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 waste products throughout the body.

• Regulation: Blood helps maintain homeostasis by regulating pH, temperature, and fluid balance.

• Protection: Blood contains cells and proteins that defend against pathogens 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.

• Comparison of Disorders:

  • Dehydration: Decreased plasma volume, increased hematocrit.

  • Hypotonic Hydration: Excess water dilutes plasma, decreases osmolarity.

  • Edema: Accumulation of fluid in tissues due to imbalance in filtration and reabsorption.

  • Hypoproteinemia: Low plasma protein levels, leading to decreased oncotic pressure and edema.

• 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 composition enable efficient oxygen and carbon dioxide exchange.

• Structure: Biconcave discs lacking nuclei and most organelles, maximizing surface area for gas exchange.

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

• Hemoglobin: The oxygen-carrying protein in RBCs. Each molecule binds up to four oxygen molecules.

• Erythropoiesis: The process of RBC formation, regulated by erythropoietin (EPO) from the kidneys. Originates from pluripotent hematopoietic stem cells in bone marrow.

• Disorders:

  • Polycythemia: Excess RBCs, increasing blood viscosity.

  • Anemias: Reduced oxygen-carrying capacity. Types include:

    • Hemorrhagic anemia: Blood loss.

    • Hemolytic anemia: RBC destruction (e.g., sickle-cell anemia).

    • Aplastic anemia: Bone marrow failure.

    • Iron-deficiency anemia: Insufficient iron for hemoglobin synthesis.

    • Pernicious anemia: Vitamin B12 deficiency, often with macrocytic RBCs.

  • Jaundice: Yellowing of skin due to bilirubin accumulation from RBC breakdown.

(17.4) Leukocytes: Defense and Immunity

Leukocytes, or white blood cells, are crucial for immune defense. They are classified based on their appearance and function.

• Types: Granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

• Leukopenia: Abnormally low WBC count, increasing infection risk.

• Leukocytosis: Elevated WBC count, often in response to infection.

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

(17.5) Platelets: Hemostasis and Clotting

Platelets are small cell fragments essential for blood clotting and prevention of blood loss.

• Significance: Platelets form the initial plug at injury sites and release factors that promote clotting.

• Disorders:

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

  • Petechiae: Small hemorrhagic spots due to capillary bleeding.

(17.6) Hemostasis: Prevention of Blood Loss

Hemostasis is the process that stops bleeding after vascular injury. It involves several coordinated steps.

• Steps of Hemostasis:

  1. Vascular Spasm: Immediate vasoconstriction to reduce blood flow.

  2. Platelet Plug Formation: Platelets adhere to exposed collagen and aggregate.

  3. Coagulation: Cascade of reactions leading to fibrin clot formation.

• Platelet Activation: Platelets release chemicals that enhance aggregation and attract more platelets.

• Clot Retraction and Repair: Clot contracts to bring wound edges together; tissue repair follows.

• Fibrinolysis: Breakdown of the clot after healing, involving plasmin and tissue plasminogen activator (tPA).

• Anticoagulants: Substances that prevent clotting, such as aspirin, heparin, and coumadin.

• Thrombus vs. Embolus:

  • Thrombus: Stationary blood clot within a vessel.

  • Embolus: A clot or fragment that travels through the bloodstream.

  • Embolism: Obstruction of a vessel by an embolus.

• 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 Formed Elements

Component	Main Constituents	Function
Plasma	Water, proteins (albumin, globulins, fibrinogen), nutrients, electrolytes, waste	Transport, osmotic balance, pH regulation
Formed Elements	Erythrocytes, leukocytes, platelets	Oxygen transport, immunity, clotting

Key Equation: Hematocrit Calculation

The hematocrit is calculated as:

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

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