📌Definition Table

Term	Definition
Circulatory system	Network of blood vessels and a pump (heart) that transports substances throughout the body.
Open circulatory system	System in which blood (hemolymph) directly bathes organs without being confined to vessels.
Closed circulatory system	System where blood flows through vessels, separated from interstitial fluid.
Double circulation	Circulation system in mammals with pulmonary (heart–lungs–heart) and systemic (heart–body–heart) circuits.
Hemoglobin	Globular protein in red blood cells that transports oxygen, showing cooperative binding.
Lymph	Fluid derived from interstitial fluid, transported in lymphatic vessels, contributing to immunity and fluid balance.

📌Introduction

Multicellular animals require efficient transport systems to overcome diffusion limitations. The circulatory system enables distribution of oxygen, nutrients, hormones, and removal of wastes like CO₂ and urea. While smaller or simple organisms (flatworms, cnidarians) rely on diffusion or gastrovascular cavities, complex organisms evolved circulatory systems. Invertebrates like insects use open systems, while vertebrates possess closed systems. Mammals have highly efficient double circulation, supporting high metabolic demands and endothermy.

📌 Open vs Closed Circulatory Systems

• Open systems (e.g., insects): hemolymph is pumped into body cavities, directly bathing organs. These systems are energetically cheaper but less efficient for rapid transport.
• Closed systems (e.g., annelids, fish, mammals): blood remains within vessels, ensuring higher pressure and faster, directed flow. This allows separation of oxygen-rich and oxygen-poor blood, crucial for sustaining active lifestyles.
• Closed systems can support large body size, complex tissues, and high metabolic activity.

🧠 Examiner Tip: When comparing open and closed systems, focus on efficiency and pressure differences, not just the presence/absence of vessels.

📌 Heart Structure and Double Circulation in Mammals

• The mammalian heart is a muscular organ with four chambers (two atria, two ventricles), ensuring complete separation of oxygenated and deoxygenated blood.
• Right side pumps deoxygenated blood to lungs (pulmonary circulation).
• Left side pumps oxygenated blood to the body (systemic circulation).
• Double circulation maintains high blood pressure in systemic circuits, ensuring efficient oxygen delivery.
• Valves prevent backflow, and coronary vessels supply the heart muscle itself.

🧬 IA Tips & Guidance: Students can dissect a mammalian heart to identify chambers, valves, and vessels. Alternatively, digital simulations can be used to trace blood flow. Classic experiments include monitoring heart rate before/after exercise to link circulation to metabolism.

📌 Blood Vessels and Pressure Regulation

• Arteries: thick-walled, elastic vessels carrying blood away from the heart under high pressure. Elastic recoil maintains pressure between beats.
• Veins: thinner walls, valves to prevent backflow, assisted by skeletal muscle contractions.
• Capillaries: one cell thick, maximizing exchange of oxygen, nutrients, and wastes between blood and tissues.
• The distribution of vessel structures reflects their roles in transport and pressure regulation.

🌐 EE Focus: An EE could investigate how exercise intensity affects blood pressure or cardiac output, or explore evolutionary adaptations in circulatory systems of animals adapted to extreme environments (e.g., diving mammals, high-altitude birds).

📌 Respiratory Pigments and Oxygen Transport

• Hemoglobin increases oxygen-carrying capacity of blood 70-fold compared to dissolved oxygen.
• Exhibits cooperative binding: binding of one oxygen molecule increases affinity for the next.
• Oxygen dissociation curves show how hemoglobin releases oxygen more readily in tissues with low oxygen and high CO₂ (Bohr effect).
• Myoglobin, in muscles, provides oxygen storage and ensures supply during high activity.

❤️ CAS Link: Students could run fitness awareness programs demonstrating how exercise improves cardiovascular efficiency, linking biological knowledge of transport to personal health.

📌 Coordination of Circulation with Other Systems

• Circulation is tightly linked to the respiratory system (oxygen uptake, CO₂ removal), digestive system (nutrient transport), and excretory system (waste removal).
• Hormones transported in blood regulate metabolism, growth, and homeostasis.
• The lymphatic system works alongside the circulatory system to return interstitial fluid and aid immune defense.

🔍 TOK Perspective: Much of our knowledge about circulation comes from models (e.g., William Harvey’s model of blood flow). TOK reflection: How do historical shifts in scientific models change our view of “established knowledge,” and what role does technology (microscopy, imaging) play in these shifts?

📝 Paper 2: Be ready to draw and label the mammalian heart, trace blood flow through double circulation, compare open vs closed systems, interpret oxygen dissociation curves, and apply knowledge to health and exercise contexts.