Pearson Edexcel A-Level Biology: Topic 1B Principles of Circulation

Principles of Circulation and Transport Needs

  • Definitions:

    • Diffusion: Net movement of particles down a concentration gradient.

    • Circulatory System: A mass transport system using fluids to move materials around the body.

1. Transport in Small Organisms

  • Single-celled organisms use diffusion for substance exchange due to:

    • Small Diffusion Distances: Enhanced by being microscopic.

    • High Surface Area to Volume Ratio (SA:V\text{SA:V}): Larger areas for absorption relative to volume.

    • Low Metabolic Demands: Fewer processes requiring limited oxygen/nutrients.

2. Transport Needs in Multicellular Organisms

  • Inadequacies of Diffusion:

    • Long diffusion distances make simple diffusion insufficient.

    • Increased metabolic activity demands efficient nutrient and gas delivery.

    • Need for specialized mass transport systems (circulatory systems) to ensure rapid transport and waste removal.

3. Types of Circulatory Systems

  • Open Circulatory Systems:

    • Blood flows through open body cavities; seen in insects.

  • Closed Circulatory Systems:

    • Blood contained within vessels; observed in mammals.

    • Single Circulation: Fish, blood passes through the heart once per circuit.

    • Double Circulation: Birds and mammals; blood passes through the heart twice:

    • Systemic Circulation: Oxygenated blood supplies body cells.

    • Pulmonary Circulation: Deoxygenated blood goes to lungs for re-oxygenation.

4. Composition and Functions of Blood

  • Blood Components:

    • Plasma: Liquid medium for transporting nutrients, hormones, and waste; acts as a buffer for pH regulation.

    • Erythrocytes (Red Blood Cells):

      • Transport oxygen via hemoglobin; biconcave for surface area efficiency.

    • Leukocytes (White Blood Cells):

      • Immune defense against pathogens.

    • Thrombocytes (Platelets):

      • Involved in blood clotting.

5. Gas Transport and Hemoglobin Dynamics

  • Hemoglobin Function:

    • Binds oxygen through cooperative binding, increasing efficiency at the lungs and tissues.

  • Carbon Dioxide Transport:

    • 5% dissolved in plasma, 10-20% as carbaminohemoglobin, majority as bicarbonate ions (HCO₃⁻).

    • Bicarbonate formation:         CO2+H2O→H2CO3→H++HCO3−\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^−

  • Bohr Effect: Shift in oxygen dissociation curve in tissues with high CO₂ concentrations, enhancing oxygen delivery.

6. Fetal Hemoglobin Dynamics

  • Higher Affinity for Oxygen: Fetal hemoglobin maintains saturation under lower oxygen concentrations compared to maternal hemoglobin.

  • Placental Gas Exchange: Efficient oxygen transfer between mother and fetus via counter-current flow.

7. Blood Clotting Cascade Mechanism

  • Purpose: Seals damaged blood vessels and minimizes blood loss.

  • Steps:

    • Platelet contact with exposed tissue; release thromboplastin and serotonin.

    • Thromboplastin activates the conversion of prothrombin to thrombin (requires Ca²⁺).

    • Thrombin converts fibrinogen to fibrin, forming a stable clot.

8. Anatomy of Blood Vessels

  • Arteries:

    • Thick walls to withstand high pressure; carry oxygenated blood (except pulmonary artery).

  • Capillaries:

    • Thin-walled (one cell thick) for efficient gas and nutrient exchange.

  • Veins:

    • Thinner walls, larger lumen, carry deoxygenated blood (except pulmonary vein); contain valves to prevent backflow.

Required Practicals

  1. Investigating the Effect of Surface Area on Diffusion: Experiment with agar cubes and diffusion of dye.

  2. Measuring Heart Rate Response to Exercise: Use a pulse oximeter pre/post activity.

  3. Blood Composition: Microscopic examination of blood slides to identify cell types.

Common Exam Points

  • Differences between single and double circulation.

  • Functions of components of blood.

  • Explanation and diagrams of the Bohr Effect.

  • Steps in blood clotting cascade.

  • Importance of maintaining pH balance in blood.

Key Equations

  • Surface Area to Volume Ratio: Surface AreaVolume\frac{\text{Surface Area}}{\text{Volume}}

  • Bicarbonate Formation:

    • CO2+H2O⇌H2CO3⇌H++HCO3−\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^−

    • Thrombin Reaction:

    • Fibrinogen→Fibrin\text{Fibrinogen} \rightarrow \text{Fibrin}