Chapter 17: Blood – Structure, Function, and Hemostasis

Blood: Structure, Function, and Hemostasis

(17.1) Functions of Blood: Transport, Regulation, and Protection

Blood is a vital connective tissue that performs several essential functions in the human body. These include transporting substances, regulating physiological parameters, and providing protection against disease.

• Transport: Blood carries oxygen, carbon dioxide, nutrients, hormones, and metabolic wastes throughout the body.

• Regulation: Blood helps regulate body temperature, pH balance, and fluid volume.

• Protection: Blood contains cells and proteins that defend against infection and prevent blood loss through clotting.

• Examples: Oxygen transport by erythrocytes, immune defense by leukocytes, and clot formation by platelets.

(17.2) Blood Components: Plasma and Formed Elements

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

• Physical Characteristics: Blood is a viscous, slightly alkaline fluid with a temperature of about 38°C and a pH of 7.35–7.45.

• Components:

  • Formed Elements: Erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes).

  • Buffy Coat: Thin layer containing leukocytes and platelets, found between plasma and erythrocytes after centrifugation.

  • Plasma: Straw-colored liquid containing water, proteins (albumin, globulins, fibrinogen), nutrients, electrolytes, and waste products.

• Plasma Constituents:

  • Proteins: Albumin, globulins, fibrinogen

  • Nitrogenous waste: Urea, creatinine

  • Nutrients: Glucose, amino acids, lipids

  • Electrolytes: Sodium, potassium, calcium, chloride

• Disorders Related to Tonicity:

  • Dehydration: Loss of water increases blood viscosity.

  • Hypotonic hydration: Excess water dilutes plasma electrolytes.

  • Edema: Fluid accumulation in tissues due to imbalances.

  • Hypoproteinemia: Low plasma protein levels reduce osmotic pressure.

• Tonicity and RBC Shape:

  • Hypotonic: Water enters RBCs, causing swelling and possible lysis.

  • Hypertonic: Water leaves RBCs, causing shrinkage (crenation).

  • Isotonic: No net water movement; RBCs retain normal shape.

(17.3) Erythrocytes: Oxygen and Carbon Dioxide Transport

Erythrocytes, or red blood cells, are specialized for gas transport. Their structure and function are critical for maintaining tissue oxygenation.

• Structure: Biconcave, anucleate cells filled with hemoglobin.

• Hematocrit: The percentage of blood volume occupied by erythrocytes. Normal values: males ~47%, females ~42%.

• Hemoglobin: Iron-containing protein that binds oxygen and carbon dioxide.

• Erythropoiesis: Production of erythrocytes from pluripotent hematopoietic stem cells in red bone marrow, stimulated by erythropoietin (EPO).

• Causes of Erythrocyte Disorders:

  • Polycythemia: Excess RBCs increase blood viscosity.

  • Anemias:

    • Hemorrhagic: Blood loss

    • Hemolytic: RBC destruction

    • Sickle-cell: Abnormal hemoglobin

    • Aplastic: Bone marrow failure

    • Iron-deficiency: Insufficient iron

    • Pernicious: Vitamin B12 deficiency (macrocytic anemia)

  • Jaundice: Accumulation of bilirubin due to RBC breakdown.

(17.4) Leukocytes: Defense Against Disease

Leukocytes, or white blood cells, are the body's primary defense against infection and foreign substances.

• Types of Leukocyte Disorders:

  • Leukopenia: Low WBC count, increases infection risk.

  • Leukocytosis: Elevated WBC count, often due to infection.

  • Leukemia: Cancer of WBC-forming tissues, leading to abnormal proliferation.

(17.5) Platelets: Hemostasis and Blood Clotting

Platelets are small cell fragments essential for preventing blood loss by forming clots at sites of vessel injury.

• Significance:

  • Megakaryocyte: Large bone marrow cell that produces platelets.

  • Thrombocytopenia: Low platelet count, increases bleeding risk.

  • Petechiae: Small hemorrhagic spots due to platelet deficiency.

(17.6) Hemostasis: Prevention of Blood Loss

Hemostasis is the process that stops bleeding through a series of coordinated steps.

• Steps of Hemostasis:

  1. Vascular spasm: Vasoconstriction reduces blood flow.

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

  3. Platelet activation: Release of chemicals to recruit more platelets.

  4. Coagulation: Formation of a fibrin mesh to stabilize the clot.

• Events After Hemostasis:

  • Clot retraction: Clot contracts to bring wound edges together.

  • Tissue repair: Platelet-derived growth factor (PDGF) stimulates healing.

  • Fibrinolysis: Breakdown of the clot by plasmin, activated by tissue plasminogen activator (tPA).

• Thrombus vs. Embolus vs. Embolism:

  • Thrombus: Stationary blood clot in a vessel.

  • Embolus: Traveling clot or debris in the bloodstream.

  • Embolism: Blockage of a vessel by an embolus.

• Anticoagulants:

  • Aspirin: Inhibits platelet aggregation.

  • Coumadin (warfarin): Inhibits vitamin K-dependent clotting factors.

  • Heparin: Enhances activity of antithrombin III, inhibiting thrombin.

• Serum vs. Plasma: Serum is plasma without clotting factors.

• Hemophilia: Genetic disorder causing deficiency of clotting factors, leading to excessive bleeding.

Key Table: Comparison of Plasma and Serum

Component	Plasma	Serum
Water	Present	Present
Proteins (including clotting factors)	Present	Absent (clotting factors removed)
Electrolytes	Present	Present
Fibrinogen	Present	Absent

Key Equations

• Hematocrit Calculation:

$\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$

• Osmosis and Tonicity:

$\text{Net Water Movement} = \text{Difference in Solute Concentration Across Membrane}$

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.