Chapter 17: Blood – Structure, Function, and Components

Blood: Overview and Characteristics

Definition and General Properties

Blood is a specialized connective tissue that plays a vital role in maintaining homeostasis and supporting life. It consists of a liquid matrix (plasma) and cellular components (formed elements).

• Physical Characteristics:

  • Sticky with a metallic odor

  • Color ranges from bright red (oxygen-rich) to deep maroon "blue" (oxygen-poor)

  • pH: 7.35–7.45 (slightly alkaline)

  • Temperature: 38°C (100.4°F), slightly higher than body temperature

  • Comprises approximately 8% of body weight

  • Average adult blood volume: 5–6 liters

Components of Whole Blood

Plasma and Formed Elements

Blood is a suspension made up of plasma (the liquid matrix) and formed elements (cellular components).

• Plasma: Makes up about 55% of whole blood; least dense component.

• Buffy coat: Contains leukocytes (white blood cells) and platelets; less than 1% of whole blood.

• Erythrocytes: Red blood cells; about 45% of whole blood; most dense component.

Example: Centrifuging a blood sample separates these components by density, allowing for analysis and diagnostic testing.

Functions of Blood

Key Roles in the Body

Blood serves several essential functions to maintain a stable internal environment and support physiological processes.

• Homeostasis: Maintains a constant environment for body tissues despite changes.

• Distribution/Transport: Transports oxygen, carbon dioxide, hormones, heat, waste, nutrients, and drugs.

• Regulation: Regulates body temperature, pH, fluid volume, and electrolyte concentrations.

• Protection: Defends against disease (antibodies, WBCs) and prevents blood loss (hemostasis).

Formed Element Properties

General Features and Production

The formed elements of blood include erythrocytes, leukocytes, and platelets, each with distinct properties and functions.

• Life Span: Short, ranging from minutes to months depending on cell type.

• Amitotic: Most formed elements do not divide (amitotic).

• Production Site: All formed elements are produced in red bone marrow.

• Adult Hematopoiesis: In adults, active red bone marrow is found in the proximal femur and humerus, ilia, sternum, vertebrae, skull, and ribs.

Major Blood Cell Types

Erythrocytes (Red Blood Cells)

Erythrocytes are the most abundant formed element and are specialized for gas transport.

• Structure: Small, flexible biconcave discs with high surface area for gas exchange; anucleate and lack organelles.

• Function: Transport oxygen and carbon dioxide via hemoglobin.

• Hemoglobin: Protein that binds oxygen; each RBC contains millions of hemoglobin molecules.

• Hematocrit: Percentage of blood volume occupied by RBCs.

Leukocytes (White Blood Cells)

Leukocytes are nucleated cells involved in immune defense and are present in much lower numbers than RBCs.

• Types: Neutrophils, Eosinophils, Basophils, Lymphocytes, Monocytes

• Function: Protect the body from infection and disease.

Platelets (Thrombocytes)

Platelets are cell fragments essential for blood clotting (hemostasis).

• Origin: Produced by fragmentation of megakaryocytes in red bone marrow.

• Function: Form platelet plugs and release chemicals for clot formation.

Blood Plasma

Composition and Functions

Plasma is the liquid matrix of blood, consisting mostly of water and dissolved solutes.

• Water: Main component, serves as solvent.

• Solutes: Includes plasma proteins, nutrients, electrolytes, gases, and waste products.

• Plasma Proteins:

  • Albumins (60%): Maintain water balance and pH.

  • Globulins (36%): Gamma globulins are antibodies; alpha and beta globulins transport fats and vitamins.

  • Fibrinogen (4%): Involved in blood clotting.

Visual Identification of Blood Cells

Microscopic Appearance

Blood smears under the microscope reveal the distinct morphology of formed elements.

• Erythrocytes: Numerous, small, pale pink discs without nuclei.

• Leukocytes: Larger, purple-stained cells with visible nuclei; different types can be identified by nuclear shape and cytoplasmic granules.

• Platelets: Small, purple-stained fragments scattered among RBCs.

Example: The provided image shows a typical blood smear with erythrocytes and several types of leukocytes.

Summary Table: Blood Components

Component	Percentage of Whole Blood	Main Function
Plasma	~55%	Transport of nutrients, waste, hormones; maintains osmotic balance
Buffy coat (Leukocytes & Platelets)	<1%	Immune defense (WBCs), clotting (platelets)
Erythrocytes	~45%	Oxygen and carbon dioxide transport

Key Equations

Hematocrit Calculation

The hematocrit is the percentage of blood volume occupied by red blood cells.

• Formula:

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

Additional info:

• Blood is classified as a connective tissue due to its origin from mesenchyme and its matrix (plasma).

• Formed elements are produced by hematopoiesis in red bone marrow.

• Plasma proteins are primarily synthesized in the liver.