Chapter 17: Blood – Structure, Function, and Clinical Relevance

Blood: The Only Fluid Tissue in the Body

Blood is a specialized connective tissue that serves as the only fluid tissue in the human body. It plays essential roles in transport, regulation, and protection, supporting the function of all organ systems.

Main Functions of Blood

• Transport: Blood transports oxygen, nutrients, metabolic wastes (such as carbon dioxide and urea), and hormones throughout the body.

• Regulation: Blood helps regulate body temperature by distributing heat, maintains normal pH (around 7.4) through buffers and bicarbonate ions, and preserves fluid volume in the circulatory system by preventing excessive fluid loss via plasma proteins.

• Protection: Blood prevents blood loss through clot formation and protects against infection via antibodies, complement proteins, and white blood cells (leukocytes).

Blood Composition

Blood consists of plasma (the liquid matrix) and formed elements (cells and cell fragments).

• Plasma: Makes up about 55% of blood volume; mostly water (90%) with dissolved solutes (ions, proteins, nutrients, gases, hormones, waste products).

• Formed Elements: Comprise about 45% of blood volume (hematocrit) and include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes).

Table: Blood Components and Their Functions

Component	Main Function(s)
Plasma (water)	Transport medium; solvent for carrying other substances
Albumin	Maintains osmotic pressure; carrier protein
Globulins	Transport proteins; immune functions (antibodies)
Fibrinogen	Clot formation
Erythrocytes (RBCs)	Transport O2 and CO2
Leukocytes (WBCs)	Defense against pathogens
Platelets (Thrombocytes)	Clotting (hemostasis)

Plasma and Plasma Proteins

Plasma is the straw-colored, sticky fluid portion of blood. It is composed of:

• Water (90%): Acts as a solvent and transport medium.

• Plasma Proteins (8% of plasma weight):

  • Albumin: Most abundant; maintains colloid osmotic pressure and transports substances.

  • Globulins: Alpha, beta (transport), and gamma (antibodies).

  • Fibrinogen: Precursor to fibrin, essential for clotting.

• Other Solutes: Electrolytes (Na+, Cl-), nutrients, gases, hormones, waste products.

Colloids refer to large molecular weight particles (mainly proteins) that remain in solution and contribute to osmotic pressure.

Formed Elements of Blood

The formed elements include erythrocytes, leukocytes, and platelets. Of these, only leukocytes are complete cells; erythrocytes lack nuclei and organelles, and platelets are cell fragments.

Erythrocytes (Red Blood Cells)

• Structure: Small (7.5 μm), biconcave discs, anucleate, lack most organelles.

• Function: Specialized for oxygen transport; shape increases surface area for gas exchange.

• Spectrin: Cytoskeletal protein that provides flexibility, allowing RBCs to traverse capillaries.

• Hemoglobin (Hb): Pigment that binds oxygen; each molecule has 4 globin subunits, each with a heme group (contains Fe2+), and each Hb can bind 4 O2 molecules.

• One RBC contains about 250 million Hb molecules, making up 97% of cell volume (excluding water).

Hematopoiesis and Erythropoiesis

• Hematopoiesis: Formation of all blood cells; occurs in red bone marrow.

• Erythropoiesis: Formation of RBCs from hemocytoblasts; involves ribosome synthesis, hemoglobin accumulation, and ejection of the nucleus before entering circulation as reticulocytes.

• Regulation: Controlled by erythropoietin (EPO), a hormone released by the kidneys in response to hypoxia (low blood O2).

Key Points:

• Too few RBCs: tissue hypoxia

• Too many RBCs: increased blood viscosity

• EPO release is triggered by low blood O2, not by the number of RBCs directly.

Destruction of Erythrocytes

• RBCs have a lifespan of ~120 days.

• Aging RBCs lose flexibility (spectrin degrades), and hemoglobin degenerates.

• The spleen removes old RBCs; iron is salvaged, and the heme group is degraded to bilirubin (excreted in bile/feces).

Disorders of Erythrocytes

Anemia

Anemia is a condition where blood has an abnormally low oxygen-carrying capacity. It is classified by cause:

• Blood loss: Hemorrhagic anemia (acute or chronic)

• Not enough RBCs produced:

  • Pernicious anemia: Autoimmune destruction of stomach mucosa; lack of intrinsic factor for vitamin B12 absorption.

  • Iron-deficiency anemia: Due to blood loss, low iron intake, or impaired absorption.

  • Renal anemia: Often due to lack of EPO from kidney disease.

  • Aplastic anemia: Destruction/inhibition of red bone marrow (drugs, radiation, viruses).

• Too many RBCs destroyed (Hemolytic anemias):

  • Thalassemias: Faulty globin chain; RBCs are thin and deficient in hemoglobin.

  • Sickle cell anemia: Abnormal hemoglobin causes RBCs to become crescent-shaped under low O2, leading to rupture and vessel blockage. Potential benefit: resistance to malaria.

Polycythemia

• Excess of circulating RBCs and elevated hematocrit; increases blood viscosity.

• Primary polycythemia: Often genetic.

• Secondary polycythemia: Due to other factors (e.g., high altitude, chronic lung disease, kidney disease).

• Dehydration can also cause a relative increase in hematocrit due to reduced plasma volume.

Blood Typing and Antigens

Surface Antigens and Blood Types

• Antigen: Any molecule that triggers a specific immune response when it enters the body.

• Antibody: Protein that binds to a specific antigen, leading to its destruction.

Table: ABO Blood Types

Blood Type	Antigens on RBCs	Antibodies in Plasma
A	A antigen	Anti-B
B	B antigen	Anti-A
AB	A and B antigens	None
O	None	Anti-A and Anti-B

Universal donor: Type O Universal recipient: Type AB

Rh Blood Groups

• Based on the presence or absence of Rh antigen (D antigen).

• Rh-positive: Antigen present; Rh-negative: Antigen absent.

• Anti-Rh antibodies are only produced after exposure to Rh-positive blood.

• No transfusion reaction on first exposure, but subsequent exposures can cause reactions.

Leukocytes (White Blood Cells)

Leukocytes are complete cells that defend the body against pathogens. There are five main classes, divided into granulocytes and agranulocytes.

Table: Classes of Leukocytes and Their Functions

Leukocyte	Type	Main Function
Neutrophils	Granulocyte	Phagocytosis of bacteria; most abundant
Eosinophils	Granulocyte	Defense against parasites; modulate allergic responses
Basophils	Granulocyte	Release histamine; involved in inflammation and allergic reactions
Lymphocytes	Agranulocyte	Adaptive immunity (B cells produce antibodies; T cells destroy infected cells)
Monocytes	Agranulocyte	Differentiate into macrophages; phagocytosis of pathogens and debris

Platelets (Thrombocytes) and Hemostasis

Platelets are cell fragments essential for blood clotting (hemostasis). Hemostasis prevents blood loss following vessel injury and involves three steps:

1. Vascular Spasm: Vasoconstriction of damaged vessel to reduce blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen fibers, become activated, and aggregate to form a temporary plug.

3. Coagulation: Fibrinogen is converted to fibrin, forming a stable clot. This process involves a cascade of clotting factors, many of which require vitamin K for synthesis.

After healing, clots are removed by fibrinolysis.

Disorders of Hemostasis

• Thromboembolism: Formation of abnormal clots (thrombus: stationary; embolus: freely floating).

• Bleeding Disorders: Deficiency of clotting factors impairs blood clotting (e.g., hemophilia).

Summary Table: Key Blood Disorders

Disorder	Main Feature	Cause
Anemia	Low O2 carrying capacity	Blood loss, low RBC production, high RBC destruction
Polycythemia	High RBC count/hematocrit	Genetic, secondary to disease or environment
Thromboembolism	Excessive clot formation	Genetic, acquired, or idiopathic
Hemophilia	Impaired clotting	Deficiency of clotting factors

Additional info: Erythropoietin (EPO) is sometimes abused by athletes to increase RBC count and oxygen-carrying capacity, but this practice is dangerous and can lead to increased risk of stroke, clotting, or heart failure due to elevated blood viscosity.