Most tested B9.2

Nervous and Respiratory Systems

This topic covers the structure and function of key animal organ systems, including the nervous, respiratory, circulatory, digestive, and excretory systems. It explains how these systems work together to enable response, gas exchange, transport, nutrition, and waste removal, which are fundamental to understanding multicellular life.

Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • The Central Nervous System (CNS), comprising the brain and spinal cord, processes information. A typical nerve pathway is: Stimulus → Receptor → Sensory Neurone → CNS (Relay Neurone) → Motor Neurone → Effector → Response.
  • Gas exchange occurs in the lungs' alveoli, which have a vast surface area, thin walls, and a rich blood supply to maximise oxygen diffusion into the blood and carbon dioxide out.
  • The double circulatory system in humans uses the four-chambered heart to pump deoxygenated blood to the lungs and oxygenated blood to the rest of the body through arteries, veins, and capillaries.
  • Blood consists of four main components: red blood cells (oxygen transport), white blood cells (immune defence), platelets (clotting), and plasma (transport medium).
  • In the digestive system, peristalsis moves food along the gut, where enzymes break it down chemically and the products are absorbed, primarily in the small intestine.
  • The kidneys are crucial for homeostasis. Their functional units, nephrons, filter the blood to remove waste products like urea and to selectively reabsorb water and useful substances, forming urine.

Formulae

SA:V Ratio = Surface Area / Volume

To explain why multicellular organisms require specialised organ systems. As an organism gets larger, its volume increases faster than its surface area (the SA:V ratio decreases), making diffusion across its outer surface insufficient for transport and exchange.

Definitions

Synapse
A microscopic gap between two neurones. An electrical impulse triggers the release of chemical neurotransmitters across the gap to initiate an impulse in the next neurone.
Reflex Arc
The neural pathway that mediates a rapid, involuntary response (a reflex). It typically involves a sensory neurone, a relay neurone in the spinal cord, and a motor neurone, bypassing conscious thought in the brain for speed.
Alveoli
Tiny air sacs at the end of the bronchioles in the lungs. They are the primary sites of gas exchange between the air and the blood, adapted with a large surface area and thin walls.
Homeostasis
The maintenance of a constant and stable internal environment in an organism, despite external changes. For example, the kidneys regulate water and salt balance in the blood.
Nephron
The microscopic structural and functional unit of the kidney, responsible for filtering blood and forming urine through filtration, reabsorption, and secretion.

Worked example

An impulse travels along a reflex arc. The sensory neurone is 60 cm long and the motor neurone is 20 cm long; the impulse travels at 80 m/s along both. The connecting relay neurone in the spinal cord is 2 mm long, and the impulse speed along it is 4 m/s. There are two synapses in the pathway, each causing a 1 ms delay. Calculate the total time for the impulse to travel from the receptor to the effector.

  1. 1

    Step 1:

    Convert all distances to metres.

    Sensory neurone = 0.6 m.

    Motor neurone = 0.2 m.

    Relay neurone = 0.002 m.

  2. 2

    Step 2:

    Calculate time along the sensory neurone.

    Time = distance / speed = 0.6 m / 80 m/s = 0.0075 s
  3. 3

    Step 3:

    Calculate time along the motor neurone.

    Time = distance / speed = 0.2 m / 80 m/s = 0.0025 s
  4. 4

    Step 4:

    Calculate time along the relay neurone.

    Time = distance / speed = 0.002 m / 4 m/s = 0.0005 s
  5. 5

    Step 5:

    Convert all times to milliseconds (ms) for easier addition.

    0.0075 s = 7.5 ms
    0.0025 s = 2.5 ms
    0.0005 s = 0.5 ms
  6. 6

    Step 6:

    Calculate total synaptic delay.

    2 synapses × 1 ms/synapse = 2 ms.

  7. 7

    Step 7:

    Sum all time components.

    Total time = 7.5 ms + 2.5 ms + 0.5 ms + 2 ms = 12.5 ms

Answer: 12.5 ms

Common mistakes

  • ×Misinterpreting diagrams of the circulatory or nervous systems. Always trace the full path, for example, confirming the direction of blood flow from atria to ventricles, or the correct sequence of neurones in a reflex arc (sensory → relay → motor).
  • ×Forgetting hidden components in a process. For nerve impulse calculations, remember to include both the time spent travelling along the neurones AND the time delay at each synapse.
  • ×Unit conversion errors are frequent. Be systematic in converting all values to a base unit (e.g., metres and seconds) before performing calculations. A common mistake is mixing cm, mm and m, or ms and s.

No-calculator tips

  • ✓Use powers of ten for unit conversions to avoid writing out long decimals. For example, 60 cm = 0.6 m = 6 x 10-1 m, and 1 ms = 10-3 s. This simplifies multiplication and division.
  • ✓Break down multi-step calculations into simpler parts. Calculate the time for each segment of a pathway separately, then sum them at the end. This reduces the chance of arithmetic errors.
  • ✓When dealing with ratios like SA:V, think in terms of simple numbers. For a cube of side length L, SA is 6L2 and V is L3, so the ratio is 6/L. This clearly shows that as L increases, the ratio decreases, without needing to calculate for complex shapes.

Read this topic in the official UAT-UK ESAT guide →

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