Blood: Structure and Function

Overview of Blood

Blood is a specialized bodily fluid that serves as the only liquid connective tissue in the human body. It is essential for transporting substances, regulating physiological parameters, and providing protection against disease and blood loss.

• Volume: Approximately 5-6 liters in males and 4-5 liters in females, accounting for about 8% of total body mass.

• Composition: Blood consists of plasma (the non-living fluid matrix) and formed elements (cells and cell fragments).

• Functions:

  • Transport of oxygen, carbon dioxide, nutrients, hormones, and metabolic wastes.

  • Protection against blood loss (hemostasis) and infection (immune system).

  • Regulation of body temperature, pH (buffering via bicarbonate), and fluid balance.

Components of Blood

Formed Elements

The formed elements of blood include erythrocytes, leukocytes, and platelets. Each type plays a distinct role in maintaining homeostasis.

• Erythrocytes (Red Blood Cells, RBCs): Oxygen-transporting cells; biconcave discs that maximize surface area for gas exchange.

• Leukocytes (White Blood Cells, WBCs): Immune cells responsible for defense against pathogens. Five major types are classified as granulocytes or agranulocytes.

• Platelets: Cell fragments involved in blood clotting and repair of damaged blood vessels.

Leukocyte Classification

• Granulocytes: Contain visible cytoplasmic granules.

  • Neutrophils: Phagocytize bacteria; most abundant WBC.

  • Eosinophils: Combat parasites and participate in allergic responses.

  • Basophils: Release histamine and heparin; involved in inflammatory responses.

• Agranulocytes: Lack visible granules.

  • Lymphocytes: Produce antibodies, destroy cancer cells, and regulate immune responses.

  • Monocytes: Differentiate into macrophages; phagocytize pathogens and debris.

Blood Plasma

Plasma is the liquid portion of blood, making up about 55% of total blood volume. It is a complex mixture of water, proteins, nutrients, electrolytes, wastes, hormones, and gases.

• Water: ~92% of plasma volume.

• Proteins: Albumins, globulins, and fibrinogen are the major plasma proteins.

  • Albumins: Smallest and most abundant; contribute to viscosity and osmolarity.

  • Globulins: Include antibodies (immune function) and transport proteins.

  • Fibrinogen: Precursor to fibrin, essential for blood clotting.

• Serum: Plasma without clotting proteins (fibrinogen).

Hematocrit and Blood Composition

Hematocrit

Hematocrit is the percentage of blood volume occupied by red blood cells. It is a key indicator of oxygen-carrying capacity.

• Normal values: ~45% RBCs, ~1% WBCs and platelets (buffy coat), ~55% plasma.

Erythrocytes and Hemoglobin

Structure and Function of Erythrocytes

Erythrocytes are biconcave, anucleate cells specialized for oxygen transport. Their shape increases surface area for gas exchange and allows flexibility in capillaries.

• Diameter: ~7.5 μm.

• Function: Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

• Metabolism: Generate ATP anaerobically; do not consume transported oxygen.

Hemoglobin

Hemoglobin is the oxygen-carrying protein in erythrocytes. It consists of four globin chains, each bound to a heme group containing iron.

• Structure: 2 alpha and 2 beta globin chains; each heme group binds one O2 molecule.

• Function: Binds oxygen reversibly; also transports some carbon dioxide.

Equation:

Erythropoiesis

Erythropoiesis is the process of erythrocyte production, primarily occurring in red bone marrow. It is regulated by the hormone erythropoietin (EPO) from the kidneys.

• Stages: Hemopoietic stem cell → Erythrocyte-CFU → Proerythroblast → Erythroblast → Reticulocyte → Mature erythrocyte.

• Regulation: Negative feedback; hypoxemia stimulates EPO release, increasing RBC production.

• Lifespan: ~120 days.

Iron Metabolism: Iron is essential for hemoglobin synthesis. Dietary iron is absorbed and transported by transferrin, stored as ferritin.

Anemia

Anemia is a condition characterized by decreased oxygen-carrying capacity of blood, often due to reduced hemoglobin or RBC count.

• Types:

  • Hemorrhagic anemia: Blood loss.

  • Hemolytic anemia: Premature RBC destruction.

  • Iron-deficiency anemia: Insufficient iron intake or absorption.

  • Pernicious anemia: Vitamin B12 deficiency.

  • Sickle cell anemia: Abnormal hemoglobin structure.

• Symptoms: Pale skin, shortness of breath, edema, reduced blood pressure, increased heart rate.

Leukocytes and Immune Function

Leukocyte Types and Functions

Leukocytes are complete cells with nuclei and organelles, less abundant than RBCs. They defend against pathogens and are classified by the presence or absence of cytoplasmic granules.

• Granulocytes:

  • Neutrophils: Phagocytize bacteria; most numerous.

  • Eosinophils: Combat parasites; modulate allergic responses.

  • Basophils: Release histamine (vasodilator) and heparin (anticoagulant).

• Agranulocytes:

  • Lymphocytes: Produce antibodies, destroy infected and cancerous cells, regulate immune responses.

  • Monocytes: Differentiate into macrophages; phagocytize pathogens and debris.

Leukopoiesis

Leukopoiesis is the production of white blood cells from hemopoietic stem cells in bone marrow. Distinct colony-forming units (CFUs) give rise to specific leukocyte lineages.

• Myeloblasts: Form neutrophils, eosinophils, basophils.

• Monoblasts: Form monocytes.

• Lymphoblasts: Form lymphocytes.

Granulocytes circulate for hours, monocytes for days, and lymphocytes for years, providing long-term immunity.

Platelets and Hemostasis

Platelet Structure and Function

Platelets are cytoplasmic fragments of megakaryocytes, essential for blood clotting and vessel repair.

• Functions:

  • Secrete vasoconstrictors to reduce blood loss.

  • Form platelet plugs to seal small breaks.

  • Release clotting factors (procoagulants).

  • Initiate clot-dissolving enzymes.

  • Attract immune cells to sites of inflammation.

  • Phagocytize bacteria.

  • Secrete growth factors for vessel repair.

Hemostasis

Hemostasis is the process of stopping bleeding, involving several steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen, aggregate, and release granule contents to recruit more platelets.

3. Coagulation: Clotting factors lead to the conversion of fibrinogen to fibrin, forming a mesh that traps blood cells.

4. Clot Retraction: The clot contracts, pulling the wound edges together.

5. Thrombolysis: Plasmin dissolves the clot after healing.

Equation (Bicarbonate Buffering):

Coagulation Pathways

• Intrinsic Pathway: Initiated by damage inside the vessel.

• Extrinsic Pathway: Initiated by external trauma.

• Both pathways converge to activate thrombin, which converts fibrinogen to fibrin.

Blood Types and Transfusion Compatibility

ABO and Rh Blood Groups

Blood types are determined by the presence of specific antigens (agglutinogens) on erythrocyte surfaces and corresponding antibodies (agglutinins) in plasma.

• ABO System:

  • Type A: A antigen, anti-B antibody.

  • Type B: B antigen, anti-A antibody.

  • Type AB: A and B antigens, no anti-A or anti-B antibodies.

  • Type O: No antigens, both anti-A and anti-B antibodies.

• Rh System:

  • Rh positive (Rh+): D antigen present.

  • Rh negative (Rh-): D antigen absent; anti-D antibodies form only after exposure.

Blood Type Compatibility Table

Transfusion Reactions

If incompatible blood is transfused, antibodies bind to foreign antigens, causing agglutination (clumping) and destruction of erythrocytes. This can lead to serious complications.

Rh Factor and Pregnancy

Rh incompatibility can occur when an Rh-negative mother carries an Rh-positive fetus. Maternal antibodies may attack fetal RBCs in subsequent pregnancies, causing hemolytic disease of the newborn.

• Prevention: Rh immune globulin injections prevent antibody formation.

Rh Factor Pregnancy Precautions Table

Summary

• Blood is a vital connective tissue with multiple functions in transport, regulation, and protection.

• Its components—plasma, erythrocytes, leukocytes, and platelets—work together to maintain homeostasis.

• Understanding blood types and compatibility is crucial for safe transfusions and pregnancy management.

Additional info: Expanded explanations and tables were inferred and added for completeness and clarity.