Blood: Structure, Function, and Clinical Relevance

Blood Overview

Composition and General Properties

Blood is a specialized connective tissue that constitutes approximately 8% of total body weight. It is composed of a liquid matrix called plasma and various formed elements. Blood is essential for the transport of substances, regulation of physiological parameters, and protection against disease.

• Plasma: The liquid component, making up about 55% of blood volume. Contains water, plasma proteins, and small solutes.

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

When blood is centrifuged, it separates into three visible layers:

• Plasma (top, ~55%)

• Buffy coat (middle,

  • Erythrocytes (bottom, ~45%)

Functions of Blood

Key Physiological Roles

Blood performs several vital functions necessary for homeostasis and survival:

• Gas Exchange: Transports oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.

• Transport of Nutrients and Wastes: Delivers nutrients to cells and removes metabolic wastes.

• Regulation of Osmosis: Maintains fluid balance between blood and tissues.

• Immune Function: Circulates leukocytes and antibodies to defend against pathogens.

• Thermoregulation: Distributes heat throughout the body to maintain temperature.

• Clot Formation: Platelets and clotting factors prevent excessive blood loss after injury.

• Acid-Base Homeostasis: Buffers in blood help maintain pH within a narrow range.

• Stabilization of Blood Pressure: Maintains adequate pressure for tissue perfusion.

Exchange of Substances Between Blood and Tissues

Capillary Exchange

Exchanges of gases, nutrients, and wastes between blood and tissues occur primarily in the capillaries, across their thin walls. The structure of capillaries facilitates efficient diffusion:

• Oxygen (O2): Loaded from alveolar air into blood in pulmonary capillaries.

• Carbon Dioxide (CO2): Unloaded from blood into alveolar air for exhalation.

• Capillary Wall: Composed of a single layer of endothelial cells and a basement membrane, allowing for rapid exchange.

Example: In the lungs, O2 diffuses from alveoli into capillaries, while CO2 diffuses from capillaries into alveoli to be exhaled.

Blood Plasma

Composition and Functions

Plasma is the straw-colored, liquid portion of blood in which cells are suspended. It is a colloidal solution containing:

• Water (about 91%): Solvent for carrying other substances.

• Plasma Proteins (about 7%):

  • Albumin: Maintains osmotic pressure and transports substances.

  • Globulins: Transport lipids, hormones, and act as antibodies.

  • Fibrinogen: Essential for blood clotting.

• Other Solutes (about 2%): Includes electrolytes (Na+, K+, Ca2+), nutrients (glucose, amino acids), gases (O2, CO2), and metabolic wastes (urea, creatinine).

Clinical Note: Liver disease (e.g., cirrhosis) can reduce plasma protein synthesis, leading to edema due to decreased oncotic pressure.

Formed Elements of Blood

Erythrocytes (Red Blood Cells)

Erythrocytes are biconcave, anucleate cells specialized for oxygen and carbon dioxide transport.

• Structure: Biconcave disc increases surface area for gas exchange; lacks nucleus and most organelles.

• Contents: Hemoglobin (oxygen-carrying protein), some enzymes (e.g., carbonic anhydrase), and minimal ATP.

• Lifespan: Approximately 100-120 days.

Hemoglobin: Each molecule consists of four globin chains and four heme groups, each binding one O2 molecule.

Leukocytes (White Blood Cells)

Leukocytes are nucleated cells involved in immune defense. They are classified as granulocytes or agranulocytes:

• Granulocytes:

  • Neutrophils: Phagocytize bacteria; most abundant WBC (60-70%).

  • Eosinophils: Combat parasites and participate in allergic responses.

  • Basophils: Release histamine and heparin during inflammatory and allergic reactions.

• Agranulocytes:

  • Lymphocytes: B cells (antibody production), T cells (cell-mediated immunity), and regulatory functions.

  • Monocytes: Differentiate into macrophages; phagocytize pathogens and present antigens to lymphocytes.

Platelets (Thrombocytes)

Platelets are small, anucleate cell fragments derived from megakaryocytes. They play a crucial role in hemostasis (blood clotting).

• Structure: Contain granules with clotting factors, enzymes, and cytoskeletal elements.

• Lifespan: 7-10 days.

Summary Table: Main Components of Blood

Component	Percentage of Blood	Main Functions
Plasma	~55%	Transport of nutrients, wastes, hormones; maintains osmotic balance
Erythrocytes	~45%	Oxygen and carbon dioxide transport
Leukocytes	<1%	Immune defense
Platelets	<1%	Blood clotting (hemostasis)

Additional info: Further details on blood cell development, disorders, and clinical applications are covered in subsequent sections of the full lecture or textbook.