Anatomy and Physiology of Blood and Blood Cells

Introduction to the Haematological System and Blood Composition

Blood is a specialized fluid connective tissue circulating continuously through the cardiovascular system. In an average adult Nigerian, total blood volume ranges between 4 to 6 litres, influenced by body size and sex. Whole blood separates into two primary components when centrifuged: fluid plasma and formed elements.

• Plasma: Makes up about 55% of total blood volume, serving as the liquid medium for transport.
• Formed Elements: Constitute roughly 45% of blood volume, packed with cellular components.

Formed elements comprise three main categories:

1. Erythrocytes: Red blood cells responsible for oxygen transport.
2. Leukocytes: White blood cells dedicated to immune defense.
3. Thrombocytes: Platelets vital for blood clotting.

Physiological Functions of Blood in the Human Body

Blood sustains life through transport, regulation, and protection. It carries oxygen from the lungs and nutrients from the digestive tract to body tissues, while removing metabolic waste products like carbon dioxide and urea to excretory organs.

Regulatory duties involve maintaining homeostasis across the internal environment. Blood helps stabilize interstitial fluid pH through chemical buffers, manages core body temperature by absorbing and distributing heat, and preserves adequate fluid volume in tissues via osmotic pressure.

Protective functions center on defense against pathogens and blood loss. Leukocytes and antibodies neutralize foreign invaders, while clotting factors prevent excessive bleeding following vascular injury.

Plasma: Composition, Proteins, and Fluid Balance

Plasma is a pale-yellow liquid consisting of approximately 90% water, which acts as a versatile solvent for dissolved substances. The remaining 10% contains dissolved proteins, electrolytes, nutrients, respiratory gases, and waste products.

Plasma proteins play critical roles in physiology and are synthesized mainly by the liver:

• Albumins: The most abundant plasma proteins, responsible for maintaining colloid osmotic pressure and drawing water back into capillaries.
• Globulins: Divided into alpha, beta, and gamma types; transport lipids and fat-soluble vitamins, while gamma globulins serve as antibodies.
• Fibrinogen: Essential soluble precursor protein converted into insoluble fibrin strands during blood clotting.

Erythrocytes (Red Blood Cells): Structure, Function, and Erythropoiesis

Erythrocytes are specialized cells designed for efficient gas transport. A mature red blood cell lacks a nucleus and cytoplasmic organelles, providing maximum interior space for hemoglobin. Its biconcave disc shape increases the surface area-to-volume ratio, facilitating rapid diffusion of oxygen and carbon dioxide across the cell membrane.

Erythropoiesis is the continuous production of red blood cells, taking place in the red bone marrow. When renal sensors detect low blood oxygen levels, the kidneys release the hormone erythropoietin (EPO). EPO stimulates progenitor cells in the bone marrow to accelerate RBC maturation. After a lifespan of roughly 120 days, worn-out erythrocytes are phagocytosed and broken down by macrophages in the spleen and liver.

Leukocytes (White Blood Cells): Types, Immunity, and Defense Mechanisms

Leukocytes protect the body against invading pathogens, abnormal cells, and foreign debris. Unlike red blood cells, they possess a nucleus and full cellular machinery. They are categorized based on staining characteristics into granulocytes and agranulocytes.

• Neutrophils: Granulocytes that act as rapid-response phagocytes, engulfing and destroying bacteria during acute infections.
• Eosinophils: Granulocytes that combat parasitic worms and modulate allergic inflammatory responses.
• Basophils: Granulocytes that release histamine and heparin to promote inflammation during immune reactions.
• Lymphocytes: Agranulocytes comprising T-cells, B-cells, and Natural Killer cells, driving adaptive immunity and antibody production.
• Monocytes: Agranulocytes that migrate into tissues and transform into macrophages to engulf cellular debris and pathogens.

Thrombocytes (Platelets) and the Haemostasis Process

Haemostasis is the physiological process that stops bleeding following vascular injury. It operates through three sequential phases:

1. Vascular Spasm: Smooth muscle in the damaged blood vessel wall contracts instantly, reducing blood flow and minimizing blood loss.
2. Platelet Plug Formation: Exposed collagen fibers at the injury site attract circulating platelets, which adhere, become activated, and release chemical messengers to recruit more platelets into a temporary plug.
3. Coagulation (Blood Clotting): A complex cascade of plasma clotting factors culminates in the conversion of soluble fibrinogen into insoluble fibrin threads. This meshwork reinforces the platelet plug and traps blood cells to form a stable clot.

Blood Grouping Systems: ABO and Rhesus Compatibility

Blood typing is determined by the presence or absence of specific inherited antigens on the surface of erythrocytes. The two most clinically significant systems are the ABO and Rhesus (Rh) systems.

• ABO System: Categorizes blood into four major types (A, B, AB, and O) based on A and B antigens. Plasma contains natural antibodies against the antigens absent on the individual's red blood cells.
• Rhesus System: Based on the presence or absence of the D antigen. Individuals with the antigen are Rh-positive, while those lacking it are Rh-negative.

Matching blood types before transfusion prevents acute haemolytic transfusion reactions, where recipient antibodies attack donor red blood cells, causing agglutination and cell destruction.