📌Definition Table

Term	Definition
Plasma	The liquid portion of blood, consisting mainly of water, proteins, electrolytes, nutrients, hormones, and waste products.
Erythrocyte	Red blood cell containing haemoglobin, specialised for oxygen and carbon dioxide transport.
Leukocyte	White blood cell involved in immune defence, including phagocytosis and antibody production.
Platelet (Thrombocyte)	Cell fragment involved in blood clotting and wound repair.
Haemostasis	The process of stopping bleeding, involving vasoconstriction, platelet plug formation, and coagulation cascade.
Fibrin	Insoluble protein fibres formed during clotting that stabilise the platelet plug.
Immunoglobulin	Antibody protein produced by B cells, binding specifically to antigens for immune defence.

📌Introduction

Blood is a specialised connective tissue that serves as the main transport medium in animals with closed circulatory systems. It delivers oxygen, nutrients, and hormones to cells, removes metabolic wastes, and plays a central role in immunity, pH balance, and temperature regulation. Blood composition reflects an organism’s physiology and environmental adaptations, and understanding its components is vital in both medical and ecological contexts.

❤️ CAS Link: Partner with a local blood donation campaign to create an educational display on blood components and their functions, encouraging community participation.

📌 Plasma

• Composition — ~90% water, with dissolved salts, proteins, nutrients, hormones, and waste products.
• Functions — Acts as a transport medium for glucose, amino acids, fatty acids, hormones, CO₂ (as bicarbonate), and urea.
• Plasma Proteins — Albumin maintains osmotic pressure; fibrinogen involved in clotting; globulins (including immunoglobulins) play immune roles.

🧠 Examiner Tip: When explaining plasma functions, explicitly link each dissolved component to its transport or homeostatic role.

📌 Erythrocytes (Red Blood Cells)

• Structure — Biconcave disc shape increases surface area for gas exchange; no nucleus or mitochondria to maximise haemoglobin content.
• Function — Transport oxygen bound to haemoglobin; transport some CO₂ bound to haemoglobin or dissolved in plasma.
• Lifespan — ~120 days, destroyed in spleen/liver; iron recycled for new haemoglobin synthesis.

🌍 Real-World Connection: Athletes training at high altitudes stimulate increased erythrocyte production to improve oxygen delivery.

📌 Leukocytes (White Blood Cells)

• Granulocytes — Neutrophils (phagocytosis of bacteria), eosinophils (defend against parasites), basophils (release histamine in inflammation).
• Agranulocytes — Lymphocytes (B and T cells for adaptive immunity), monocytes (differentiate into macrophages for phagocytosis).
• Numbers rise during infection; low counts can indicate immune suppression.

📌Transport of Gases and Nutrients

• Oxygen — Bound to haemoglobin in erythrocytes; loading in lungs, unloading in tissues (Bohr effect in active tissues).
• Carbon Dioxide — Mostly as bicarbonate ions in plasma, some bound to haemoglobin, small amount dissolved.
• Nutrients — Absorbed in intestines, transported in plasma to cells for metabolism or storage.

📝 Paper 2: Data Response Tip: When explaining the hydrothermal vent hypothesis, always include energy source, protection from UV, and chemical gradients for full marks

📌 Immunity and Defence

• Innate Immunity — Non-specific; includes skin barrier, phagocytosis, inflammation.
• Adaptive Immunity — Specific; B lymphocytes produce antibodies, T lymphocytes target infected cells.
• Memory cells ensure faster, stronger secondary responses.

📌 Homeostatic Roles of Blood

• Temperature Regulation — Redistribution of blood flow to skin or internal organs.
• pH Buffering — Plasma proteins and bicarbonate buffer system maintain pH near 7.4.
• Osmoregulation — Plasma proteins help maintain water balance between blood and tissues.