Blood: Structure, Function, and Clinical Aspects

Overview of Blood Functions

Blood is a specialized connective tissue that serves as the body's internal transport system. It performs essential functions in transport, regulation, and protection.

• Transport: Delivers oxygen and nutrients to cells, removes metabolic wastes, and transports hormones.

• Regulation: Maintains body temperature, pH balance, and fluid volume.

• Protection: Prevents blood loss (clotting) and fights infection (immune cells and proteins).

Composition of Blood

Blood consists of plasma (the liquid matrix) and formed elements (cells and cell fragments). When centrifuged, blood separates into three layers:

• Plasma: ~55% of blood; contains water, proteins, nutrients, hormones, and waste products.

• Buffy coat: <1% of blood; contains leukocytes (white blood cells) and platelets.

• Erythrocytes: ~45% of blood; red blood cells (RBCs) responsible for oxygen transport.

Physical Characteristics and Volume

• Blood is more viscous than water and has a metallic taste.

• Color varies with oxygen content: bright red (oxygen-rich) to dark red (oxygen-poor).

• pH: 7.35–7.45; volume: 5–6 L in males, 4–5 L in females (~8% of body weight).

Blood Plasma

Plasma is a straw-colored, sticky fluid making up about 90% water. It contains over 100 dissolved solutes, including:

• Electrolytes: Sodium, potassium, calcium, chloride, bicarbonate, etc.

• Plasma proteins: Albumin (60%), globulins (36%), fibrinogen (4%).

• Nutrients, gases, hormones, wastes, and nonprotein nitrogenous substances.

Formed Elements of Blood

Types of Formed Elements

The formed elements include erythrocytes (RBCs), leukocytes (WBCs), and platelets. RBCs lack nuclei and most organelles; platelets are cell fragments.

Erythrocytes (Red Blood Cells)

Structure and Function

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

• Diameter: ~7.5 μm; thickness: 2.5 μm

• Filled with hemoglobin (Hb), which binds oxygen and carbon dioxide

• Lack mitochondria; generate ATP anaerobically

Hemoglobin Structure and Function

• Each hemoglobin molecule consists of four polypeptide chains (two alpha, two beta) and four heme groups, each with a central iron ion.

• Each iron ion binds one O2 molecule; each RBC contains ~250 million Hb molecules.

• Oxygen loading in lungs forms oxyhemoglobin (bright red); unloading in tissues forms deoxyhemoglobin (dark red).

• About 20% of CO2 binds to Hb, forming carbaminohemoglobin.

Erythropoiesis: Formation of Red Blood Cells

Erythropoiesis occurs in red bone marrow and involves several developmental stages:

• Hematopoietic stem cell → Myeloid stem cell → Proerythroblast → Basophilic erythroblast → Polychromatic erythroblast → Orthochromatic erythroblast → Reticulocyte → Erythrocyte

• Reticulocyte count is a clinical indicator of RBC production rate.

Regulation of Erythropoiesis

Erythropoiesis is regulated by the hormone erythropoietin (EPO), primarily produced by the kidneys in response to hypoxia (low oxygen levels).

• Stimuli for EPO release: decreased RBC count, decreased hemoglobin, decreased O2 availability.

• Testosterone increases EPO production, contributing to higher hematocrit in males.

Dietary Requirements for Erythropoiesis

• Amino acids, lipids, carbohydrates for cell synthesis

• Vitamin B12 and folic acid for DNA synthesis

• Iron for hemoglobin synthesis (stored as ferritin/hemosiderin, transported by transferrin)

Life Cycle and Fate of Erythrocytes

RBCs live 100–120 days. Old RBCs are destroyed in the spleen; components are recycled:

• Iron is stored and reused

• Heme is degraded to bilirubin (excreted in bile)

• Globin is broken down to amino acids

Erythrocyte Disorders

• Anemia: Blood's oxygen-carrying capacity is too low. Causes include blood loss, decreased RBC production, or increased RBC destruction.

• Polycythemia: Excess RBCs increase blood viscosity; can be caused by bone marrow cancer or high altitude.

• Sickle-cell anemia: Genetic disorder causing abnormal hemoglobin (HbS), leading to sickled RBCs and poor oxygen delivery.

Leukocytes (White Blood Cells)

Leukocytes are complete cells that defend the body against infection. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes, monocytes

Granulocytes

• Neutrophils: Most abundant; phagocytize bacteria; multilobed nucleus.

• Eosinophils: Kill parasitic worms; role in allergy/asthma; bilobed nucleus, red granules.

• Basophils: Release histamine; rarest WBC; bilobed nucleus, large purple granules.

Agranulocytes

• Lymphocytes: Second most numerous; crucial for immunity; T cells (attack infected cells), B cells (produce antibodies).

• Monocytes: Largest WBC; become macrophages in tissues; phagocytize pathogens and debris.

Leukopoiesis: Formation of White Blood Cells

Leukopoiesis is stimulated by interleukins and colony-stimulating factors (CSFs). Hematopoietic stem cells differentiate into myeloid or lymphoid lines, giving rise to various WBCs.

Leukocyte Disorders

• Leukopenia: Abnormally low WBC count; often drug-induced.

• Leukemia: Cancerous overproduction of abnormal WBCs; classified by cell type and progression rate.

• Infectious mononucleosis: Viral disease (Epstein-Barr virus); excess atypical lymphocytes.

Platelets (Thrombocytes)

Platelets are cell fragments derived from megakaryocytes. They play a key role in hemostasis by forming temporary plugs to seal vessel breaks.

• Normal count: 150,000–400,000/μL

• Formation (thrombopoiesis) is regulated by thrombopoietin.

Summary Table: Formed Elements of Blood

Hemostasis: Prevention of Blood Loss

Hemostasis is a rapid, localized process that stops bleeding. It involves three steps:

• Step 1: Vascular spasm – Vasoconstriction reduces blood flow.

• Step 2: Platelet plug formation – Platelets adhere to exposed collagen and aggregate.

• Step 3: Coagulation – Fibrin mesh stabilizes the plug, forming a clot.

Coagulation Pathways

Coagulation involves a cascade of clotting factors, leading to the conversion of fibrinogen to fibrin. There are two initial pathways:

• Intrinsic pathway: Triggered by factors within blood; slower.

• Extrinsic pathway: Triggered by tissue factor outside blood; faster.

• Both pathways converge to activate factor X, forming prothrombin activator.

Clot Retraction and Fibrinolysis

• Clot retraction: Platelets contract, pulling wound edges together.

• Fibrinolysis: Plasmin digests fibrin, dissolving the clot after repair.

Disorders of Hemostasis

• Thromboembolic disorders: Unwanted clot formation (thrombus, embolus).

• Bleeding disorders: Thrombocytopenia (low platelets), hemophilia (clotting factor deficiency), impaired liver function.

• Disseminated intravascular coagulation (DIC): Both clotting and bleeding occur.

Blood Transfusion and Blood Groups

ABO and Rh Blood Groups

• ABO system: Based on presence/absence of A and B antigens on RBCs.

• Rh system: Presence of D antigen (Rh+); important in transfusion and pregnancy.

• Transfusion reactions occur if incompatible blood is given, leading to agglutination and hemolysis.

Blood Tests and Clinical Assessment

• Hematocrit: Percentage of RBCs in blood; low in anemia.

• Differential WBC count: Proportions of each WBC type; helps diagnose infections and diseases.

• Prothrombin time (PT): Assesses clotting ability.

• Comprehensive metabolic panel (CMP): Checks blood chemistry for organ function.

• Complete blood count (CBC): Measures formed elements, hemoglobin, and hematocrit.

Developmental Aspects of Blood

• Fetal blood cells form in yolk sac, liver, and spleen; red bone marrow becomes primary site by the seventh month.

• Fetal hemoglobin (HbF) has higher O2 affinity than adult hemoglobin (HbA).

• Blood diseases increase with age, often due to cardiovascular or immune system disorders.