The haemopoietic system, also known as the hematopoietic system, is responsible for the production and regulation of blood cells. It encompasses the organs and tissues involved in the production of blood cells (haemopoiesis) and their regulation, as well as the circulatory system that carries the blood throughout the body.

1. Composition of Blood

Blood is a specialized bodily fluid that plays a critical role in transporting nutrients, gases, and waste products. Blood is made up of two primary components: plasma and formed elements (cells and cell fragments).

1. Plasma

• Definition: Plasma is the liquid portion of blood, making up about 55% of total blood volume. It is a yellowish fluid composed primarily of water, proteins, electrolytes, and other substances.
• Components of Plasma:
  1. Water (about 90%): Acts as the solvent for transport of various substances.
  2. Plasma Proteins (7%):
     • Albumins: Maintain osmotic pressure and contribute to the transport of hormones, fatty acids, and drugs.
     • Globulins: Involved in immune responses (antibodies) and transport of lipids and metals.
     • Fibrinogen: Essential for blood clotting (coagulation).
  3. Electrolytes (such as sodium, potassium, chloride, bicarbonate): Maintain fluid balance and pH levels.
  4. Nutrients (glucose, amino acids, fatty acids): Provide energy and building blocks for cells.
  5. Waste Products (urea, carbon dioxide): Removed from the body via excretion.
  6. Hormones and Enzymes: Regulate various bodily functions and metabolic processes.

1. Formed Elements (Blood Cells and Cell Fragments)

The formed elements make up about 45% of the blood volume and consist of three major types:

1. Red Blood Cells (RBCs) or Erythrocytes:
   • Structure: Biconcave, disc-shaped cells without a nucleus.
   • Function: Primarily responsible for the transport of oxygen from the lungs to tissues and the transport of carbon dioxide from tissues to the lungs.
   • Lifespan: Approximately 120 days in circulation.
2. White Blood Cells (WBCs) or Leukocytes:
   • Structure: Nucleated cells that are involved in the body’s immune response.
   • Function: Protect the body against infections and foreign invaders. They are categorized into:
     • Granulocytes: Neutrophils, eosinophils, and basophils.
     • Agranulocytes: Lymphocytes (T-cells, B-cells) and monocytes.
   • Lifespan: Varies depending on the type (from days to years for memory cells).
3. Platelets (Thrombocytes):
   • Structure: Small, irregular cell fragments derived from megakaryocytes.
   • Function: Essential for blood clotting and the prevention of blood loss after injury.
   • Lifespan: Around 7-10 days.

2. Functions of Blood

Blood performs a variety of essential functions in the body, which can be broadly classified into the following categories:

1. Transport Functions

1. Oxygen Transport:
   • RBCs, through the hemoglobin protein, transport oxygen from the lungs to tissues and organs.
2. Carbon Dioxide Transport:
   • Blood carries carbon dioxide, a waste product of cellular respiration, from the tissues to the lungs for exhalation.
3. Nutrient Transport:
   • Blood transports nutrients (such as glucose, amino acids, fatty acids, vitamins, and minerals) absorbed from the digestive system to tissues and organs.
4. Hormone Transport:
   • Blood carries hormones from endocrine glands to target organs, regulating body processes such as growth, metabolism, and reproduction.
5. Waste Product Removal:
   • Blood carries waste products such as urea, creatinine, and bilirubin to the kidneys, liver, and other excretory organs for elimination from the body.

1. Regulatory Functions

1. Temperature Regulation:
   • Blood helps in regulating the body’s internal temperature by distributing heat generated by metabolism to different parts of the body. It also assists in cooling the body by adjusting the flow of blood to the skin.
2. pH Balance:
   • Blood helps maintain the pH of the body within a narrow range (around 7.35-7.45) through buffer systems, such as bicarbonate, that neutralize acids and bases.
3. Fluid and Electrolyte Balance:
   • Plasma proteins, such as albumin, maintain osmotic pressure, which helps in the regulation of fluid distribution between blood vessels and tissues. Electrolytes in blood, such as sodium and potassium, help maintain cell function and fluid balance.

1. Protective Functions

1. Immune Defense:
   • WBCs play a critical role in defending the body against infections, foreign substances, and abnormal cells (such as cancer cells). They participate in the immune response through mechanisms like phagocytosis (engulfing pathogens) and antibody production.
2. Blood Clotting:
   • Platelets and plasma proteins (e.g., fibrinogen) work together to form blood clots in response to injury, preventing excessive blood loss. This process is known as hemostasis.

3. Haemopoiesis (Hematopoiesis)

Haemopoiesis is the process by which blood cells are formed in the body. It primarily occurs in the bone marrow in adults, while in embryos and infants, it also takes place in other organs like the liver and spleen.

• Process: Hematopoietic stem cells in the bone marrow differentiate into various types of blood cells:
  1. Erythropoiesis: Formation of red blood cells.
  2. Leukopoiesis: Formation of white blood cells.
  3. Thrombopoiesis: Formation of platelets.

The production of blood cells is regulated by a complex interaction of hormones and growth factors:

• Erythropoietin (EPO): Stimulates RBC production in response to low oxygen levels.
• Granulocyte Colony-Stimulating Factor (G-CSF): Stimulates the production of granulocytes (neutrophils, eosinophils, basophils).
• Thrombopoietin (TPO): Stimulates platelet production.
• Interleukins and other cytokines: Regulate the production and function of various types of WBCs.

Summary of Composition and Functions of Blood

Conclusion

The haemopoietic system plays a vital role in maintaining the body’s homeostasis by producing blood cells and ensuring their proper function in the body. Blood is essential for transport, regulation, and protection, facilitating numerous critical physiological processes. Understanding the composition and functions of blood helps in recognizing the importance of maintaining a healthy circulatory and immune system, as well as in diagnosing and treating blood-related disorders.