Blood, the Heart and Circulation

Blood Composition, Formed Elements, and Hemostasis

Whole blood functions as a vital transport fluid that consists of formed elements suspended within a liquid extracellular matrix known as plasma. Plasma makes up approximately 55 percent of total blood volume and is predominantly water. In healthy individuals, the normal pH range of human arterial blood is tightly maintained between 7.35 and 7.45. Suspended within the plasma are vital proteins, the most abundant of which is albumin. Albumin plays a critical role in circulatory physiology by maintaining colloid osmotic pressure, which ensures that water remains properly balanced between the bloodstream and the surrounding tissues.

The formed elements include erythrocytes, leukocytes, and thrombocytes, all generated through the process of blood cell production called hematopoiesis, which occurs primarily in red bone marrow. Erythrocytes, or red blood cells, are specialized biconcave, anucleate cells dedicated to the transport of oxygen and carbon dioxide. Packed inside each erythrocyte is hemoglobin, an iron-containing protein that reversibly binds oxygen. Because erythrocytes lack a nucleus and possess a biconcave shape, they provide an efficient vehicle for carrying respiratory gases throughout the body.

Leukocytes, or white blood cells, are nucleated cells that defend the body against pathogens. They are categorized based on their appearance into granulocytes and agranulocytes. Granulocytes contain visible cytoplasmic granules and comprise neutrophils, eosinophils, and basophils. Neutrophils are the most abundant white blood cells, serving as rapid phagocytic responders to bacterial infection. Eosinophils defend against parasitic worm infections and moderate allergic inflammatory responses. Basophils release histamine to promote inflammation and secrete heparin to inhibit blood clotting. In contrast, agranulocytes lack prominent specific cytoplasmic granules and consist of monocytes and lymphocytes. Monocytes circulate in the blood before migrating into tissues to mature into active phagocytic macrophages. Lymphocytes include B cells, T cells, and natural killer cells, which coordinate and mediate both adaptive and innate immunity.

Thrombocytes, commonly called platelets, are small cell fragments derived from large bone marrow cells called megakaryocytes. Platelets are essential for preventing blood loss by forming platelet plugs at sites of vascular injury. When a vessel is damaged, the body initiates hemostasis, the physiological process that stops bleeding. Hemostasis proceeds through three coordinated steps: vascular spasm, platelet plug formation, and coagulation, which limit blood loss and form a temporary seal while the vessel undergoes repair.