Blood Composition and Function

Overview of Blood Components

Blood is a specialized connective tissue essential for transporting substances, defending against pathogens, and maintaining homeostasis. It consists of two main components: plasma and formed elements.

• Plasma: Makes up approximately 55% of blood volume. It is the liquid matrix that suspends blood cells and transports nutrients, wastes, and other molecules.

• Formed Elements: Comprise about 45% of blood volume and include red blood cells, white blood cells, and platelets.

Plasma Composition

Plasma is primarily water but contains various dissolved substances crucial for physiological processes.

• Water: ~90% of plasma; serves as a solvent and medium for transport.

• Solutes: ~1% of plasma; include:

  • Glucose and amino acids: Provide nutrition and are used for protein synthesis.

  • Nitrogenous wastes: Such as urea, transported to excretory organs.

  • Ions: Electrolytes (e.g., Na+, K+, Ca2+) important for acid-base and electrolyte homeostasis.

  • Gases: Small amounts of O2 and CO2.

• Plasma Proteins: ~9% of plasma; include:

  • Albumin: Maintains blood's colloid osmotic pressure, helping retain water in the bloodstream.

  • Immune proteins (antibodies): Produced by leukocytes; function in immunity.

  • Clotting proteins: Essential for stopping bleeding (e.g., fibrinogen).

  • Transport proteins: Bind and carry hydrophobic molecules (e.g., hormones, lipids).

Formed Elements

The formed elements are the cellular components of blood, each with specialized functions.

• Red Blood Cells (Erythrocytes): Transport oxygen from the lungs to tissues and assist in carbon dioxide removal.

• White Blood Cells (Leukocytes): Defend the body against infection and foreign substances.

• Platelets (Thrombocytes): Cell fragments that play a key role in blood clotting.

White Blood Cells (Leukocytes)

Classification of Leukocytes

Leukocytes are divided into two main categories based on the presence or absence of cytoplasmic granules:

• Granulocytes: Contain specific cytoplasmic granules released upon activation.

• Agranulocytes: Lack specific cytoplasmic granules.

Granulocytes

• Neutrophils:

  • Appearance: 3-5 lobed nucleus; granules often not visible.

  • Function: Perform chemotaxis to migrate toward infection sites, kill bacteria, and enhance inflammation. Also perform phagocytosis to engulf bacteria.

• Basophils:

  • Appearance: S-shaped nucleus; many dark purple-staining granules.

  • Function: Release histamine (mediates inflammation) and heparin (prevents blood clotting).

• Eosinophils:

  • Appearance: Bilobed, "headphone-shaped" nucleus; red-staining granules.

  • Function: Immune response to parasitic infections; perform phagocytosis of parasites and inflamed tissue.

Agranulocytes

• Lymphocytes:

  • Appearance: Large spherical nucleus; thin rim of light blue cytoplasm.

  • Function: Produce antibodies to bind and remove antigens; activate immune system components to destroy abnormal cells.

• Monocytes:

  • Appearance: U-shaped nucleus; light-blue to purple cytoplasm.

  • Function: Differentiate into macrophages that ingest dead cells, bacteria, antigens, and debris; activate other leukocytes by displaying phagocytosed antigens.

Blood Groups

ABO and Rh Blood Group Systems

Blood groups are determined by the presence or absence of specific antigens on the surface of red blood cells. These antigens are inherited genetically and are critical for safe blood transfusions.

• ABO Group:

  • Eight types: AB+, AB-, A+, A-, B+, B-, O+, O-

  • Determined by the presence of A and/or B antigens.

• Rh Group:

  • Presence of Rh antigen (D antigen) = positive (+); absence = negative (-).

Key Terms

• Antigen: A molecule on the surface of cells that can trigger an immune response.

• Antibody: A protein produced by lymphocytes that binds to a specific antigen.

• Phagocytosis: The process by which a cell engulfs particles such as bacteria or debris.

• Chemotaxis: Movement of cells toward chemical signals, often at sites of infection or inflammation.

Example: Blood Transfusion Compatibility

A person with type O- blood can donate to any other blood type (universal donor), but can only receive O- blood. A person with AB+ blood can receive blood from any type (universal recipient) because they have no anti-A, anti-B, or anti-Rh antibodies.