Blood is a specialized connective tissue composed of a liquid component called plasma, which suspends and carries various dissolved substances. Within this plasma, three types of structures are suspended, often referred to as the “formed elements.” One element is technically a cell fragment rather than a complete cell. Each formed element possesses a unique structure that enables it to perform specific functions, including the transport of gases, defense against foreign invaders, and repair of the circulatory system.
Red Blood Cells (Erythrocytes): Oxygen Carriers
Red blood cells, or erythrocytes, are the most abundant formed element in the blood. Their primary role is to transport oxygen from the lungs to tissues and carry carbon dioxide back to the lungs for exhalation. This specialized function is reflected in their unique structure.
The mature erythrocyte is a biconcave disc. This shape increases the cell’s surface area-to-volume ratio, maximizing gas diffusion across its membrane. The cell also lacks a nucleus and most other organelles (anucleated).
The absence of internal structures provides maximum space for hemoglobin, the iron-containing protein that binds to oxygen. Each red blood cell contains approximately 270 million hemoglobin molecules. The iron atom within the heme group reversibly binds to oxygen, forming bright red oxyhemoglobin in the lungs.
Once the red blood cell reaches tissues where oxygen concentration is low, the hemoglobin releases its bound oxygen. Hemoglobin also carries about 20% of the carbon dioxide back toward the lungs. Erythrocytes typically complete a 120-day lifespan.
The flexibility of the cell’s membrane, maintained by proteins like spectrin, allows the biconcave disc to deform and squeeze through the narrowest capillaries without rupturing.
White Blood Cells (Leukocytes): The Immune Defense
White blood cells, or leukocytes, circulate to identify and neutralize pathogens, clear cellular debris, and mediate inflammatory responses. Unlike erythrocytes, leukocytes are complete cells, possessing a nucleus and other organelles, and are capable of active movement. They can leave the bloodstream and migrate into tissues, a process called diapedesis, to patrol and launch defensive actions.
Leukocytes are classified into two main categories based on the presence or absence of visible granules in their cytoplasm: granulocytes and agranulocytes. Granulocytes (neutrophils, eosinophils, and basophils) have lobed nuclei and contain enzyme-filled sacs released during an immune response.
Neutrophils are the most numerous leukocyte and are the first responders to bacterial infections and localized inflammation. They are highly phagocytic, destroying bacteria and other foreign particles. Eosinophils primarily target parasitic worms and help dampen allergic reactions.
Basophils are the least common granulocyte and play a role in allergic and inflammatory responses by releasing histamine, which dilates blood vessels to increase blood flow. The agranulocytes category includes lymphocytes and monocytes, which lack cytoplasmic granules and have a spherical or kidney-shaped nucleus.
Lymphocytes are the central coordinators of adaptive immunity, providing targeted defense against specific invaders. T lymphocytes directly attack virus-infected and tumor cells. B lymphocytes differentiate into plasma cells that produce specific antibodies. Monocytes circulate briefly before migrating into tissues, where they differentiate into macrophages.
Macrophages are large phagocytic cells that clean up cellular debris, dead neutrophils, and damaged tissue. They also function as antigen-presenting cells, processing foreign materials and displaying them to T lymphocytes to activate the immune response.
Platelets (Thrombocytes): Essential for Clotting
Platelets, or thrombocytes, are small, irregular, non-nucleated cellular fragments derived from large bone marrow cells called megakaryocytes. They are packed with chemicals and proteins necessary for their primary function: hemostasis, the process of stopping bleeding following damage to a blood vessel.
When a blood vessel wall is injured, platelets respond immediately to prevent blood loss. The initial step is adhesion, where platelets stick to the exposed collagen fibers at the injury site. Attached platelets then become activated, changing shape and releasing chemical messengers.
These chemical signals attract more platelets, leading to aggregation where they stick to one another to form a temporary platelet plug. This plug serves as a physical seal for the breach in the vessel wall, providing immediate primary hemostasis.
Simultaneously, activated platelets release factors that initiate the coagulation cascade involving plasma proteins. This cascade culminates in the conversion of soluble fibrinogen into a meshwork of insoluble fibrin protein strands. The fibrin mesh reinforces the temporary platelet plug, creating a stable blood clot that seals the injury until tissue repair is complete.
The lifespan of a platelet is short, circulating for about 8 to 10 days before being removed by the spleen and liver.