Comprehensive Study Notes: Blood (Chapter 19) – Structure, Function, and Clinical Relevance

Blood: Structure and Function

Overview of Blood

Blood is the body's only liquid connective tissue, accounting for about 8% of total body mass (5-6 liters in males, 4-5 liters in females). Unlike typical connective tissues, blood lacks collagen and elastic fibers. It consists of formed elements (cells and cell fragments) suspended in plasma (the non-living fluid matrix).

• Formed elements: Erythrocytes (RBCs), Leukocytes (WBCs), Platelets

• Plasma: Water, proteins, nutrients, electrolytes, hormones, gases, and wastes

Functions of Blood

Blood performs three primary functions:

1. Transport: Oxygen, carbon dioxide, nutrients, wastes, and hormones

2. Protection: Clotting (hemostasis) and immune defense against pathogens

3. Regulation: Body temperature, pH (buffering via bicarbonate), and fluid volume

Blood Composition

Plasma and Formed Elements

Blood is composed of plasma (about 55% of volume) and formed elements (about 45% of volume).

• Plasma: Mostly water (92%), with proteins (albumins, globulins, fibrinogen), nutrients, electrolytes, and wastes

• Formed elements:

  • Erythrocytes (RBCs): Oxygen transport

  • Leukocytes (WBCs): Immune defense

  • Platelets: Clotting

Plasma Proteins

Plasma contains three major categories of proteins:

• Albumins: Smallest and most abundant; contribute to viscosity, osmolarity, and influence blood pressure

• Globulins: Antibodies and transport proteins; alpha, beta, and gamma types

• Fibrinogen: Precursor to fibrin, essential for clot formation

Formed Elements: Erythrocytes (RBCs)

Structure and Function

Erythrocytes are small, biconcave, non-nucleated cells specialized for gas transport. Their shape maximizes surface area for efficient exchange of oxygen and carbon dioxide.

• Do not consume the oxygen they transport (anaerobic metabolism)

• Renewed by division of precursor cells in bone marrow

Hemoglobin

Hemoglobin is the oxygen-carrying protein in erythrocytes.

• Composed of four globin chains (2 alpha, 2 beta)

• Each chain is bound to a heme group containing iron

• Each iron atom binds reversibly to one O2 molecule

Erythropoiesis: RBC Production

Erythropoiesis is the process of RBC formation, occurring in the red bone marrow.

• Stimulated by erythropoietin (EPO) from the kidneys

• Development involves reduction in cell size, synthesis of hemoglobin, and loss of nucleus

• Average lifespan of RBC: ~120 days

Iron Metabolism

Iron is essential for hemoglobin synthesis.

• Dietary iron (Fe3+) is converted to Fe2+ (usable form) by stomach acid

• Transported in blood by transferrin; stored in liver as ferritin

Erythrocyte Death and Removal

Old erythrocytes are removed by macrophages in the spleen and liver. Hemoglobin is broken down into heme and globin; iron is recycled, and bilirubin is excreted in bile.

Anemia

Anemia is a condition of reduced oxygen-carrying capacity of blood.

• Causes: Decreased hemoglobin, decreased hematocrit, abnormal hemoglobin

• Types: Hemorrhagic, hemolytic, iron-deficiency, pernicious, sickle cell

• Symptoms: Fatigue, pale skin, shortness of breath, edema, increased heart rate

Formed Elements: Leukocytes (WBCs)

Types and Functions

Leukocytes are complete cells with nuclei and organelles, responsible for immune defense.

• Granulocytes:

  • Neutrophils: Phagocytize bacteria, release antimicrobial chemicals

  • Eosinophils: Attack parasites, phagocytize antigen-antibody complexes

  • Basophils: Release histamine (inflammation) and heparin (anticoagulant)

• Agranulocytes:

  • Lymphocytes: Destroy cancer/virus-infected cells, secrete antibodies, immune memory

  • Monocytes: Differentiate into macrophages, phagocytize pathogens and debris

Leukopoiesis: WBC Production

Leukocytes are produced from hemopoietic stem cells in bone marrow.

• Myeloblasts form granulocytes

• Monoblasts form monocytes

• Lymphoblasts form lymphocytes

Formed Elements: Platelets and Hemostasis

Platelet Structure and Function

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

• Secrete vasoconstrictors, form platelet plugs, release clotting factors

• Initiate clot-dissolving enzymes, attract immune cells, repair blood vessels

Hemostasis: Steps in Blood Clotting

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 and aggregate

3. Coagulation: Fibrin mesh forms, trapping blood cells and stabilizing the clot

4. Clot Retraction and Thrombolysis: Clot contracts and is eventually dissolved

Blood Typing and Matching

ABO and Rh Blood Groups

Blood types are determined by antigens (agglutinogens) on erythrocyte surfaces and 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 antibodies (universal recipient)

  • Type O: No antigens, anti-A and anti-B antibodies (universal donor)

• Rh System:

  • Rh+ has D antigen; Rh- lacks D antigen

  • Anti-D antibodies form only after exposure to Rh+ blood

Transfusion Reactions

Transfusion of incompatible blood leads to agglutination (clumping) and destruction of erythrocytes, which can be life-threatening.

Rh Factor and Pregnancy

Rh incompatibility can cause hemolytic disease of the newborn if an Rh- mother carries an Rh+ fetus. Preventive treatment with Rh immune globulin is used to avoid antibody formation.

Summary Table: Blood Components

Component	Main Function	Key Features
Erythrocytes (RBCs)	Oxygen transport	Biconcave, no nucleus, hemoglobin
Leukocytes (WBCs)	Immune defense	Complete cells, several types
Platelets	Clotting	Cell fragments, form plugs
Plasma	Transport medium	Water, proteins, solutes

Key Equations

Bicarbonate Buffer System: $\mathrm{H_2CO_3 \rightleftharpoons HCO_3^- + H^+}$ Hematocrit Calculation: $\mathrm{Hematocrit = \frac{Volume\ of\ RBCs}{Total\ Blood\ Volume} \times 100\%}$ Oxygen Binding to Hemoglobin: $\mathrm{Hb + O_2 \rightleftharpoons HbO_2}$ Iron Transport: $\mathrm{Fe^{2+} + Transferrin \rightarrow Fe^{2+}-Transferrin\ Complex}$ Coagulation Cascade (simplified): $\mathrm{Fibrinogen \xrightarrow{Thrombin} Fibrin}$ Agglutination Reaction: $\mathrm{Antibody + Antigen \rightarrow Agglutination}$

Additional info:

• Blood is classified as a connective tissue due to its origin from mesenchyme and its function in connecting body systems via transport.

• Serum is plasma without clotting factors (fibrinogen).

• Blood typing is crucial for safe transfusions and organ transplants.