Blood: Composition and Physical Characteristics

Overview of Blood

Blood is a specialized fluid connective tissue essential for the transport of substances, regulation of physiological parameters, and protection against disease. It consists of formed elements suspended in plasma.

• Formed elements:

  • Erythrocytes (Red Blood Cells, RBCs): Transport oxygen and carbon dioxide.

  • Leukocytes (White Blood Cells, WBCs): Defend against pathogens.

  • Platelets: Cell fragments involved in clotting.

• Physical characteristics:

  • Color: Scarlet (O2-rich) to dark red (O2-poor)

  • Viscosity: More dense and viscous than water

  • pH: 7.35–7.45

  • Temperature: ~38°C

  • Volume: 5–6 L (♂), 4–5 L (♀); ~8% of body weight

Main Functions of Blood

Blood performs three primary functions: distribution, regulation, and protection.

• Distribution:

  • Transports oxygen, nutrients, metabolic wastes, and hormones throughout the body.

• Regulation:

  • Maintains body temperature via heat distribution, conservation, and dissipation.

  • Regulates pH and ion composition (e.g., plasma proteins, bicarbonate reserve).

  • Ensures adequate fluid volume in tissues.

• Protection:

  • Prevents blood loss via clotting (platelets, plasma proteins).

  • Defends against infection (complement system, WBCs, antibodies).

Plasma: Composition and Functions

Plasma Characteristics

Plasma is the straw-colored, liquid component of blood, making up about 55% of its volume. It is composed of:

• 90% water

• 10% solutes (proteins, electrolytes, nutrients, gases, hormones, waste products)

Plasma proteins are produced mainly in the liver (except gamma globulins) and serve various functions:

• Albumin: 60% of plasma proteins; maintains osmotic pressure, transports molecules, acts as a buffer.

• Globulins: Transport proteins and antibodies.

• Fibrinogen: Essential for blood clotting.

Additional info: The major osmotic ion in plasma is sodium (Na+).

Formed Elements: Erythrocytes, Leukocytes, and Platelets

Erythrocytes (Red Blood Cells)

Erythrocytes are biconcave, anucleate discs (~7.5 μm diameter) optimized for gas transport.

• Structure: No nucleus or organelles; filled with hemoglobin.

• Function: Transport O2 (97% of oxygen carried) and CO2 (20% of carbon dioxide carried).

• Life cycle: ~100–120 days; produced in bone marrow, destroyed in spleen.

• Hemoglobin: Protein with four polypeptide chains (2α, 2β), each with a heme group containing iron (Fe) that binds O2.

Equation for oxygen binding:

Additional info: Spectrin is a cytoskeletal protein that maintains RBC shape and flexibility.

Leukocytes (White Blood Cells)

Leukocytes are complete cells involved in immune defense. Types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

• Function: Protect against infection and foreign substances.

Platelets

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

• Structure: Small, disc-shaped, contain granules with clotting factors.

• Function: Form platelet plugs and release chemicals for clotting.

• Lifespan: ~10 days

Hemostasis: Blood Clotting Mechanisms

Steps of Hemostasis

Hemostasis is the process that stops bleeding after vessel injury, involving three key steps:

1. Vascular spasm: Vasoconstriction reduces blood flow.

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

3. Coagulation: Cascade of reactions forms a fibrin mesh, stabilizing the clot.

Platelet activation: Involves release of ADP, serotonin, and thromboxane A2 (promotes aggregation and spasm).

Anticoagulants: Prostacyclin and nitric oxide prevent unnecessary platelet aggregation.

Coagulation Pathways

Coagulation occurs via intrinsic and extrinsic pathways, both leading to the formation of prothrombin activator.

• Intrinsic pathway: Initiated by damage inside the vessel; slower.

• Extrinsic pathway: Initiated by tissue factor from damaged tissue; faster.

Key reactions:

Clot Retraction and Fibrinolysis

After clot formation, the clot retracts and is eventually dissolved.

• Clot retraction: Platelets contract, pulling fibrin strands and drawing wound edges together.

• Fibrinolysis: Plasmin (from plasminogen, activated by tPA) digests fibrin, removing the clot.

Equation for fibrinolysis:

Blood Disorders

Anemia and Polycythemia

Anemia: Reduced oxygen-carrying capacity due to low RBC count, decreased hemoglobin, or abnormal hemoglobin (e.g., sickle cell anemia).

Polycythemia: Excess RBCs increase blood viscosity; can be primary (polycythemia vera), secondary (due to hypoxia), or artificial (EPO abuse).

Hemostatic Disorders

• Thrombus: Clot in an unbroken vessel.

• Embolus: Clot that breaks free and travels in the bloodstream.

• Thrombocytopenia: Low platelet count, leading to bleeding risk.

• Hemophilia: Hereditary deficiency in clotting factors (e.g., factor VIII or IX).

Blood Groups and Transfusion Reactions

ABO and Rh Blood Group Systems

Blood types are determined by the presence of antigens (agglutinogens) on RBC surfaces and antibodies (agglutinins) in plasma.

Rh system: Rh+ individuals have the D antigen; Rh- individuals do not. Anti-Rh antibodies are produced only after exposure.

Additional info: Type O is the universal donor; type AB is the universal recipient.

Transfusion Reactions

Transfusion of incompatible blood leads to agglutination (clumping) and hemolysis (destruction) of RBCs, which can block vessels and damage kidneys.

• Agglutination: Antibodies bind to foreign antigens, causing clumping.

• Hemolysis: RBCs rupture, releasing hemoglobin.

• Clinical consequences: Reduced oxygen delivery, kidney failure, shock.

Summary Table: Blood Components and Functions