Blood is a fluid within the human body that circulates to deliver necessary substances and remove waste products. This transport system comprises a liquid matrix, plasma, and various solid components suspended within it. These solid components, the formed elements, perform diverse and specialized functions fundamental to maintaining overall health.
Overall Composition of Blood
The formed elements of blood are cellular and cell-fragment components suspended within the plasma. Plasma is primarily water, transporting nutrients, hormones, and waste products. The three main categories of formed elements are erythrocytes, responsible for carrying oxygen; leukocytes, involved in the body’s defense mechanisms; and thrombocytes, which stop bleeding. These components account for about 45% of total blood volume, with plasma making up the remaining 55%.
Red Blood Cells
Erythrocytes, or red blood cells, are the most numerous formed elements, responsible for oxygen transport. These cells have a biconcave disc shape, which increases their surface area for gas exchange and allows them to flex through narrow capillaries. Mature red blood cells lack a nucleus and other organelles, making more space for hemoglobin, the iron-containing protein that binds to oxygen. Hemoglobin enables efficient oxygen delivery from the lungs to the body’s tissues. Red blood cells circulate for approximately 120 days before being removed by the spleen and liver.
White Blood Cells
Leukocytes, or white blood cells, are a diverse group of cells integral to the body’s immune system, defending against infections and foreign invaders. These cells are less numerous than red blood cells but are larger and retain their nuclei. Leukocytes are categorized into two main groups based on the presence or absence of visible granules in their cytoplasm: granulocytes and agranulocytes.
Granulocytes include neutrophils, eosinophils, and basophils.
Neutrophils
Neutrophils are the most abundant white blood cells. They are characterized by a multi-lobed nucleus and fine, faintly staining granules, primarily targeting bacterial infections through phagocytosis.
Eosinophils
Eosinophils have a bi-lobed nucleus and large, reddish-orange granules. They play a role in allergic reactions and combating parasitic worms.
Basophils
Basophils, the least common white blood cells, possess a bi-lobed nucleus often obscured by large, dark purple granules. They release histamine and heparin in inflammatory and allergic responses.
Agranulocytes, lacking prominent cytoplasmic granules, consist of lymphocytes and monocytes.
Lymphocytes
Lymphocytes have a large, round nucleus that fills most of the cell. They are central to specific immunity, with B cells producing antibodies and T cells directly attacking infected cells.
Monocytes
Monocytes are the largest white blood cells, featuring a kidney-shaped or horseshoe-shaped nucleus. They differentiate into macrophages in tissues, becoming powerful phagocytes that engulf pathogens and cellular debris.
Platelets
Thrombocytes, or platelets, are small, irregular-shaped cell fragments, not complete cells. These fragments originate from large cells called megakaryocytes in the bone marrow. Platelets circulate for about 8 to 10 days before being removed. Their primary function is hemostasis, the process of stopping bleeding.
When a blood vessel is injured, platelets quickly adhere to the damaged site and aggregate to form a temporary plug. They also release various chemicals that promote further platelet aggregation and activate clotting factors. This initiates the coagulation cascade, leading to the formation of a stable fibrin clot that reinforces the initial platelet plug.
Where Blood Cells Are Formed
All formed elements of the blood originate from a common source through a process called hematopoiesis, or hemopoiesis. This continuous production of blood cells primarily occurs within the red bone marrow in adults, found in bones such as the pelvis, sternum, and vertebrae. The precursor cells for all blood cell types are multipotent hematopoietic stem cells. These stem cells possess the ability to self-renew and differentiate into any of the specialized blood cell lineages.
Under the influence of various growth factors and signaling molecules, hematopoietic stem cells undergo a series of divisions and maturation steps. This complex differentiation pathway ensures a steady supply of new red blood cells, white blood cells, and platelets to replace old or damaged cells and to respond to the body’s changing demands.