Haemopoietic System

HAEMOPOIETIC SYSTEM

Introduction

The haemopoietic system is the group of organs and tissues that produce blood cells. This system includes the bone marrow, spleen, lymph nodes, and thymus gland. These structures work together to form the cellular components of blood, which include red blood cells, white blood cells, and platelets.

Blood is a specialized fluid connective tissue that transports oxygen and nutrients to all body tissues and removes carbon dioxide and waste products.

Important facts about blood:

• Total blood volume in an adult = about 6 liters
• Specific gravity = 1.055
• pH = about 7.4 (slightly alkaline)
• Blood forms about 8% of total body weight
• It circulates continuously, helping maintain a stable internal environment (homeostasis)

FUNCTIONS OF BLOOD

Blood performs several essential functions necessary for survival:

1. Transportation

• Nutrients (glucose, amino acids, vitamins, minerals) absorbed from the intestine are carried to tissues.
• Hormones secreted by endocrine glands are transported to target organs.
• Blood carries oxygen from the lungs to tissues and carries carbon dioxide back to the lungs.

2. Provides Oxygen to Cells

Blood picks up oxygen from the lungs and delivers it to all tissues of the body through red blood cells containing hemoglobin.

3. Protection

• White blood cells fight infections through phagocytosis and antibody formation.
• Blood helps maintain adequate levels of water and electrolytes.

4. Regulation

Blood helps regulate:

• Body temperature
• pH balance
• Water content of cells

5. Homeostasis

Homeostasis means keeping the internal body environment stable despite external changes. Blood plays a major role by distributing heat, transporting chemicals, and maintaining fluid balance.

COMPOSITION OF BLOOD

Blood has two major components:

1. Plasma (liquid portion)
2. Formed elements (RBCs, WBCs, and platelets)

Table: Major Components of Blood

1. PLASMA

Plasma is the yellowish fluid that makes up about 55% of total blood volume. It is essential for transporting nutrients, hormones, and waste products throughout the body.

• Plasma contains 90–92% water.
• When fibrinogen is removed from plasma, the remaining fluid is called serum.
  Serum = Plasma – Fibrinogen

Contents of Plasma

Amino Acids

These are building blocks of proteins, required for growth and repair.

Nitrogenous Waste

Examples: urea, uric acid, creatinine
These are waste products of protein metabolism, carried by blood to the kidneys for excretion.

Nutrients

Glucose, amino acids, fatty acids—used for energy production, growth, and repair.

Hormones

Chemicals released by glands that travel through blood to regulate body functions.

Plasma Proteins

These contribute to osmotic pressure and transport functions.

Gases

Oxygen, carbon dioxide, and nitrogen are transported in plasma (often bound to hemoglobin in RBCs).

2. FORMED ELEMENTS

Formed elements include:

a)     Red Blood Cells (RBCs / Erythrocytes)

b)     White Blood Cells (WBCs / Leukocytes)

c)      Platelets (Thrombocytes)

Red Blood Cells (RBCs / Erythrocytes)

RBCs are biconcave, disc-shaped, non-nucleated cells about 7.5–8.7 μm in diameter. Their red color comes from hemoglobin.

Characteristics

• RBC count in men: 4.35–5.65 million/mcL
• RBC count in women: 3.92–5.13 million/mcL
• Lifespan: about 120 days
• Destroyed in spleen → called the “Graveyard of RBCs”

Functions of RBCs

1. Transport of Oxygen

• Hemoglobin combines with oxygen to form oxyhemoglobin.
  About 97% of oxygen is carried this way.

2. Transport of Carbon Dioxide

• Hemoglobin binds to CO₂ forming carbaminohemoglobin.
  About 30% of CO₂ is transported by RBCs.

DEVELOPMENT AND LIFE SPAN OF RBCs (ERYTHROPOIESIS)

RBCs are formed in bone marrow from stem cells called haemocytoblasts. The whole process takes about 7 days.

Key Steps in Erythropoiesis

Destruction of RBCs

Aged RBCs are broken down in spleen and liver.

• Iron is reused for new RBC formation.
• Heme → converted to bilirubin → excreted in bile.

White Blood Cells (WBCs / Leukocytes)

WBCs protect the body from infection and foreign substances.
Normal count: 7,000–8,000 per mm³.

• Formed in bone marrow
• Lifespan varies from hours to years depending on type
• Found in the “buffy coat” layer after centrifugation

Functions of WBCs

• Phagocytosis of pathogens
• Production of antibodies
• Inflammation and defense
• Pus formation at infected sites
• Removal of dead cells

Types of White Blood Cells

WBCs are divided into two major groups:

1. Granulocytes

These contain granules in the cytoplasm and have multilobed nuclei.

2. Agranulocytes

Monocytes also release interleukins, which:

• Stimulate globulin production
• Activate T-lymphocytes
• Raise body temperature via hypothalamus

Platelets (Thrombocytes)

Platelets are small, irregular cell fragments formed in bone marrow.

• No nucleus
• Size: 2–4 microns
• Lifespan: 8–11 days
• Normal count: 1,50,000–4,00,000 per microliter

Functions of Platelets

BLOOD CLOTTING

Blood clotting (coagulation) is the process by which liquid blood turns into a gel-like clot to prevent blood loss after an injury. It is an essential part of hemostasis, which stops bleeding and allows healing to begin.

Blood normally contains several inactive clotting factors (procoagulants). When a blood vessel is damaged, these factors become activated, leading to clot formation.

MECHANISM OF BLOOD CLOTTING

Blood clotting involves three major mechanisms:

1. Vasoconstriction

• When a blood vessel is injured, the smooth muscles in its walls constrict.
• This reduces blood flow and slows blood loss.
• Platelets stick to the damaged site, making the surface sticky.

2. Platelet Plug Formation

Platelets form a temporary “plug” at the injury site by three steps:

This plug closes small injuries temporarily.

3. Blood Coagulation (Clotting)

• Blood becomes a semi-solid clot due to conversion of soluble fibrinogen into insoluble fibrin.
• Fibrin threads form a mesh that traps RBCs and seals the wound.

STAGES OF BLOOD CLOTTING

Stage 1: Formation of Prothrombin Activator

This is the most important initiating step. Prothrombin activator is formed by two pathways:

Both pathways require calcium ions (Ca²⁺) and clotting factors.

Clotting Factors

Stage 2: Conversion of Prothrombin into Thrombin

• Prothrombin activator formed in Stage 1 converts prothrombin → thrombin.
• Thrombin is the key enzyme in blood clotting.

Once thrombin is formed, clot formation becomes rapid and almost irreversible.

Stage 3: Conversion of Fibrinogen into Fibrin

• Thrombin converts fibrinogen (soluble) into fibrin (insoluble).
• Fibrin strands form a mesh to trap blood cells and form a solid clot.
• Factor XIII (fibrin stabilizing factor) strengthens and tightens the clot.

BLOOD GROUP

Blood groups were discovered in 1901 by Karl Landsteiner, who received the Nobel Prize in 1930. Blood grouping helps ensure safe blood transfusions and avoid dangerous reactions.

Blood groups are determined by the presence of antigens on RBCs and antibodies in plasma.

• Antigens = proteins on the surface of RBCs
• Antibodies = proteins in plasma that attack foreign antigens

Blood Group System

There are 4 major blood groups based on the ABO system:

• A
• B
• AB
• O

Each can be either Rh-positive (Rh⁺) or Rh-negative (Rh⁻), giving a total of 8 groups.

IMPORTANCE OF BLOOD GROUPING

Blood grouping is essential for:

• Safe blood transfusions (to prevent mismatched reactions)
• Preventing haemolytic disease of the newborn (Rh incompatibility)
• Paternity testing
• Medicolegal investigations

ABO SYSTEM

The ABO system is based on the presence or absence of antigens A and B on RBCs.

Compatibility of Blood Groups

DISORDER OF BLOOD

Blood disorders affect the composition or function of RBCs, WBCs, platelets, or clotting factors.

They may cause excessive bleeding, infections, fatigue, or slow healing.

1. DISORDERS OF RED BLOOD CELLS

a. Anemia

Anemia is a condition where the body has too few healthy RBCs or reduced hemoglobin, leading to decreased oxygen transport.

Symptoms:

• Fatigue
• Breathlessness
• Reduced appetite
• Pale skin

Types of Anemia

b. Polycythemia

A condition where the body produces too many RBCs, making blood thick and increasing the risk of clotting.

2. DISORDERS OF WHITE BLOOD CELLS

a. Leukocytosis

Increased WBC count (>10,000/µL).
Occurs in infections, inflammation, and certain cancers.

Types:

• Neutrophilia – Elevated neutrophils
• Eosinophilia – Elevated eosinophils (due to allergies, parasites)
• Leukemia – Cancer of blood-forming tissues producing immature WBCs

b. Leukopenia

Low WBC count (<3500/µL)
Occurs due to viral infections, bone marrow damage, or drugs.

3. DISORDERS OF PLATELETS

Thrombocytopenia

A decrease in platelet count, causing:

• Easy bruising
• Bleeding gums
• Slow clotting

4. DISORDERS OF CLOTTING

a. Hemophilia

An inherited disorder where the body lacks certain clotting factors (commonly factor VIII).
Causes prolonged bleeding even after minor injuries.

b. Thrombosis

Formation of abnormal blood clots inside blood vessels.
Can reduce blood flow and may lead to:

• Stroke
• Heart attack