Blood: Structure, Function, and Components

Overview of Blood

Blood is a specialized connective tissue that serves as the internal transport system of the body. It is composed of cellular elements suspended in a liquid matrix (plasma) and plays essential roles in transport, regulation, and protection.

• Transport: Delivers oxygen, nutrients, and hormones; removes metabolic wastes.

• Regulation: Maintains body temperature, pH, and fluid volume.

• Protection: Prevents blood loss and fights infection.

Functions of Blood

Transport Functions

Blood is responsible for transporting various substances throughout the body:

• Oxygen and Nutrients: Delivered to body cells for cellular metabolism.

• Metabolic Wastes: Transported to lungs (for CO2 removal) and kidneys (for nitrogenous waste elimination).

• Hormones: Carried from endocrine glands to target organs.

Regulatory Functions

Blood helps maintain homeostasis by:

• Body Temperature: Absorbing and distributing heat throughout the body.

• pH Balance: Using buffers (such as bicarbonate ions) to maintain normal pH (7.35–7.45).

• Fluid Volume: Maintaining adequate fluid volume in the circulatory system to support efficient blood flow.

Protective Functions

Blood protects the body by:

• Preventing Blood Loss: Platelets and plasma proteins initiate clot formation to seal breaks in blood vessels.

• Preventing Infection: Blood carries immune agents such as antibodies, complement proteins, and white blood cells (leukocytes).

Composition of Blood

Blood as Connective Tissue

Blood consists of two main components:

• Formed Elements: Cellular components including red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

• Plasma: The liquid extracellular matrix, primarily water with dissolved solutes (proteins, nutrients, gases, hormones, wastes, ions).

Note: Tissue fibers (fibrin) are only visible when blood clots.

Separation of Blood Components

When a sample of whole blood is centrifuged, it separates into three distinct layers:

Hematocrit: The percentage of blood volume occupied by erythrocytes. Normal values: 47% ± 5% in males, 42% ± 5% in females.

Physical Characteristics of Blood

• Viscosity: Blood is about five times more viscous than water, mainly due to RBCs.

• Color: Scarlet red when oxygen-rich; dark red when oxygen-poor.

• pH: 7.35–7.45 (slightly alkaline).

• Volume: 5–6 L in males, 4–5 L in females (about 8% of body weight).

Plasma Composition

Plasma is a straw-colored, sticky fluid making up about 55% of blood volume. It is approximately 90% water and contains over 100 dissolved solutes:

• Nutrients, gases, hormones, wastes, proteins, inorganic ions

• Plasma Proteins: Most abundant solutes by weight (~8% of plasma)

  • Albumin (~60%): Blood buffer, carrier for molecules, maintains osmotic pressure.

  • Globulins (~36%): Transport lipids, ions, and fat-soluble vitamins; some are antibodies.

  • Fibrinogen: Involved in clotting (converted to fibrin during coagulation).

Formed Elements

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

• White Blood Cells (Leukocytes): Defend against disease.

• Platelets (Thrombocytes): Cell fragments involved in clotting.

Formed elements are suspended in plasma and have unique features. RBCs lack nuclei and most organelles; platelets are cell fragments. Most formed elements survive only a few days and are replaced by stem cells in red bone marrow.

Example: Centrifuged Blood Sample

When a blood sample is centrifuged, the denser erythrocytes settle at the bottom, the buffy coat (WBCs and platelets) forms a thin middle layer, and plasma remains on top. This separation is used clinically to assess hematocrit and diagnose blood disorders.

Additional info: Later sections of the chapter (not shown in these slides) typically cover erythrocyte structure and function, hemoglobin, leukocyte types, hemostasis, and blood disorders.