blood is a specialized fluid tissue that circulates throughout the cardiovascular system, delivering vital substances to tissues and removing waste products. It consists of liquid plasma and cellular formed elements. The continuous production of these cellular components is known as hematopoiesis, a process that occurs primarily in red bone marrow. Blood plasma accounts for the fluid portion and is composed of approximately ninety percent water, along with electrolytes, nutrients, and proteins. The major plasma proteins suspended in this fluid are albumin, globulins, and fibrinogen. When a collected blood specimen is allowed to clot, the fluid separated from the clot is called serum. Serum differs from plasma because it lacks fibrinogen and has reduced levels of clotting factors that were consumed during the coagulation process.

The formed elements of blood include erythrocytes, leukocytes, and thrombocytes. Erythrocytes, or red blood cells, are biconcave discs that lack a nucleus when mature. Their primary function is oxygen transport, made possible by hemoglobin, an iron-containing protein that binds and transports oxygen. Leukocytes, or white blood cells, protect the body against pathogens and infection. Leukocytes are divided into granulocytes and agranulocytes. Among the granulocytes, neutrophils are the most abundant and serve as the first line of cellular defense through phagocytosis; eosinophils defend against parasitic infections and modulate allergic responses; and basophils release histamine to promote inflammation and heparin to inhibit blood clotting. The agranulocytes include monocytes, which can differentiate into macrophages in tissues, and lymphocytes: B and T cells mediate adaptive immunity, while natural killer cells contribute to innate immunity. Finally, thrombocytes, or platelets, are cell fragments derived from megakaryocytes that are essential for hemostasis.

In the outpatient medical office, the medical assistant routinely handles blood specimens under provider supervision. When a provider orders a complete blood count, the medical assistant collects venous blood in tubes containing EDTA. If coagulation testing is ordered, the medical assistant collects blood in tubes containing sodium citrate. Beyond venipuncture, the medical assistant performs point-of-care hemoglobin and hematocrit testing using CLIA-waived analyzers as delegated. When diagnostic testing reveals laboratory-reported critical blood cell count results, the medical assistant promptly relays these findings to the supervising provider according to office protocol, maintaining professional scope by never attempting to independently interpret results.