6. Blood and Haemopoietic System: A Complete Guide to Composition and Functions of Blood

BLOOD AND HAEMOPOIETIC SYSTEM

Welcome, future healthcare professionals!

As a pharmacy educator with years of experience teaching human anatomy and physiology, I have always emphasized that blood is the river of life. It is the only tissue that touches every organ, every cell, and every system in the body. Without blood, oxygen cannot reach the brain, nutrients cannot feed the tissues, and waste cannot be removed. Understanding blood is essential for understanding health and disease.

In this comprehensive guide, I will take you on a journey through the haemopoietic system—the system responsible for producing blood cells. We will explore the composition, functions, and clinical significance of blood. By the end of this article, you will have a solid understanding of one of the most clinically important topics in anatomy and physiology. Let us begin.

WHAT IS THE HAEMOPOIETIC SYSTEM?

The haemopoietic (haematopoietic) system is the system of organs and tissues involved in the production of cellular blood components. It includes the bone marrow, spleen, thymus, and lymph nodes. Haematology is the branch of medical science concerned with the study of blood, blood-forming tissues, and blood disorders.

Blood is a connective tissue that plays a vital role in carrying out various life processes and protecting the body against diseases. Around 8% of total body weight is blood—approximately 4-5 litres in adult females and 5-6 litres in adult males.

COMPOSITION OF BLOOD

Blood is a connective tissue with a liquid matrix containing cells and cell fragments. It comprises two main components:

1. Blood Plasma: The clear extracellular fluid (approximately 55% of blood).
2. Formed Elements: Red blood cells (RBCs), white blood cells (WBCs), and platelets (approximately 45% of blood).

PART 1: BLOOD PLASMA

Blood plasma is the golden-yellow liquid portion of blood, constituting about 55% of blood volume (approximately 2.7-3.0 litres in an average human).

Composition of Plasma:

• Water: 92%
• Plasma Proteins: 8%
• Other Substances: Hormones, carbon dioxide, electrolytes, vitamins, nutrients, and waste products (in trace amounts).

Plasma Proteins

Plasma Protein	Source	Primary Function
Albumin	Liver	Maintains osmotic pressure; transports substances
Globulins	Liver, plasma cells	Immunity (antibodies); transport lipids
Fibrinogen	Liver	Blood clotting (forms fibrin)
Prothrombin	Liver	Blood clotting factor

Functions of Blood Plasma Proteins

1. Oncotic Pressure (Colloid Osmotic Pressure): Plasma proteins, especially albumin, help keep fluid within blood vessels.
2. Transport: Act as carriers for hormones, vitamins, metals, and drugs.
3. Immunity: Immunoglobulins (globulins) recognize and neutralize pathogens.
4. Clotting: Fibrinogen and clotting factors are essential for blood clot formation.
5. Buffering: Help maintain the pH balance of blood (7.35-7.45).
6. Acute Phase Response: Production of acute-phase proteins during inflammation.
7. Enzyme Activity: Some plasma proteins have enzymatic activity.

PART 2: FORMED ELEMENTS

The formed elements are cells and cell fragments suspended in plasma. They are classified into three types:

1. Erythrocytes (Red Blood Cells)
2. Leukocytes (White Blood Cells)
3. Thrombocytes (Platelets)

Red Blood Cells (Erythrocytes)

RBCs are discotic-shaped cells (biconcave discs) that constitute 99% of the blood and carry haemoglobin molecules. Each cell is composed of:

• 65% water
• 35% solids (33% haemoglobin + 2% stromal meshwork)

Normal RBC Count:

• Males: 4.7-6.1 million cells/µL
• Females: 4.2-5.4 million cells/µL

Functions of RBCs:

1. Oxygen Transporter: Haemoglobin binds oxygen in the lungs and releases it to tissues.
2. Release of ATP and Vessel Dilation: Shear stress causes ATP release, relaxing and dilating vessel walls.
3. Release of S-Nitrosothiols: On deoxygenation, release S-nitrosothiols to dilate vessels.
4. Immunity: Release free radicals that rupture bacterial cell walls.

White Blood Cells (Leukocytes)

WBCs are the defenders of the body, fighting against external organisms and foreign substances. They are classified into granulocytes (with granules) and agranulocytes (without granules).

Cell Type	Percentage	Characteristics	Function
Neutrophils	60-70%	5-7 lobed nucleus; hydrolytic enzymes	Phagocytosis (digests bacteria)
Eosinophils	1-5%	2-3 lobed nucleus; stained with acid dyes	Combat parasites; allergic responses
Basophils	<1%	S-shaped 2-3 lobed nucleus	Secrete heparin, histamine, serotonin
Monocytes	1-6%	Ovoid/kidney-shaped nucleus; largest	Transform into macrophages; phagocytosis
Lymphocytes	20-40%	Small with large round nuclei	B-cells (antibodies), T-cells (cell-mediated immunity)

Functions of WBCs:

1. Phagocytosis: Digestion of foreign particles and bacteria by neutrophils.
2. Antibody Formation: Formation of B- and γ-serum globulin.
3. Fibroblasts Formation: Transform into fibroblasts and help in the repair process.
4. Heparin Secretion: Secreted by basophils; blocks intravascular clotting.
5. Anti-histamine Function: Defensive function against allergic conditions.

Platelets (Thrombocytes)

Platelets are small, non-nucleated discs (diameter 2-4µm) produced from the cytoplasm of megakaryocytes in red bone marrow.

Normal Platelet Count: 2,00,000-3,50,000/mm³

Functions of Platelets:

1. Vasoconstriction: Secrete serotonin to constrict damaged vessels.
2. Platelet Plug Formation: Adhere and aggregate to form a temporary seal.
3. Coagulation (Blood Clotting): Prothrombin activator → thrombin → fibrin (clot).
4. Fibrinolysis: Breakdown of clots; plasminogen converts to plasmin to dissolve fibrin.

FUNCTIONS OF BLOOD

Blood performs several essential functions that are critical for survival:

1. Transportation: Transports oxygen (O₂), carbon dioxide (CO₂), nutrients, wastes, and hormones.
2. Thermoregulation: Transfers heat during blood circulation.
3. Hydraulic Functions: Blood flow restriction causes engorgement (e.g., erection).
4. Vehicle for Hormones: Carries hormones, vitamins, and essential chemicals.
5. Maintenance of Ion Balance: Maintains ionic balance between cells and tissues.
6. Regulation of Water Balance: Freely interchangeable with interstitial fluid.
7. Regulation of Acid-Base Balance: Plasma proteins and haemoglobin act as buffers.
8. Regulation of Body Temperature (Homeostasis): Maintains balance between heat loss and gain.
9. Storage Function: Reservoir of water, proteins, glucose, sodium, and potassium.
10. Defensive Function: WBCs protect against bacteria and antigens; lymphocytes develop immunity.

SUMMARY OF FORMED ELEMENTS

Component	Other Name	Normal Count	Primary Function
Erythrocytes	Red Blood Cells	4.2-6.1 million/µL	Oxygen transport (haemoglobin)
Leukocytes	White Blood Cells	4,000-11,000/µL	Immunity and defense
Thrombocytes	Platelets	2,00,000-3,50,000/µL	Blood clotting (haemostasis)

A TEACHER’S PRACTICAL INSIGHTS

Over my years of teaching, I have developed a few key insights about blood that I always share with my students:

• “Blood Is a Window to Health”: A simple blood test can reveal anemia, infection, clotting disorders, and even cancer. This is why blood analysis is one of the most common diagnostic tools.
• Clinical Relevance: Understanding blood composition is essential for understanding anemia, leukemia, haemophilia, thrombosis, and infections.
• Use Mnemonics: “Never Let Monkeys Eat Bananas” helps remember the WBC types in order of abundance: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils.
• Think About Haemostasis: Blood clotting is a delicate balance between clot formation and clot breakdown. Disorders on either side can be life-threatening.

FREQUENTLY ASKED QUESTIONS (FAQs)

1. What is the composition of blood?

Blood is composed of blood plasma (55%) and formed elements (45%). Formed elements include RBCs, WBCs, and platelets.

2. What is the function of haemoglobin?

Haemoglobin is the oxygen-carrying protein in RBCs. It binds oxygen in the lungs and releases it to tissues throughout the body.

3. What are the three types of formed elements in blood?

The three types of formed elements are erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets).

4. What is the difference between plasma and serum?

Plasma is the liquid portion of blood containing clotting factors. Serum is plasma without clotting factors (fibrinogen is removed during clotting).

5. What are the functions of white blood cells?

WBCs defend the body against infection through phagocytosis, antibody formation, heparin secretion, and anti-histamine functions.

6. What is haemostasis?

Haemostasis is the process by which bleeding is stopped. It involves vasoconstriction, platelet plug formation, and blood clotting (coagulation).

7. What is the normal platelet count?

The normal platelet count is 2,00,000-3,50,000 per mm³. Low platelet count (thrombocytopenia) increases bleeding risk; high platelet count (thrombocytosis) increases clotting risk.

SUMMARY

Blood is the river of life—a connective tissue that transports oxygen, nutrients, hormones, and waste products while defending the body against disease. It is composed of plasma (55%) and formed elements (45%), including RBCs, WBCs, and platelets.

The haemopoietic system—including bone marrow, spleen, thymus, and lymph nodes—produces these cellular components. Understanding blood is essential for healthcare professionals because blood disorders—such as anemia, leukemia, haemophilia, and infections—are among the most common clinical conditions.

As I always tell my students: “Blood is the only tissue that tells the story of the entire body. Learn to read it, and you will understand health and disease.”

REFERENCES & FURTHER READING

• Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
• Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
• Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
• Hoffbrand, A. V., & Moss, P. A. H. (2016). Essential Haematology (7th ed.). Wiley-Blackwell.
• National Heart, Lung, and Blood Institute (NHLBI). (2022). Blood Disorders and Haematology Resources. Retrieved from NHLBI Official Website.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.