Composition and Functions of Blood

Erythrocytes: The Oxygen Carriers

Erythrocytes, commonly known as red blood cells (RBCs), make up 99% of all formed elements. They are uniquely adapted for their role in gas exchange.

Structure and Adaptation

RBCs are shaped like flattened biconcave discs. This specific geometry provides two major advantages:

• Increased Surface Area: Maximizes the efficiency of gas diffusion.
• Flexibility: Allows the cells to bend and squeeze through narrow capillaries.

Unlike most cells, mature RBCs lack a nucleus and mitochondria, which allows more room for their primary cargo: hemoglobin. Each individual RBC contains approximately 280 million hemoglobin molecules.

Hemoglobin and Gas Transport

Hemoglobin is a complex protein consisting of four polypeptide chains (two alpha and two beta chains), each containing a heme molecule. The state of the hemoglobin determines the color of the blood and the type of gas being carried:

• Oxyhemoglobin: Hemoglobin bound to oxygen, giving blood a bright red appearance.
• Deoxyhemoglobin: Hemoglobin that has released its oxygen, appearing burgundy or reddish-blue (a condition known as cyanosis).
• Carbaminohemoglobin: Hemoglobin bound to carbon dioxide, which remains attached until the RBC reaches the lungs.

Clinical Note: Anemia A low concentration of hemoglobin or a low number of RBCs results in a condition called anemia. A specific genetic variation known as sickle-cell anemia is caused by a mutation in the beta polypeptide chains of the hemoglobin molecule.

Erythropoiesis: Making New Cells

Because RBCs only live for about 120 days, the body must constantly produce new ones through a process called erythropoiesis. This process involves the maturation of myeloid stem cells in the bone marrow (and in the liver and spleen during fetal development). Erythropoiesis is stimulated by hormones when the body experiences hypoxic (low oxygen) conditions. When old RBCs reach the end of their lifespan, they are engulfed by white blood cells in the liver, spleen, or bone marrow; the heme is converted into bilirubin, while other components are recycled.

Leukocytes and Platelets: Defense and Repair

While RBCs dominate the volume, other formed elements provide essential defense and clotting services.

Leukocytes (White Blood Cells)

Leukocytes are the primary agents of the immune system. Unlike RBCs, they possess nuclei and mitochondria, and they exhibit amoeboid movement, allowing them to squeeze through capillary walls (a process called diapedesis) to reach infection sites.

Leukocytes are divided into two main groups:

1. Granular Leukocytes: These contain visible granules in their cytoplasm.
   • Neutrophils: The most common WBC; they are highly phagocytic and use "positive chemotaxis" to be attracted to chemical signals from bacteria.
   • Eosinophils: Involved in attacking allergens and parasites.
   • Basophils: The rarest WBC; they release histamine (to cause vasodilation) and heparin (to prevent clotting).
2. Agranular Leukocytes:
   • Monocytes: The largest WBCs, which act as powerful phagocytes.
   • Lymphocytes: Essential for specific immunity, including T-cells (cell defense), B-cells (antibody production), and Natural Killer (NK) cells (destroying abnormal/cancerous cells).

Platelets (Thrombocytes)

Platelets are not whole cells but rather small fragments of larger cells called megakaryocytes. They lack a nucleus and have a short lifespan of 5–9 days. Their primary role is to facilitate clotting by forming a "platelet plug" and releasing serotonin to cause vasoconstriction, which reduces blood flow to a damaged area.