Blood: Components, Formed Elements, and Functions

Blood and the Cardiovascular System

Overview

Blood is a specialized connective tissue that plays a vital role in transporting substances, defending against pathogens, and maintaining homeostasis. It consists of plasma and formed elements, each with distinct functions and properties.

Composition of Blood

Main Components

• Plasma: The liquid, extracellular matrix of blood, making up about 55% of total blood volume.

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

Blood is composed of both a fluid portion (plasma) and cellular elements (formed elements).

Plasma

Properties and Components

• Definition: Plasma is a pale yellow, sticky fluid that is about 90% water.

• Solutes: Contains electrolytes (e.g., Na+, Cl-), nutrients (glucose, amino acids, lipids), gases (O2, CO2), metabolic wastes (urea, ammonia), and proteins.

• Main Proteins:

  • Albumin: Most abundant; maintains osmotic pressure and prevents water from leaving blood vessels.

  • Globulins: Includes antibodies and transport proteins for lipids, iron, and copper.

  • Fibrinogen: Essential for blood clotting; involved in chemical reactions for coagulation.

Formed Elements of Blood

Classification

• Erythrocytes (Red Blood Cells, RBCs): Oxygen-transporting cells; most numerous of the formed elements.

• Leukocytes (White Blood Cells, WBCs): Defense cells; protect the body from infectious agents.

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

Red Blood Cells (RBCs)

Structure and Function

• Shape: Biconcave discs, increasing surface area for gas exchange.

• Count: 4.8–5.2 million cells per cubic millimeter in adults.

• Organelles: Lack mitochondria and nuclei, maximizing space for hemoglobin.

• Hemoglobin: Oxygen-carrying protein; each molecule has four subunits, each with a heme group containing iron for oxygen binding.

Equation:

$\text{Hemoglobin} + \text{O}_2 \rightarrow \text{Oxyhemoglobin}$

White Blood Cells (WBCs)

Types and Functions

WBCs are less numerous than RBCs (4,800–10,800 cells/μL) and are crucial for immune defense. They are classified into two main groups:

• Granulocytes: Contain visible cytoplasmic granules.

  • Neutrophils: Most abundant; multilobed nucleus; phagocytize bacteria.

  • Eosinophils: Bilobed nucleus; combat parasitic infections and mediate allergic responses.

  • Basophils: Least abundant; bilobed nucleus; release histamine for inflammation.

• Agranulocytes: Lack visible granules.

  • Lymphocytes: Large, spherical nucleus; key cells of the immune system; produce antibodies and attack foreign cells.

  • Monocytes: Kidney-shaped nucleus; differentiate into macrophages for phagocytosis.

Mnemonic: "Never Let Monkeys Eat Bananas" (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils) — from most to least abundant.

Differential WBC Count Table

Type	Relative Abundance (%)	Main Function
Neutrophils	50–70%	Phagocytosis of bacteria
Lymphocytes	20–45%	Immune response, antibody production
Monocytes	4–8%	Phagocytosis, become macrophages
Eosinophils	1–4%	Combat parasites, allergic reactions
Basophils	0.5–1%	Release histamine, inflammation

Platelets (Thrombocytes)

Structure and Function

• Definition: Small, disc-shaped cell fragments derived from megakaryocytes.

• Function: Essential for blood clotting; form temporary plugs in vessel injury.

Formation of Blood Cells (Hematopoiesis)

Process and Sites

• Definition: Hematopoiesis is the process by which blood cells are formed.

• Location: Occurs in red bone marrow.

• Stem Cells: All blood cells originate from a common hemopoietic stem cell.

• Lineages:

  • Lymphoid stem cells: Give rise to lymphocytes.

  • Myeloid stem cells: Give rise to all other blood cells (RBCs, platelets, granulocytes, monocytes).

Clinical Considerations

Common Disorders

• Polycythemia: Excess of erythrocytes; increases blood viscosity.

• Anemia: Deficiency of RBCs or hemoglobin; reduces oxygen-carrying capacity.

• Sickle Cell Anemia: Genetic disorder causing abnormal hemoglobin and misshapen RBCs.

• Leukemia: Cancer of WBCs; abnormal proliferation of leukocytes.

• Thrombocytopenia: Low platelet count; increases risk of bleeding.

Example: Blood Separation by Centrifugation

When blood is centrifuged, it separates into three layers:

• Plasma: Top layer; about 55% of blood volume.

• Buffy coat: Middle thin layer; contains WBCs and platelets (<1% of blood volume).

• RBCs: Bottom layer; about 45% of blood volume.

Equation for Hematocrit:

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

Additional info: Academic context and definitions were expanded for clarity and completeness. Table values and functions inferred from standard physiology sources.