Blood: An Overview

Introduction to Blood

Blood is a specialized connective tissue that serves as the body’s internal transport system, playing vital roles in transport, regulation, and protection. It consists of cellular elements suspended in a liquid matrix called plasma.

• Transport: Delivers oxygen, nutrients, hormones; removes wastes.

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

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

Composition of Blood

Major Components

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

• Plasma: ~55% of whole blood; least dense component.

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

• Erythrocytes: ~45% of blood (hematocrit); most dense component.

Physical Characteristics and Volume

• Viscous, opaque fluid with a metallic taste.

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

• pH: 7.35–7.45.

• Volume: 5–6 L in males, 4–5 L in females (about 8% of body weight).

Blood Plasma

Plasma is a straw-colored, sticky fluid composed mostly of water (about 90%) and over 100 dissolved solutes, including nutrients, gases, hormones, wastes, proteins, and inorganic ions.

• Plasma proteins: Most abundant solutes by weight; produced mainly by the liver.

• Albumin: Major plasma protein; acts as a blood buffer, carrier, and maintains osmotic pressure.

Formed Elements of Blood

Overview

The formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Most are short-lived and do not divide; they are replaced by stem cells in red bone marrow.

Erythrocytes (Red Blood Cells)

Structure

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

• Diameter: ~7.5 μm; thickness: ~2.5 μm.

• Lack nuclei and most organelles.

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

Function

• Dedicated to respiratory gas transport.

• Hemoglobin binds reversibly with oxygen; each Hb molecule can carry four oxygen molecules.

• Normal Hb values: Males 13–18 g/100 mL; Females 12–16 g/100 mL.

Hemoglobin Structure

Hemoglobin consists of four polypeptide chains (two alpha, two beta) each bound to a heme group with a central iron ion that binds oxygen.

Gas Exchange

• Oxygen loading (lungs): Produces oxyhemoglobin (bright red).

• Oxygen unloading (tissues): Produces deoxyhemoglobin (dark red).

• CO2 loading (tissues): 20% of CO2 binds to Hb, forming carbaminohemoglobin.

Erythropoiesis: Formation of Red Blood Cells

Erythropoiesis is the process of red blood cell production, occurring in red bone marrow. It involves several stages from stem cell to mature erythrocyte.

• Hematopoietic stem cell → Myeloid stem cell → Proerythroblast → Erythroblast stages → Reticulocyte → Erythrocyte

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

Regulation of Erythropoiesis

Maintained by hormonal control (mainly erythropoietin, EPO) and adequate dietary nutrients (iron, vitamin B12, folic acid).

• Erythropoietin (EPO): Released by kidneys in response to hypoxia (low oxygen).

• Testosterone enhances EPO production (higher hematocrit in males).

Dietary Requirements

• Iron: Essential for hemoglobin synthesis; stored as ferritin and hemosiderin, transported by transferrin.

• Vitamin B12 and folic acid: Required for DNA synthesis in rapidly dividing cells.

Fate and Destruction of Erythrocytes

RBCs have a lifespan of 100–120 days. Old RBCs are removed by macrophages in the spleen, liver, and bone marrow. Hemoglobin is broken down into heme (degraded to bilirubin), iron (recycled), and globin (amino acids reused).

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.

• Sickle-cell anemia: Genetic disorder causing abnormal hemoglobin (HbS) and sickled RBCs.

Leukocytes (White Blood Cells)

General Structure and Function

Leukocytes are complete cells with nuclei and organelles. They defend the body against infection and disease, and can leave the bloodstream to enter tissues (diapedesis).

• Normal count: 4,800–10,800/μL.

• Leukocytosis: WBC count above normal, usually in response to infection.

Types of Leukocytes

• Granulocytes: Neutrophils, eosinophils, basophils (contain visible granules).

• Agranulocytes: Lymphocytes, monocytes (lack visible granules).

Granulocytes

• Neutrophils: Most abundant; phagocytize bacteria.

• Eosinophils: Attack parasitic worms; modulate allergic responses.

• Basophils: Release histamine; involved in inflammation.

Agranulocytes

• Lymphocytes: T cells (attack infected/tumor cells), B cells (produce antibodies).

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

Leukopoiesis: Formation of White Blood Cells

Leukopoiesis is stimulated by interleukins and colony-stimulating factors. Hematopoietic stem cells differentiate into myeloid or lymphoid lineages, giving rise to various leukocytes.

Leukocyte Disorders

• Leukemia: Cancerous overproduction of abnormal WBCs.

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

• Infectious mononucleosis: Viral disease causing excess atypical lymphocytes.

Platelets (Thrombocytes)

Structure and Function

Platelets are cell fragments derived from megakaryocytes. They play a crucial role in hemostasis by forming temporary plugs to seal vessel breaks and releasing chemicals for clotting.

• Normal count: 150,000–400,000/μL.

• Formation (thrombopoiesis) regulated by thrombopoietin.

Hemostasis: Prevention of Blood Loss

Overview

Hemostasis is a rapid, localized process to stop bleeding, involving three steps:

1. Vascular spasm: Vasoconstriction of damaged vessel.

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

3. Coagulation: Fibrin mesh forms, stabilizing the plug.

Coagulation Pathways

Coagulation involves a cascade of clotting factors, leading to the conversion of fibrinogen to fibrin. There are intrinsic and extrinsic pathways, both leading to the formation of prothrombin activator.

Clot Retraction and Fibrinolysis

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

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

Disorders of Hemostasis

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

• Bleeding disorders: Impaired clotting (thrombocytopenia, hemophilia, impaired liver function).

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

Blood Transfusions and Blood Typing

Blood Groups

Blood types are determined by the presence or absence of antigens (agglutinogens) on RBC membranes. The most clinically important are the ABO and Rh groups.

• ABO system: Type A (A antigen), Type B (B antigen), Type AB (both), Type O (neither).

• Rh system: Presence (+) or absence (–) of D antigen.

Transfusion Reactions

• Occur if mismatched blood is transfused; recipient’s antibodies attack donor RBCs.

• Can cause agglutination, hemolysis, and renal failure.

• Type O is the universal donor; type AB is the universal recipient (for ABO system only).

Blood Tests and Clinical Assessment

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

• Blood glucose: Assesses diabetes.

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

• Prothrombin time (PT) and platelet count: Assess hemostasis.

• Complete blood count (CBC): Evaluates 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 oxygen affinity than adult hemoglobin (HbA).

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