Transport Systems in Animal Biology

May 21, 2025

OCR (A) Biology A-Level: Transport in Animals

Circulatory Systems

  • Types of Circulatory Systems
    • Open system: Found in insects
    • Closed system: Found in fish and mammals
      • Single circulatory system: Heart with two chambers, blood passes through heart once per circuit.
      • Double circulatory system: Heart with four chambers, blood passes through heart twice per circuit.

Important Blood Vessel Structures and Functions

  • Arteries
    • Carry blood away from the heart.
    • Thick walls to withstand high pressure.
    • Contain elastic tissue for stretching and recoiling.
    • Smooth muscle to vary blood flow.
    • Lined with smooth endothelium to reduce friction.
  • Arterioles
    • Branch off arteries.
    • Thinner and less muscular walls.
    • Feed blood into capillaries.
  • Capillaries
    • Smallest blood vessels.
    • Site of metabolic exchange.
    • One cell thick for fast substance exchange.
  • Venules
    • Larger than capillaries but smaller than veins.
  • Veins
    • Carry blood to the heart.
    • Wide lumen for increased blood volume.
    • Thin walls since blood is under low pressure.
    • Valves to prevent backflow.
    • Little elastic tissue or smooth muscle due to lack of pulse.

Tissue Fluid and Its Functions

  • Contains dissolved oxygen and nutrients.
  • Provides tissues with essential solutes and exchanges for waste.
  • Formed by hydrostatic pressure forcing blood fluid out of capillaries.
  • Osmotic pressure pushes fluid back into capillaries.
  • Lymphatic system returns remaining tissue fluid, containing lymph fluid and nodes that filter bacteria.

Mammalian Heart and Cardiac Cycle

  • Myogenic Heart: Initiates its own contraction.

    • Sinoatrial Node (SAN): Pacemaker in the right atrium, initiates contraction.
    • Atrioventricular Node (AVN): Conducts excitation to ventricles via the bundle of His and Purkyne fibres.
  • Cardiac Cycle Stages

    1. Atrial Systole: Atria contract, AV valves open, blood flows into ventricles.
    2. Ventricular Systole: Ventricles contract, AV valves close, semilunar valves open, blood ejected.
    3. Cardiac Diastole: Atria and ventricles relax, semilunar valves close, blood drawn into heart.

Haemoglobin and Oxygen Transport

  • Structure: Water-soluble protein with two alpha and two beta chains, each containing a haem group.

  • Function: Binds and carries oxygen, released as needed.

  • Oxygen Affinity and Partial Pressure

    • High partial pressure increases haemoglobin's oxygen affinity (loading in lungs).
    • Low partial pressure decreases affinity (unloading in tissues).
  • Dissociation Curves

    • Show haemoglobin saturation changes with partial pressure.
  • Fetal vs. Adult Haemoglobin

    • Fetal haemoglobin has higher oxygen affinity to compensate for decreased oxygen saturation in placenta.
  • Bohr Effect

    • Presence of CO2 decreases oxygen affinity, promoting oxygen release.