Blood and Circulation: Composition, Haemopoiesis, and Clotting

1. Introduction to Blood

Blood is a specialized connective tissue that circulates throughout the body, carrying essential substances to cells and removing waste products. It is a vital fluid responsible for transporting oxygen, nutrients, hormones, and other crucial elements, while also playing a key role in immunity and thermoregulation. The study of blood and its disorders is known as hematology.

2. Composition of Blood

Blood is a complex mixture composed of two main components: plasma and blood cells (formed elements). These components can be separated by centrifugation. Typically, blood is about 55% plasma and 45% formed elements. Plasma itself constitutes about 90% water and 10% solutes.

2.1. Plasma

Plasma is the liquid matrix of blood, and it is straw-colored. It is primarily composed of water, which acts as a solvent for many substances. The dissolved solutes in plasma include proteins, electrolytes, nutrients, waste products, hormones, and gases.

• Proteins: Plasma proteins are synthesized mainly in the liver. The three major types are:
  • Albumin: The most abundant plasma protein, it is crucial for maintaining osmotic pressure (oncotic pressure) in the blood, which helps to keep water within the blood vessels. It also acts as a carrier for various substances, such as fatty acids and hormones.
  • Globulins: These proteins are involved in immunity (gamma globulins are antibodies) and transport (alpha and beta globulins transport lipids, metal ions, and fat-soluble vitamins).
  • Fibrinogen: This protein is essential for blood clotting. Once activated, it forms fibrin, a mesh-like structure that traps blood cells to form a clot.
• Electrolytes: These are inorganic ions vital for cellular function and maintaining blood pH. Key electrolytes include sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), chloride (Cl^-), bicarbonate (HCO₃^-), and phosphate (PO₄^3-).
• Nutrients: These are absorbed from the digestive system and transported to tissues for energy, growth, and repair. They include glucose, amino acids, fatty acids, vitamins, and minerals.
• Waste Products: These are by-products of metabolism that need to be transported to excretory organs. Key waste products include urea, uric acid, creatinine, and bilirubin.
• Hormones: These are chemical messengers produced by endocrine glands and transported by blood to target organs.
• Gases: Dissolved gases include oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂). Oxygen is primarily transported by red blood cells, but a small amount is dissolved in plasma. Carbon dioxide is transported in three forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions in plasma.

2.2. Formed Elements (Blood Cells)

These are the cellular components of blood, each with specific functions. They are produced in the bone marrow.

• Erythrocytes (Red Blood Cells - RBCs):
  • These are biconcave discs, anucleated in mammals, and lack most organelles. This shape increases their surface area for gas exchange and allows them to squeeze through narrow capillaries.
  • Their primary function is to transport oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. This is achieved through the presence of hemoglobin, a protein that binds oxygen.
  • The normal count in adults is about 4.5-5.5 million cells per cubic millimeter (mm³).
  • Their lifespan is about 120 days, after which they are removed by macrophages in the spleen and liver.
• Leukocytes (White Blood Cells - WBCs):
  • These are larger than RBCs and possess a nucleus and organelles. They are fewer in number than RBCs (4,000-11,000 per mm³).
  • WBCs are part of the immune system, defending the body against pathogens and foreign substances.
  • They are classified into two main groups: granulocytes and agranulocytes, based on the presence or absence of granules in their cytoplasm.
  • Granulocytes:
    • Neutrophils: The most abundant type (40-70% of WBCs). They are phagocytic cells that engulf bacteria and cellular debris.
    • Eosinophils: (1-4% of WBCs). They combat parasitic infections and modulate allergic inflammatory responses.
    • Basophils: (0-1% of WBCs). They release histamine and heparin, contributing to inflammatory and allergic responses.
  • Agranulocytes:
    • Lymphocytes: (20-45% of WBCs). They are crucial for the specific immune response, including the production of antibodies (B lymphocytes) and direct cell killing (T lymphocytes).
    • Monocytes: (2-8% of WBCs). They are the largest WBCs and are phagocytic. They differentiate into macrophages in tissues, which engulf pathogens and cellular debris.
• Thrombocytes (Platelets):
  • These are small, irregular cell fragments derived from megakaryocytes in the bone marrow.
  • They are essential for hemostasis, the process of stopping bleeding. They aggregate at the site of injury and release factors that initiate blood clotting.
  • Normal count ranges from 150,000-400,000 per mm³.
  • Their lifespan is about 7-10 days.

3. Haemopoiesis (Hematopoiesis)

Haemopoiesis is the process of blood cell formation. In adult mammals, it occurs primarily in the red bone marrow of certain bones, such as the sternum, vertebrae, ribs, pelvis, and the epiphyses of long bones. In the fetus, haemopoiesis initially occurs in the yolk sac, then in the liver and spleen, before shifting to the bone marrow before birth.

3.1. Stem Cells and Differentiation

All blood cells originate from a common pluripotent hematopoietic stem cell (HSC). These stem cells have the remarkable ability to self-renew and differentiate into various progenitor cells, which then mature into specific blood cell types.

• Myeloid Stem Cells: These give rise to erythrocytes, megakaryocytes (which produce platelets), granulocytes (neutrophils, eosinophils, basophils), and monocytes.
• Lymphoid Stem Cells: These give rise to lymphocytes (B cells, T cells, NK cells).

3.2. Regulation of Haemopoiesis

Haemopoiesis is a tightly regulated process, influenced by various growth factors and hormones known as colony-stimulating factors (CSFs) and interleukins. These regulators stimulate the proliferation and differentiation of specific cell lines.

• Erythropoietin (EPO): A hormone produced mainly by the kidneys in response to hypoxia (low oxygen levels). EPO stimulates the bone marrow to increase the production of red blood cells.
• Thrombopoietin (TPO): A hormone produced by the liver and kidneys that stimulates the production of platelets from megakaryocytes.
• Colony-Stimulating Factors (CSFs): These stimulate the production of specific types of white blood cells, such as granulocyte-CSF (G-CSF) and macrophage-CSF (M-CSF).
• Interleukins: A group of cytokines that play a role in immune responses and also influence the proliferation and differentiation of various blood cells.

4. Blood Clotting (Coagulation)

Blood clotting, or coagulation, is a critical process that prevents excessive blood loss when a blood vessel is injured. It involves a complex cascade of enzymatic reactions leading to the formation of a fibrin clot, which seals the damaged vessel. This process is carefully regulated to prevent spontaneous clotting within intact vessels.

4.1. Hemostasis: The Steps to Stop Bleeding

Hemostasis involves several coordinated events:

1. Vascular Spasm: Immediately after a blood vessel is damaged, the smooth muscle in its wall contracts, constricting the injured vessel and reducing blood flow. This is a direct response to injury and the release of substances like serotonin by platelets.
2. Platelet Plug Formation: Platelets circulating in the blood adhere to the exposed collagen fibers at the site of injury. They become activated, changing shape, releasing chemicals (like ADP and thromboxane A2) that attract more platelets, and forming a temporary plug. This plug helps to reduce bleeding temporarily.
3. Coagulation (Formation of a Blood Clot): This is the most complex phase, involving a series of protein factors (clotting factors) that are normally inactive in the blood. The cascade ultimately leads to the conversion of fibrinogen into fibrin.
   • Fibrinogen to Fibrin: The enzyme thrombin converts soluble fibrinogen into insoluble fibrin strands. These strands form a meshwork that traps blood cells (RBCs, WBCs, and platelets), forming a stable clot.
   • Clotting Factors: There are about 13 major clotting factors, designated by Roman numerals (Factor I to Factor XIII). Most of these are synthesized in the liver and require Vitamin K for their synthesis.
   • Intrinsic and Extrinsic Pathways: The coagulation cascade can be initiated by two pathways:
     • Intrinsic Pathway: Activated by damage to the blood vessel wall itself or by contact with foreign surfaces. It is a slower process.
     • Extrinsic Pathway: Activated by tissue factor (thromboplastin) released from damaged tissues outside the blood vessel. It is a faster process.
     Both pathways converge at the activation of Factor X, which then leads to the common pathway.
   • Common Pathway: Activated Factor X converts prothrombin into thrombin. Thrombin then converts fibrinogen into fibrin. This pathway also leads to the activation of Factor XIII, which stabilizes the fibrin mesh by cross-linking the fibrin strands.
4. Clot Retraction and Repair: After the clot has formed, platelets contract, pulling the edges of the damaged vessel closer together. This process, called clot retraction, squeezes out serum (plasma minus clotting factors). Simultaneously, healing begins, and new endothelial cells and connective tissue grow to repair the vessel wall.

4.2. Anticoagulation and Fibrinolysis

The body has natural mechanisms to prevent excessive clotting and to dissolve clots once they are no longer needed.

• Natural Anticoagulants:
  • Heparin: A substance produced by basophils and mast cells that inhibits the action of thrombin.
  • Antithrombin III: A protein that inactivates thrombin and other clotting factors.
  • Protein C and Protein S: These proteins, along with their receptor (thrombomodulin), inactivate specific clotting factors.
• Fibrinolysis: This is the process of dissolving a clot. The enzyme plasmin breaks down the fibrin mesh. Plasminogen, an inactive precursor, circulates in the blood and is converted to plasmin by activators like tissue plasminogen activator (t-PA), which is released by endothelial cells.

4.3. Blood Group Antigens and Transfusion Reactions

The surface of red blood cells contains genetically determined antigens, which define blood groups. The most important are the ABO and Rh systems.

• ABO System: Based on the presence or absence of A and B antigens on RBCs.
  • Type A: Has A antigens, anti-B antibodies.
  • Type B: Has B antigens, anti-A antibodies.
  • Type AB: Has both A and B antigens, no antibodies. (Universal recipient)
  • Type O: Has neither A nor B antigens, both anti-A and anti-B antibodies. (Universal donor)
• Rh System: Based on the presence (Rh positive) or absence (Rh negative) of the D antigen. Rh-negative individuals do not naturally have anti-Rh antibodies but can develop them if exposed to Rh-positive blood.

Transfusing incompatible blood types can lead to a severe transfusion reaction, where antibodies in the recipient's plasma bind to antigens on the transfused RBCs, causing them to agglutinate (clump) and hemolyze (burst). This can be life-threatening.