Organ systems

The central nervous system (CNS) is the brain and spinal cord. Signals travel along sensory, relay and motor neurones, crossing gaps called synapses. A reflex arc gives a fast, automatic response: receptor → sensory neurone → relay (CNS) → motor neurone → effector.

The breathing system (including the thorax) brings air to the lungs. Ventilation moves air in and out; gas exchange swaps oxygen into the blood and carbon dioxide out. A large surface area : volume ratio (the alveoli) makes exchange efficient.

The circulatory system has the heart, blood vessels and blood. The three vessels: arteries (carry blood from the heart, thick walls), veins (return blood, have valves) and capillaries (thin walls for exchange).

Blood components and roles: red blood cells carry oxygen; white blood cells make antibodies and carry out phagocytosis; platelets clot the blood; plasma transports dissolved substances (hormones, antibodies, urea, CO₂) and distributes heat. Multicellular organisms need exchange surfaces and a transport system because of their small surface area : volume ratio.

The digestive system breaks food down. Peristalsis pushes food along; digestion breaks large molecules into small ones; absorption takes them into the blood; egestion removes undigested waste.

The excretory system, mainly the kidneys (with the nephron as the working unit), removes waste and helps maintain a constant internal environment (homeostasis).

• Mixing up the vessels: arteries away from the heart, veins back to it.
• Forgetting why exchange surfaces matter (the surface area : volume ratio).