Biology (5090)
Topic 4 of 4Cambridge O Levels

Human Body Systems

Circulatory, respiratory, and nervous systems

The human body has several organ systems that work together to keep you alive.


Circulatory System:

  • Heart: Double pump — right side pumps to lungs, left side pumps to body
  • Blood vessels: Arteries (away from heart, thick walls, high pressure), Veins (to heart, thinner walls, have valves), Capillaries (tiny, one cell thick for exchange)
  • Blood: Red blood cells (carry O₂), White blood cells (fight infection), Platelets (clotting), Plasma (liquid, carries dissolved substances)
  • Double circulation: Heart → Lungs → Heart → Body → Heart

    • Respiratory System:

  • Air path: Nose → Trachea → Bronchi → Bronchioles → Alveoli
  • Alveoli: Tiny air sacs where gas exchange happens
  • O₂ diffuses from alveoli into blood; CO₂ diffuses from blood into alveoli
  • Alveoli adaptations: huge surface area, thin walls (one cell), rich blood supply

    • Aerobic Respiration: Glucose + Oxygen → Carbon Dioxide + Water + Energy

    C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy


    Anaerobic Respiration: Without oxygen (during intense exercise)

    Glucose → Lactic Acid + Some Energy

    Creates an "oxygen debt" — you breathe heavily after exercise to repay it.

    Key Points to Remember

    • 1Heart is a double pump: right→lungs, left→body
    • 2Arteries = thick walls, away from heart. Veins = valves, to heart
    • 3Gas exchange at alveoli: O₂ in, CO₂ out by diffusion
    • 4Aerobic: glucose + O₂ → CO₂ + H₂O + energy

    Pakistan Example

    Cricketers' Fitness & Respiration

    When Babar Azam sprints between wickets, his muscles need energy fast. At first, aerobic respiration provides energy: Glucose + O₂ → CO₂ + H₂O + Energy. But during an intense sprint, his body can't deliver oxygen fast enough, so anaerobic respiration kicks in: Glucose → Lactic Acid + less energy. The lactic acid buildup causes the burning feeling in muscles. After the sprint, he breathes heavily — this is repaying the oxygen debt by breaking down lactic acid. His heart rate increases to pump more oxygenated blood to muscles. That's why cricketers train their cardiovascular fitness!

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