Blood: Structure, Function, and Clinical Significance

Module 19.1 Overview of Blood

Blood is a specialized connective tissue essential for transport, regulation, and protection in the body. Understanding its components and functions is foundational for anatomy and physiology students.

• Major Components of Blood:

  • Whole blood: The total volume of blood, consisting of plasma and formed elements.

  • Plasma: The liquid matrix, making up about 55% of blood volume; contains water, proteins, and solutes.

  • Formed elements: Cellular components including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

  • Erythrocytes: Red blood cells responsible for oxygen transport.

  • Leukocytes: White blood cells involved in immune defense.

  • Platelets: Cell fragments important for clotting.

  • Buffy coat: Thin layer between plasma and erythrocytes after centrifugation, containing leukocytes and platelets.

  • Hematocrit: Percentage of blood volume occupied by erythrocytes; normal range is about 37–54%.

• Blood Volume: Average adult has 5 liters of blood, about 8% of body weight.

• Proportion of Centrifuged Blood:

  • Plasma: ~55%

  • Buffy coat: <1%

  • Erythrocytes: ~45%

• Seven Basic Functions of Blood:

  1. Transport of gases (O2, CO2), nutrients, hormones, and wastes

  2. Regulation of pH and ion composition

  3. Restriction of fluid loss (clotting)

  4. Defense against toxins and pathogens

  5. Stabilization of body temperature

  6. Regulation of osmotic pressure

  7. Communication between organs via hormones

• Plasma Composition:

  • Water: ~90%

  • Proteins: ~7%

  • Solutes (electrolytes, nutrients, wastes): ~2–3%

• Plasma Proteins:

  • Albumins: Maintain osmotic pressure; most abundant.

  • Gamma-globulins (antibodies): Immune defense.

  • Transport proteins: Alpha- and beta-globulins, lipoproteins; carry substances like iron, lipids.

  • Clotting proteins: Fibrinogen and others; essential for coagulation.

• Example: Albumin helps retain water in the bloodstream, preventing edema.

Module 19.2 Erythrocytes and Oxygen Transport

Erythrocytes are specialized for oxygen transport, and their production and destruction are tightly regulated.

• Structure and Function of Erythrocytes:

  • Biconcave shape increases surface area for gas exchange.

  • Lack nuclei and organelles; filled with hemoglobin.

• Hemoglobin:

  • Protein with four subunits, each containing a heme group (iron-containing pigment).

  • Each heme binds one O2 molecule.

• Hematopoiesis and Erythropoiesis:

  • Hematopoiesis: Formation of all blood cells from hematopoietic stem cells.

  • Erythropoiesis: Formation of erythrocytes.

  • Steps: Hematopoietic stem cell → myeloid cell line → erythrocyte CFU → erythroblast → reticulocyte → erythrocyte.

  • Erythropoietin: Hormone produced by kidneys; stimulates erythroblast maturation.

• Erythrocyte Death and Recycling:

  • Lifespan: ~120 days; limited by lack of organelles.

  • Destroyed in spleen; hemoglobin broken into heme, globin, and iron.

  • Transferrin: Protein that transports recycled iron.

  • Heme Disposal: Heme converted to biliverdin (green), then bilirubin (yellow).

• Anemia: Condition of reduced oxygen-carrying capacity.

  • Causes: Decreased erythrocytes, decreased hemoglobin, abnormal hemoglobin.

• Types of Anemia:

  • Iron-deficiency anemia: Low iron, reduced hemoglobin.

  • Pernicious anemia: Vitamin B12 deficiency.

  • Hemolytic anemia: Excessive erythrocyte destruction.

  • Aplastic anemia: Bone marrow failure.

  • Sickle-cell disease: Abnormal hemoglobin causes cell deformation.

• Example: Sickle-cell disease leads to blocked capillaries and pain crises.

Module 19.3 Leukocytes and Immune Function

Leukocytes are the cellular basis of immune defense, with diverse types and developmental pathways.

• Five Major Types of Leukocytes:

  • Neutrophils: Phagocytosis of bacteria.

  • Eosinophils: Defense against parasites; modulate allergic responses.

  • Basophils: Release histamine; mediate inflammation.

  • Monocytes: Become macrophages; phagocytosis.

  • Lymphocytes: T- and B-cells; adaptive immunity.

• Leukopoiesis: Formation of leukocytes from stem cells.

• Granulocytes vs. Agranulocytes:

  • Granulocytes: Neutrophils, eosinophils, basophils; contain granules.

  • Agranulocytes: Monocytes, lymphocytes; lack granules.

• Development of Formed Elements:

  • Hematopoietic stem cell → myeloid cell line (→ erythrocytes, platelets, neutrophils, eosinophils, basophils, monocytes) or lymphoid cell line (→ T-lymphocytes, B-lymphocytes).

  • Myeloblast → granulocytes; monoblast → monocytes; lymphoblast → lymphocytes.

• Example: Neutrophils are the most abundant leukocyte and first responders to infection.

Module 19.4 Platelets

Platelets are cell fragments essential for blood clotting and repair of damaged vessels.

• Function: Initiate clotting by forming platelet plugs and releasing clotting factors.

• Formation:

  • Hematopoietic stem cell → myeloid cell line → megakaryoblast → megakaryocyte → platelets (fragments).

• Example: Platelets aggregate at injury sites to prevent blood loss.

Module 19.5 Hemostasis

Hemostasis is the process of stopping blood loss, involving multiple phases and specialized proteins.

• Hemostasis vs. Coagulation:

  • Hemostasis: Overall process of stopping bleeding.

  • Coagulation: Specific phase where blood forms a clot.

• Phases of Hemostasis:

  1. Vascular phase: Vessel constriction (vascular spasm).

  2. Platelet plug formation: Platelets adhere, activate, and aggregate.

  3. Coagulation phase: Cascade of clotting factors converts fibrinogen to fibrin, forming a stable clot.

  4. Clot retraction: Clot tightens and shrinks.

  5. Thrombolysis: Clot breakdown via plasminogen activation to plasmin.

• Intrinsic vs. Extrinsic Pathways:

  • Intrinsic (contact) activation: Initiated by damage inside vessel.

  • Extrinsic (tissue factor) activation: Initiated by external tissue damage.

  • Common pathway: Both converge to activate factor X, leading to fibrin formation.

• Role of Calcium and Vitamin K: Essential cofactors for clotting factor activation.

• Hemophilia: Genetic disorder of clotting factor deficiency.

• Thrombosis: Formation of abnormal clots.

  • Thrombus: Stationary clot.

  • Thromboembolism: Clot that breaks free and travels.

  • Embolism: Obstruction caused by a traveling clot.

• Example: Hemophilia A is caused by deficiency of factor VIII.

Module 19.6 Blood Typing and Matching

Blood typing is crucial for safe transfusions and understanding immune reactions to blood antigens.

• Key Terms:

  • Blood transfusion: Transfer of blood from one individual to another.

  • Blood groups: Classification based on antigens on erythrocytes.

  • Antigen: Surface molecule triggering immune response.

  • Antibody: Protein targeting specific antigens.

  • Agglutination: Clumping of cells due to antibody-antigen reaction.

  • Hemolysis: Destruction of erythrocytes.

• Surface Antigens: Determine blood group compatibility.

• ABO Blood Groups:

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

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

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

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

• Rh Blood Groups:

  • Rh+: D antigen present; no anti-Rh antibodies unless exposed.

  • Rh−: D antigen absent; can develop anti-Rh antibodies after exposure.

• Development of Antibodies:

  • Anti-A and anti-B antibodies are naturally occurring.

  • Anti-Rh antibodies develop only after exposure to Rh antigen.

• Blood Type Compatibility:

  • Incorrect transfusion leads to agglutination and hemolysis.

  • Type O− is universal donor; AB+ is universal recipient.

• Example: A person with type B− blood can receive B− or O− blood.

Blood Group Compatibility Table

Additional info: Table entries inferred for completeness and clarity.

Key Equations

• Hematocrit Calculation:

• Oxygen Carrying Capacity: